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THE ELEMENTS OF GEOLOGY
BY WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
PREFACE
Geology is a science of such rapid growth that no apology is
expected when from time to time a new text-book is added to those
already in the field. The present work, however, is the outcome of
the need of a text-book of very simple outline, in which causes
and their consequences should be knit together as closely as
possible,--a need long felt by the author in his teaching, and
perhaps by other teachers also. The author has ventured,
therefore, to depart from the common usage which subdivides
geology into a number of departments,--dynamical, structural,
physiographic, and historical,--and to treat in immediate
connection with each geological process the land forms and the
rock structures which it has produced.
It is hoped that the facts of geology and the inferences drawn
from them have been so presented as to afford an efficient
discipline in inductive reasoning. Typical examples have been used
to introduce many topics, and it has been the author's aim to give
due proportion to both the wide generalizations of our science and
to the concrete facts on which they rest.
There have been added a number of practical exercises such as the
author has used for several years in the class room. These are not
made so numerous as to displace the problems which no doubt many
teachers prefer to have their pupils solve impromptu during the
recitation, but may, it is hoped, suggest their use.
In historical geology a broad view is given of the development of
the North American continent and the evolution of life upon the
planet. Only the leading types of plants and animals are
mentioned, and special attention is given to those which mark the
lines of descent of forms now living.
By omitting much technical detail of a mineralogical and
paleontological nature, and by confining the field of view almost
wholly to our own continent, space has been obtained to give to
what are deemed for beginners the essentials of the science a
fuller treatment than perhaps is common.
It is assumed that field work will be introduced with the
commencement of the study. The common rocks are therefore briefly
described in the opening chapters. The drift also receives early
mention, and teachers in the northern states who begin geology in
the fall may prefer to take up the chapter on the Pleistocene
immediately after the chapter on glaciers.
Simple diagrams have been used freely, not only because they are
often clearer than any verbal statement, but also because they
readily lend themselves to reproduction on the blackboard by the
pupil. The text will suggest others which the pupil may invent. It
is hoped that the photographic views may also be used for
exercises in the class room.
The generous aid of many friends is recognized with special
pleasure. To Professor W. M. Davis of Harvard University there is
owing a large obligation for the broad conceptions and luminous
statements of geologic facts and principles with which he has
enriched the literature of our science, and for his stimulating
influence in education. It is hoped that both in subject-matter
and in method the book itself makes evident this debt. But besides
a general obligation shared by geologists everywhere, and in
varying degrees by perhaps all authors of recent American text-
books in earth science, there is owing a debt direct and personal.
The plan of the book, with its use of problems and treatment of
land forms and rock structures in immediate connection with the
processes which produce them, was submitted to Professor Davis,
and, receiving his approval, was carried into effect, although
without the sanction of precedent at the time. Professor Davis
also kindly consented to read the manuscript throughout, and his
many helpful criticisms and suggestions are acknowledged with
sincere gratitude.
Parts of the manuscript have been reviewed by Dr. Samuel Calvin
and Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W.
Beyer of the Iowa College of Agriculture and Mechanic Arts; Dr. U.
S. Grant of Northwestern University; Professor J. A. Udden of
Augustana College, Illinois; Dr. C. H. Gordon of the New Mexico
State School of Mines; Principal Maurice Ricker of the High
School, Burlington, Iowa; and the following former students of the
author who are engaged in the earth sciences: Dr. W. C. Alden of
the United States Geological Survey and the University of Chicago;
Mr. Joseph Sniffen, instructor in the Academy of the University of
Chicago, Morgan Park; Professor Martin Iorns, Fort Worth
University, Texas; Professor A. M. Jayne, Dakota University;
Professor G. H. Bretnall, Monmouth College, Illinois; Professor
Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,
instructor in the Iowa State Normal College; Principal C. C. Gray
of the High School, Fargo, North Dakota; and Mr. Charles Persons
of the High School, Hannibal, Missouri. A large number of the
diagrams of the book were drawn by Mr. W. W. White of the Art
School of Cornell College. To all these friends, and to the many
who have kindly supplied the illustrations of the text, whose
names are mentioned in an appended list, the writer returns his
heartfelt thanks.
WILLIAM HARMON NORTON
CORNELL COLLEGE, MOUNT VERNON, IOWA
JULY, 1905
INTRODUCTORY NOTE
During the preparation of this book Professor Norton has
frequently discussed its plan with me by correspondence, and we
have considered together the matters of scope, arrangement, and
presentation.
As to scope, the needs of the young student and not of the expert
have been our guide; the book is therefore a text-book, not a
reference volume.
In arrangement, the twofold division of the subject was chosen
because of its simplicity and effectiveness. The principles of
physical geology come first; the several chapters are arranged in
what is believed to be a natural order, appropriate to the
greatest part of our country, so that from a simple beginning a
logical sequence of topics leads through the whole subject. The
historical view of the science comes second, with many specific
illustrations of the physical processes previously studied, but
now set forth as part of the story of the earth, with its many
changes of aspect and its succession of inhabitants. Special
attention is here given to North America, and care is taken to
avoid overloading with details.
With respect to method of presentation, it must not be forgotten
that the text-book is only one factor in good teaching, and that
in geology, as in other sciences, the teacher, the laboratory, and
the local field are other factors, each of which should play an
appropriate part. The text suggests observational methods, but it
cannot replace observation in field or laboratory; it offers
certain exercises, but space cannot be taken to make it a
laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the
illustrative descriptions that a good and experienced teacher
should supply. Frequent use is made of induction and inference in
order that the student may come to see how reasonable a science is
geology, and that he may avoid the too common error of thinking
that the opinions of "authorities" are reached by a private road
that is closed to him. The further extension of this method of
presentation is urged upon the teacher, so that the young
geologist may always learn the evidence that leads to a
conclusion, and not only the conclusion itself.
W. M. DAVIS
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
JULY, 1905
CONTENTS
INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY
PART I
EXTERNAL GEOLOGICAL AGENCIES
I. THE WORK OF THE WEATHER
II. THE WORK OF GROUND WATER
III. RIVERS AND VALLEYS
IV. RIVER DEPOSITS
V. THE WORK OF GLACIERS
VI. THE WORK OF THE WIND
VII. THE SEA AND ITS SHORES
VIII. OFFSHORE AND DEEP-SEA DEPOSITS
PART II
INTERNAL GEOLOGICAL AGENCIES
IX. MOVEMENTS OF THE EARTH'S CRUST
X. EARTHQUAKES
XI. VOLCANOES
XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
XIII. METAMORPHISM AND MINERAL VEINS
PART III
HISTORICAL GEOLOGY
XIV. THE GEOLOGICAL RECORD
XV. THE PRE-CAMBRIAN SYSTEMS
XVI. THE CAMBRIAN
XVII. THE ORDOVICIAN AND SILURIAN
XVIII. THE DEVONIAN
XIX. THE CARBONIFEROUS
XX. THE MESOZOIC
XXI. THE TERTIARY
XXII. THE QUATERNARY
INDEX
THE ELEMENTS OF GEOLOGY
INTRODUCTION
THE SCOPE AND AIM OF GEOLOGY
Geology deals with the rocks of the earth's crust. It learns from
their composition and structure how the rocks were made and how
they have been modified. It ascertains how they have been brought
to their present places and wrought to their various topographic
forms, such as hills and valleys, plains and mountains. It studies
the vestiges which the rocks preserve of ancient organisms which
once inhabited our planet. Geology is the history of the earth and
its inhabitants, as read in the rocks of the earth's crust.
To obtain a general idea of the nature and method of our science
before beginning its study in detail, we may visit some valley,
such as that illustrated in the frontispiece, on whose sides are
rocky ledges. Here the rocks lie in horizontal layers. Although
only their edges are exposed, we may infer that these layers run
into the upland on either side and underlie the entire district;
they are part of the foundation of solid rock which everywhere is
found beneath the loose materials of the surface.
The ledges of the valley of our illustration are of sandstone.
Looking closely at the rock we see that it is composed of myriads
of grains of sand cemented together. These grains have been worn
and rounded. They are sorted also, those of each layer being about
of a size. By some means they have been brought hither from some
more ancient source. Surely these grains have had a history before
they here found a resting place,--a history which we are to learn
to read.
The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses
of stone in a wall were laid before the courses which rest upon
them.
We have no reason to believe that the lowest layers which we see
here were the earliest ever formed. Indeed, some deep boring in
the vicinity may prove that the ledges rest upon other layers of
rock which extend downward for many hundreds of feet below the
valley floor. Nor may we conclude that the highest layers here
were the latest ever laid; for elsewhere we may find still later
layers lying upon them.
A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as
these are of kinds whose nearest living relatives now have their
home in the sea, we infer that it was on the flat sea floor that
the sandstone was laid. Its present position hundreds of feet
above sea level proves that it has since emerged to form part of
the land; while the flatness of the beds shows that the movement
was so uniform and gentle as not to break or strongly bend them
from their original attitude.
The surface of some of these layers is ripple-marked. Hence the
sand must once have been as loose as that of shallow sea bottoms
and sea beaches to-day, which is thrown into similar ripples by
movements of the water. In some way the grains have since become
cemented into firm rock.
Note that the layers on one side of the valley agree with those on
the other, each matching the one opposite at the same level. Once
they were continuous across the valley. Where the valley now is
was once a continuous upland built of horizontal layers; the
layers now show their edges, or OUTCROP, on the valley sides
because they have been cut by the valley trench.
The rock of the ledges is crumbling away. At the foot of each step
of rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in
the future.
Through the valley runs a stream. The waters of rains which have
fallen on the upper parts of the stream's basin are now on their
way to the river and the sea. Rock fragments and grains of sand
creeping down the valley slopes come within reach of the stream
and are washed along by the running water. Here and there they
lodge for a time in banks of sand and gravel, but sooner or later
they are taken up again and carried on. The grains of sand which
were brought from some ancient source to form these rocks are on
their way to some new goal. As they are washed along the rocky bed
of the stream they slowly rasp and wear it deeper. The valley will
be deeper in the future; it has been less deep in the past.
In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the
rocks, and the land forms to which they have been sculptured, the
record of a long succession of past changes involving the origin
of sand grains and their gathering and deposit upon the bottom of
some ancient sea, the cementation of their layers into solid rock,
the uplift of the rocks to form a land surface, and, last of all,
the carving of a valley in the upland. Everywhere, in the fields,
along the river, among the mountains, by the seashore, and in the
desert, we may discover slow changes now in progress and the
record of similar changes in the past. Everywhere we may catch
glimpses of a process of gradual change, which stretches backward
into the past and forward into the future, by which the forms and
structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long
process is geology. Geology treats of the natural changes now
taking place upon the earth and within it, the agencies which
produce them, and the land forms and rock structures which result.
It studies the changes of the present in order to be able to read
the history of the earth's changes in the past.
The various agencies which have fashioned the face of the earth
may. be divided into two general classes. In Part I we shall
consider those which work upon the earth from without, such as the
weather, running water, glaciers, the wind, and the sea. In Part
II we shall treat of those agencies whose sources are within the
earth, and among whose manifestations are volcanoes and
earthquakes and the various movements of the earth's crust. As we
study each agency we shall notice not only how it does its work,
but also the records which it leaves in the rock structures and
the land forms which it produces. With this preparation we shall
be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.
PART I
EXTERNAL GEOLOGICAL AGENCIES
CHAPTER I
THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed
a process which is going forward in all lands. Everywhere the
rocks are crumbling away; their fragments are creeping down
hillsides to the stream ways and are carried by the streams to the
sea, where they are rebuilt into rocky layers. When again the
rocks are lifted to form land the process will begin anew; again
they will crumble and creep down slopes and be washed by streams
to the sea. Let us begin our study of this long cycle of change at
the point where rocks disintegrate and decay under the action of
the weather. In studying now a few outcrops and quarries we shall
learn a little of some common rocks and how they weather away.
STRATIFICATION AND JOINTING. At the sandstone ledges we saw that
the rock was divided into parallel layers. The thicker layers are
known as STRATA, and the thin leaves into which each stratum may
sometimes be split are termed LAMINAE. To a greater or less degree
these layers differ from each other in fineness of grain, showing
that the material has been sorted. The planes which divide them
are called BEDDING PLANES.
Besides the bedding planes there are other division planes, which
cut across the strata from top to bottom. These are found in all
rocks and are known as joints. Two sets of joints,
running at about right angles to each other, together with the
bedding planes, divide the sandstone into quadrangular blocks.
SANDSTONE. Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of
the grains are of a clear glassy mineral called quartz. These
quartz grains are very hard and will scratch the steel of a knife
blade. They are not affected by acid, and their broken surfaces
are irregular like those of broken glass.
The grains of sandstone are held together by some cement. This may
be calcareous, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly ferruginous,--hydrated iron
oxide, or iron rust, forming the bond, somewhat as in the case of
iron nails which have rusted together. The strongest and most
lasting cement is siliceous, and sand rocks whose grains are
closely cemented by silica, the chemical substance of which quartz
is made, are known as quartzites.
We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view
its surface is more or less discolored and the grains are loose
and may be rubbed off with the finger. On gentle slopes the rock
is covered with a soil composed of sand, which evidently is
crumbled sandstone, and dark carbonaceous matter derived from the
decay of vegetation. Clearly it is by the dissolving of the cement
that the rock thus breaks down to loose sand. A piece of sandstone
with calcareous cement, or a bit of old mortar, which is really an
artificial stone also made of sand cemented by lime, may be
treated in a test tube with hydrochloric acid to illustrate the
process.
A LIMESTONE QUARRY. Here also we find the rock stratified and
jointed (Fig. 2). On the quarry face the rock is distinctly seen
to be altered for some distance from its upper surface. Below the
altered zone the rock is sound and is quarried for building; but
the altered upper layers are too soft and broken to be used for
this purpose. If the limestone is laminated, the laminae here have
split apart, although below they hold fast together. Near the
surface the stone has become rotten and crumbles at the touch,
while on the top it has completely broken down to a thin layer of
limestone meal, on which rests a fine reddish clay.
Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed
in a test tube with hydrochloric acid dissolves with brisk
effervescence, leaving the insoluble impurities, which were
disseminated through it, at the bottom of the tube as a little
clay.
We can now understand the changes in the upper layers of the
quarry. At the surface of the rock the limestone has completely
dissolved, leaving the insoluble residue as a layer of reddish
clay. Immediately below the clay the rock has disintegrated into
meal where the cement between the limestone grains has been
removed, while beneath this the laminae are split apart where the
cement has been dissolved only along the planes of lamination
where the stone is more porous. As these changes in the rock are
greatest at the surface and diminish downward, we infer that they
have been caused by agents working downward from the surface.
At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that
the rock is altered even down to the quarry floor.
A SHALE PIT. Let us now visit some pit where shale--a laminated
and somewhat hardened clay--is quarried for the manufacture of
brick. The laminae of this fine-grained rock may be as thin as
cardboard in places, and close joints may break the rock into
small rhombic blocks. On the upper surface we note that the shale
has weathered to a clayey soil in which all traces of structure
have been destroyed. The clay and the upper layers of the shale
beneath it are reddish or yellow, while in many cases the color of
the unaltered rock beneath is blue.
THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are
the leading types of the great group of stratified, or
sedimentary, rocks. This group includes all rocks made of
sediments, their materials having settled either in water upon the
bottoms of rivers, lakes, or seas, or on dry land, as in the case
of deposits made by the wind and by glaciers. Sedimentary rocks
are divided into the fragmental rocks--which are made of
fragments, either coarse or fine--and the far less common rocks
which are constituted of chemical precipitates.
The sedimentary rocks are divided according to their composition
into the following classes:
1. The arenaceous, or quartz rocks, including beds of loose sand
and gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).
2. The calcareous, or lime rocks, including limestone and a soft
white rock formed of calcareous powder known as chalk.
3. The argillaceous, or clay rocks, including muds, clays, and
shales. These three classes pass by mixture into one another. Thus
there are limy and clayey sandstones, sandy and clayey limestones,
and sandy and limy shales.
GRANITE. This familiar rock may be studied as an example of the
second great group of rocks,--the unstratified, or igneous rocks.
These are not made of cemented sedimentary grains, but of
interlocking crystals which have crystallized from a molten mass.
Examining a piece of granite, the most conspicuous crystals which
meet the eye are those of feldspar. They are commonly pink, white,
or yellow, and break along smooth cleavage planes which reflect
the light like tiny panes of glass. Mica may be recognized by its
glittering plates, which split into thin elastic scales. A third
mineral, harder than steel, breaking along irregular surfaces like
broken glass, we identify as quartz.
How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying
the loose formations of the surface, and in many parts of the
northern states where granite bowlders and pebbles more or less
decayed may be found in a surface sheet of stony clay called the
drift. Of the different minerals composing granite, quartz alone
remains unaltered. Mica weathers to detached flakes which have
lost their elasticity. The feldspar crystals have lost their
luster and hardness, and even have decayed to clay. Where long-
weathered granite forms the country rock, it often may be cut with
spade or trowel for several feet from the surface, so rotten is
the feldspar, and here the rock is seen to break down to a clayey
soil containing grains of quartz and flakes of mica.
These are a few simple illustrations of the surface changes which
some of the common kinds of rocks undergo. The agencies by which
these changes are brought about we will now take up under two
divisions,--CHEMICAL AGENCIES producing rock decay and MECHANICAL
AGENCIES producing rock disintegration.
THE CHEMICAL WORK OF WATER
As water falls on the earth in rain it has already absorbed from
the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks
into the ground and becomes what is termed ground water, it takes
into solution from the soil humus acids and carbon dioxide, both
of which are constantly being generated there by the decay of
organic matter. So both rain and ground water are charged with
active chemical agents, by the help of which they corrode and rust
and decompose all rocks to a greater or less degree. We notice now
three of the chief chemical processes concerned in weathering,--
solution, the formation of carbonates, and oxidation.
SOLUTION. Limestone, although so little affected by pure water
that five thousand gallons would be needed to dissolve a single
pound, is easily dissolved in water charged with carbon dioxide.
In limestone regions well water is therefore "hard." On boiling
the water for some time the carbon dioxide gas is expelled, the
whole of the lime carbonate can no longer be held in solution, and
much of it is thrown down to form a crust or "scale" in the kettle
or in the tubes of the steam boiler. All waters which flow over
limestone rocks or soak through them are constantly engaged in
dissolving them away, and in the course of time destroy beds of
vast extent and great thickness.
The upper surface of limestone rocks becomes deeply pitted, as we
saw in the limestone quarry, and where the mantle of waste has
been removed it may be found so intricately furrowed that it is
difficult to traverse.
Beds of rock salt buried among the strata are dissolved by seeping
water, which issues in salt springs. Gypsum, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched
with the finger nail, is readily taken up by water, giving to the
water of wells and springs a peculiar hardness difficult to
remove.
The dissolving action of moisture may be noted on marble
tombstones of some age, marble being a limestone altered by heat
and pressure and composed of crystalline grains. By assuming that
the date on each monument marks the year of its erection, one may
estimate how many years on the average it has taken for weathering
to loosen fine grains on the polished surface, so that they may be
rubbed off with the finger, to destroy the polish, to round the
sharp edges of tool marks in the lettering, and at last to open
cracks and seams and break down the stone. We may notice also
whether the gravestones weather more rapidly on the sunny or the
shady side, and on the sides or on the top.
The weathered surface of granular limestone containing shells
shows them standing in relief. As the shells are made of
crystalline carbonate of lime, we may infer whether the carbonate
of lime is less soluble in its granular or in its crystalline
condition.
THE FORMATION OF CARBONATES. In attacking minerals water does more
than merely take them into solution. It decomposes them, forming
new chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken
up by the action of water containing carbon dioxide, forming
alkaline carbonates. These carbonates are freely soluble and
contribute potash and soda to soils and river waters. By the
removal of the soluble ingredients of feldspar there is left the
silicate of alumina, united with water or hydrated, in the
condition of a fine plastic clay which, when white and pure, is
known as KAOLIN and is used in the manufacture of porcelain.
Feldspathic rocks which contain no iron compounds thus weather to
whitish crusts, and even apparently sound crystals of feldspar,
when ground to thin slices and placed under the microscope, may be
seen to be milky in color throughout because an internal change to
kaolin has begun.
OXIDATION. Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty
films. Oxygen and water have here united with the iron, forming
hydrated iron oxide. The effects of oxidation may be seen in the
alteration of many kinds of rocks and in red and yellow colors of
soils and subsoils.
Pyrite is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and
sulphur (iron sulphide). Under the attack of the weather it takes
up oxygen, forming iron sulphate (green vitriol), a soluble
compound, and insoluble hydrated iron oxide, which as a mineral is
known as limonite. Several large masses of iron sulphide were
placed some years ago on the lawn in front of the National Museum
at Washington. The mineral changed so rapidly to green vitriol
that enough of this poisonous compound was washed into the ground
to kill the roots of the surrounding grass.
AGENTS OF MECHANICAL DISINTEGRATION
HEAT AND COLD. Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool
and contract. When the difference in temperature between day and
night is considerable, the repeated strains of sudden expansion
and contraction at last become greater than the rocks can bear,
and they break, for the same reason that a glass cracks when
plunged into boiling water (Fig. 5).
Rocks are poor conductors of heat, and hence their surfaces may
become painfully hot under the full blaze of the sun, while the
interior remains comparatively cool. By day the surface shell
expands and tends to break loose from the mass of the stone. In
cooling in the evening the surface shell suddenly contracts on the
unyielding interior and in time is forced off in scales.
Many rocks, such as granite, are made up of grains of various
minerals which differ in color and in their capacity to absorb
heat, and which therefore contract and expand in different ratios.
In heating and cooling these grains crowd against their neighbors
and tear loose from them, so that finally the rock disintegrates
into sand.
The conditions for the destructive action of heat and cold are
most fully met in arid regions when vegetation is wanting for lack
of sufficient rain. The soil not being held together by the roots
of plants is blown away over large areas, leaving the rocks bare
to the blazing sun in a cloudless sky. The air is dry, and the
heat received by the earth by day is therefore rapidly radiated at
night into space. There is a sharp and sudden fall of temperature
after sunset, and the rocks, strongly heated by day, are now
chilled perhaps even to the freezing point.
In the Sahara the thermometer has been known to fall 131 degrees
F. within a few hours. In the light air of the Pamir plateau in
central Asia a rise of 90 degrees F. has been recorded from seven
o'clock in the morning to one o'clock in the afternoon. On the
mountains of southwestern Texas there are frequently heard
crackling noises as the rocks of that arid region throw off scales
from a fraction of an inch to four inches in thickness, and loud
reports are made as huge bowlders split apart. Desert pebbles
weakened by long exposure to heat and cold have been shivered to
fine sharp-pointed fragments on being placed in sand heated to 180
degrees F. Beds half a foot thick, forming the floor of limestone
quarries in Wisconsin, have been known to buckle and arch and
break to fragments under the heat of the summer sun.
FROST. By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more
or less porous and all contain more or less water in their pores.
Workers in stone call this "quarry water," and speak of a stone as
"green" before the quarry water has dried out. Water also seeps
along joints and bedding planes and gathers in all seams and
crevices. Water expands in freezing, ten cubic inches of water
freezing to about eleven cubic inches of ice. As water freezes in
the rifts and pores of rocks it expands with the irresistible
force illustrated in the freezing and breaking of water pipes in
winter. The first rift in the rock, perhaps too narrow to be seen,
is widened little by little by the wedges of successive frosts,
and finally the rock is broken into detached blocks, and these
into angular chip-stone by the same process.
It is on mountain tops and in high latitudes that the effects of
frost are most plainly seen. "Every summit" says Whymper, "amongst
the rock summits upon which I have stood has been nothing but a
piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen,
in Kamchatka, and in other frigid lands large areas are thickly
strewn with sharp-edged fragments into which the rock has been
shattered by frost.
ORGANIC AGENTS
We must reckon the roots of plants and trees among the agents
which break rocks into pieces. The tiny rootlet in its search for
food and moisture inserts itself into some minute rift, and as it
grows slowly wedges the rock apart. Moreover, the acids of the
root corrode the rocks with which they are in contact. One may
sometimes find in the soil a block of limestone wrapped in a mesh
of roots, each of which lies in a little furrow where it has eaten
into the stone.
Rootless plants called lichens often cover and corrode rocks as
yet bare of soil; but where lichens are destroying the rock less
rapidly than does the weather, they serve in a way as a
protection.
CONDITIONS FAVORING DISINTEGRATION AND DECAY. The
disintegration of rocks under frost and temperature changes
goes on most rapidly in cold and arid climates, and where
vegetation is scant or absent. On the contrary, the decay of rocks
under the chemical action of water is favored by a warm, moist
climate and abundant vegetation. Frost and heat and cold can only
act within the few feet from the surface to which the necessary
temperature changes are limited, while water penetrates and alters
the rocks to great depths.
The pupil may explain.
In what ways the presence of joints and bedding planes assists in
the breaking up and decay of rocks under the action of the
weather.
Why it is a good rule of stone masons never to lay stones on edge,
but always on their natural bedding planes.
Why stones fresh from the quarry sometimes go to pieces in early
winter, when stones which have been quarried for some months
remain uninjured.
Why quarrymen in the northern states often keep their quarry
floors flooded during winter.
Why laminated limestone should not be used for curbstone.
Why rocks composed of layers differing in fineness of grain and in
ratios of expansion do not make good building stone.
Fine-grained rocks with pores so small that capillary attraction
keeps the water which they contain from readily draining away are
more apt to hold their pores ten elevenths full of water than are
rocks whose pores are larger. Which, therefore, are more likely to
be injured by frost?
Which is subject to greater temperature changes, a dark rock or
one of a light color? the north side or the south side of a
valley?
THE MANTLE OF ROCK WASTE
We have seen that rocks are everywhere slowly wasting away. They
are broken in pieces by frost, by tree roots, and by heat and
cold. They dissolve and decompose under the chemical action of
water and the various corrosive substances which it contains,
leaving their insoluble residues as residual clays and sands upon
the surface. As a result there is everywhere forming a mantle of
rock waste which covers the land. It is well to imagine how the
country would appear were this mantle with its soil and vegetation
all scraped away or had it never been formed. The surface of the
land would then be everywhere of bare rock as unbroken as a quarry
floor.
THE THICKNESS OF THE MANTLE. In any locality the thickness of the
mantle of rock waste depends as much on the rate at which it is
constantly being removed as on the rate at which it is forming. On
the face of cliffs it is absent, for here waste is removed as fast
as it is made. Where waste is carried away more slowly than it is
produced, it accumulates in time to great depth.
The granite of Pikes Peak is disintegrated to a depth of twenty
feet. In the city of Washington granite rock is so softened to a
depth of eighty feet that it can be removed with pick and shovel.
About Atlanta, Georgia, the rocks are completely rotted for one
hundred feet from the surface, while the beginnings of decay may
be noticed at thrice that depth. In places in southern Brazil the
rock is decomposed to a depth of four hundred feet.
In southwestern Wisconsin a reddish residual clay has an average
depth of thirteen feet on broad uplands, where it has been removed
to the least extent. The country rock on which it rests is a
limestone with about ten per cent of insoluble impurities. At
least how thick, then, was that portion of the limestone which has
rotted down to the clay?
DISTINGUISHING CHARACTERISTICS OF RESIDUAL WASTE. We must learn to
distinguish waste formed in place by the action of the weather
from the products of other geological agencies. Residual waste is
unstratified. It contains no substances which have not been
derived from the weathering of the parent rock. There is a gradual
transition from residual waste into the unweathered rock beneath.
Waste resting on sound rock evidently has been shifted and was not
formed in place.
In certain regions of southern Missouri the land is covered with a
layer of broken flints and red clay, while the country rock is
limestone. The limestone contains nodules of flint, and we may
infer that it has been by the decay and removal of thick masses of
limestone that the residual layer of clay and flints has been left
upon the surface. Flint is a form of quartz, dull-lustered,
usually gray or blackish in color, and opaque except on thinnest
edges, where it is translucent.
Over much of the northern states there is spread an unstratified
stony clay called the drift. It often rests on sound rocks. It
contains grains of sand, pebbles, and bowlders composed of many
different minerals and rocks that the country rock cannot furnish.
Hence the drift cannot have been formed by the decay of the rock
of the region. A shale or limestone, for example, cannot waste to
a clay containing granite pebbles. The origin of the drift will be
explained in subsequent chapters.
The differences in rocks are due more to their soluble than to
their insoluble constituents. The latter are few in number and are
much the same in rocks of widely different nature, being chiefly
quartz, silicate of alumina, and iron oxide. By the removal of
their soluble parts very many and widely different rocks rot down
to a residual clay gritty with particles of quartz and colored red
or yellow with iron oxide.
In a broad way the changes which rocks undergo in weathering are
an adaptation to the environment in which they find themselves at
the earth's surface,--an environment different from that in which
they were formed under sea or under ground. In open air, where
they are attacked by various destructive agents, few of the rock-
making minerals are stable compounds except quartz, the iron
oxides, and the silicate of alumina; and so it is to one or more
of these comparatively insoluble substances that most rocks are
reduced by long decay.
Which produces a mantle of finer waste, frost or chemical decay?
which a thicker mantle? In what respects would you expect that the
mantle of waste would differ in warm humid lands like India, in
frozen countries like Alaska, and in deserts such as the Sahara?
THE SOIL. The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the
surface, where the waste has weathered longest, it is gradually
made fine enough to support the growth of plants, and is then
known as soil. The coarser waste beneath is sometimes spoken of as
subsoil. Soil usually contains more or less dark, carbonaceous,
decaying organic matter, called humus, and is then often termed
the humus layer. Soil forms not only on waste produced in place
from the rock beneath, but also on materials which have been
transported, such as sheets of glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather
is more rapid and effective on the fragments of the mantle of
waste than on the rocks from which waste is being formed. Why?
Any fresh excavation of cellar or cistern, or cut for road or
railway, will show the characteristics of the humus layer. It may
form only a gray film on the surface, or we may find it a layer a
foot or more thick, dark, or even black, above, and growing
gradually lighter in color as it passes by insensible gradations
into the subsoil. In some way the decaying vegetable matter
continually forming on the surface has become mingled with the
material beneath it.
HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and
the subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying
substance to fertilize it. Leaves and stems falling on the surface
are turned under by several agents. Earthworms and other animals
whose home is in the waste drag them into their burrows either for
food or to line their nests. Trees overthrown by the wind, roots
and all, turn over the soil and subsoil and mingle them together.
Bacteria also work in the waste and contribute to its enrichment.
The animals living in the mantle do much in other ways toward the
making of soil. They bring the coarser fragments from beneath to
the surface, where the waste weathers more rapidly. Their burrows
allow air and water to penetrate the waste more freely and to
affect it to greater depths.
ANTS. In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build
mounds which may reach as high above it. In some parts of Paraguay
and southern Brazil these mounds, like gigantic potato hills,
cover tracts of considerable area.
In search for its food--the dead wood of trees--the so-called
white ant constructs runways of earth about the size of gas pipes,
reaching from the base of the tree to the topmost branches. On the
plateaus of central Africa explorers have walked for miles through
forests every tree of which was plastered with these galleries of
mud. Each grain of earth used in their construction is moistened
and cemented by slime as it is laid in place by the ant, and is
thus acted on by organic chemical agents. Sooner or later these
galleries are beaten down by heavy rains, and their fertilizing
substances are scattered widely by the winds.
EARTHWORMS. In temperate regions the waste is worked over largely
by earthworms. In making their burrows worms swallow earth in
order to extract from it any nutritive organic matter which it may
contain. They treat it with their digestive acids, grind it in
their stony gizzards, and void it in castings on the surface of
the ground. It was estimated by Darwin that in many parts of
England each year, on every acre, more than ten tons of earth pass
through the bodies of earthworms and are brought to the surface,
and that every few years the entire soil layer is thus worked over
by them.
In all these ways the waste is made fine and stirred and enriched.
Grain by grain the subsoil with its fresh mineral ingredients is
brought to the surface, and the rich organic matter which plants
and animals have taken from the atmosphere is plowed under. Thus
Nature plows and harrows on "the great world's farm" to make ready
and ever to renew a soil fit for the endless succession of her
crops.
The world processes by which rocks are continually wasting away
are thus indispensable to the life of plants and animals. The
organic world is built on the ruins of the inorganic, and because
the solid rocks have been broken down into soil men are able to
live upon the earth.
SOLAR ENERGY. The source of the energy which accomplishes all this
necessary work is the sun. It is the radiant energy of the sun
which causes the disintegration of rocks, which lifts vapor into
the atmosphere to fall as rain, which gives life to plants and
animals. Considering the earth in a broad way, we may view it as a
globe of solid rock,--the lithosphere,--surrounded by two mobile
envelopes: the envelope of air,--THE ATMOSPHERE, and the envelope
of water,--THE HYDROSPHERE. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are
dissolved in all waters,--and both air and water enter and work
upon the solid earth. By their action upon the lithosphere they
have produced a third envelope,--the mantle of rock waste.
This envelope also is in movement, not indeed as a whole, but
particle by particle. The causes which set its particles in
motion, and the different forms which the mantle comes to assume,
we will now proceed to study.
MOVEMENTS OF THE MANTLE OF ROCK WASTE
At the sandstone ledges which we first visited we saw not only
that the rocks were crumbling away, but also that grains and
fragments of them were creeping down the slopes of the valley to
the stream and were carried by it onward toward the sea. This
process is going on everywhere. Slowly it may be, and with many
interruptions, but surely, the waste of the land moves downward to
the sea. We may divide its course into two parts,--the path to the
stream, which we will now consider, and its carriage onward by the
stream, which we will defer to a later chapter.
GRAVITY. The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull
of the earth's mass and follows it when free to do so. All
agencies which produce waste tend to set its particles free and in
motion, and therefore cooperate with gravity. On cliffs, rocks
fall when wedged off by frost or by roots of trees, and when
detached by any other agency. On slopes of waste, water freezes in
chinks between stones, and in pores between particles of soil, and
wedges them apart. Animals and plants stir the waste, heat expands
it, cold contracts it, the strokes of the raindrops drive loose
particles down the slope and the wind lifts and lets them fall. Of
all these movements, gravity assists those which are downhill and
retards those which are uphill. On the whole, therefore, the
downhill movements prevail, and the mantle of waste, block by
block and grain by grain, creeps along the downhill path.
A slab of sandstone laid on another of the same kind at an angle
of 17 degrees and left in the open air was found to creep down the
slope at the rate of a little more than a millimeter a month.
Explain why it did so.
RAIN. The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of
the blows of the falling drops, and washes them down all slopes to
within reach of permanent streams. On surfaces unprotected by
vegetation, as on plowed fields and in arid regions, the rain
wears furrows and gullies both in the mantle of waste and in
exposures of unaltered rock (Fig. 17).
At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the
hillside is steepest the soil may be exceedingly thin, or quite
absent, because removed about as fast as formed. Against the walls
of an abbey built on a slope in Wales seven hundred years ago, the
creeping waste has gathered on the uphill side to a depth of seven
feet. The slow-flowing sheet of waste is often dammed by fences
and walls, whose uphill side gathers waste in a few years so as to
show a distinctly higher surface than the downhill side,
especially in plowed fields where the movement is least checked by
vegetation.
TALUS. At the foot of cliffs there is usually to be found a slope
of rock fragments which clearly have fallen from above. Such a
heap of waste is known as talus. The amount of talus in any place
depends both on the rate of its formation and the rate of its
removal. Talus forms rapidly in climates where mechanical
disintegration is most effective, where rocks are readily broken
into blocks because closely jointed and thinly bedded rather than
massive, and where they are firm enough to be detached in
fragments of some size instead of in fine grains. Talus is removed
slowly where it decays slowly, either because of the climate or
the resistance of the rock. It may be rapidly removed by a stream
flowing along its base.
In a moist climate a soluble rock, such as massive limestone, may
form talus little if any faster than the talus weathers away. A
loose-textured sandstone breaks down into incoherent sand grains,
which in dry climates, where unprotected by vegetation, may be
blown away as fast as they fall, leaving the cliff bare to the
base. Cliffs of such slow-decaying rocks as quartzite and granite
when closely jointed accumulate talus in large amounts.
Talus slopes may be so steep as to reach THE ANGLE OF REPOSE, i.e.
the steepest angle at which the material will lie. This angle
varies with different materials, being greater with coarse and
angular fragments than with fine rounded grains. Sooner or later a
talus reaches that equilibrium where the amount removed from its
surface just equals that supplied from the cliff above. As the
talus is removed and weathers away its slope retreats together
with the retreat of the cliff, as seen in Figure 9.
GRADED SLOPES. Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges.
On the steeper slopes it is coarser and in more rapid movement
than on slopes more gentle, but mountain sides and hills and
plains alike come to be mantled with sheets of waste which
everywhere is creeping toward the streams. Such unbroken slopes,
worn or built to the least inclination at which the waste supplied
by weathering can be urged onward, are known as GRADED SLOPES.
Of far less importance than the silent, gradual creep of waste,
which is going on at all times everywhere about us, are the
startling local and spasmodic movements which we are now to
describe.
AVALANCHES. On steep mountain sides the accumulated snows of
winter often slip and slide in avalanches to the valleys below.
These rushing torrents of snow sweep their tracks clean of waste
and are one of Nature's normal methods of moving it along the
downhill path.
LANDSLIDES. Another common and abrupt method of delivering waste
to streams is by slips of the waste mantle in large masses. After
long rains and after winter frosts the cohesion between the waste
and the sound rock beneath is loosened by seeping water
underground. The waste slips on the rock surface thus lubricated
and plunges down the mountain side in a swift roaring torrent of
mud and stones.
We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved
in the sudden movement. Such slips occur when valleys have been
rapidly deepened by streams or glaciers and their sides have not
yet been graded. A favorable condition is where the strata dip
(i.e. incline downwards) towards the valley (Fig. 11), or are
broken by joint planes dipping in the same direction. The upper
layers, including perhaps the entire mountain side, have been cut
across by the valley trench and are left supported only on the
inclined surface of the underlying rocks. Water may percolate
underground along this surface and loosen the cohesion between the
upper and the underlying strata by converting the upper surface of
a shale to soft wet clay, by dissolving layers of a limestone, or
by removing the cement of a sandstone and converting it into loose
sand. When the inclined surface is thus lubricated the overlying
masses may be launched into the valley below. The solid rocks are
broken and crushed in sliding and converted into waste consisting,
like that of talus, of angular unsorted fragments, blocks of all
sizes being mingled pellmell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.
At Gohna, India, in 1893, the face of a spur four thousand feet
high, of the lower ranges of the Himalayas, slipped into the gorge
of the headwaters of the Ganges River in successive rock falls
which lasted for three days. Blocks of stone were projected for a
mile, and clouds of limestone dust were spread over the
surrounding country. The debris formed a dam one thousand feet
high, extending for two miles along the valley. A lake gathered
behind this barrier, gradually rising until it overtopped it in a
little less than a year. The upper portion of the dam then broke,
and a terrific rush of water swept down the valley in a wave
which, twenty miles away, rose one hundred and sixty feet in
height. A narrow lake is still held by the strong base of the dam.
In 1896, after forty days of incessant rain, a cliff of sandstone
slipped into the Yangtse River in China, reducing the width of the
channel to eighty yards and causing formidable rapids.
At Flims, in Switzerland, a prehistoric landslip flung a dam
eighteen hundred feet high across the headwaters of the Rhine. If
spread evenly over a surface of twenty-eight square miles, the
material would cover it to a depth of six hundred and sixty feet.
The barrier is not yet entirely cut away, and several lakes are
held in shallow basins on its hummocky surface.
A slide from the precipitous river front of the citadel hill of
Quebec, in 1889, dashed across Champlain Street, wrecking a number
of houses and causing the death of forty-five persons. The strata
here are composed of steeply dipping slate.
In lofty mountain ranges there may not be a single valley without
its traces of landslides, so common there is this method of the
movement of waste, and of building to grade over-steepened slopes.
ROCK SCULPTURE BY WEATHERING
We are now to consider a few of the forms into which rock masses
are carved by the weather.
BOWLDERS OF WEATHERING. In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers
have been broken along their joints and bedding planes are no
longer angular, as are those of the layers below. The edges and
corners of these blocks have been worn away by the weather. Such
rounded cores, known as bowlders of weathering, are often left to
strew the surface.
DIFFERENTIAL WEATHERING. This term covers all cases in which a
rock mass weathers differently in different portions. Any weaker
spots or layers are etched out on the surface, leaving the more
resistant in relief. Thus massive limestones become pitted where
the weather drills out the weaker portions. In these pits, when
once they are formed, moisture gathers, a little soil collects,
vegetation takes root, and thus they are further enlarged until
the limestone may be deeply honeycombed.
On the sides of canyons, and elsewhere where the edges of strata
are exposed, the harder layers project as cliffs, while the softer
weather back to slopes covered with the talus of the harder layers
above them. It is convenient to call the former cliff makers and
the latter slope makers.
Differential weathering plays a large part in the sculpture of the
land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in
the valleys and mountains of eastern Pennsylvania. But in such
instances the lowering of the surface of the weaker rock is also
due to the wear of streams, and especially to the removal by them
from the land of the waste which covers and protects the rocks
beneath.
Rocks owe their weakness to several different causes. Some, such
as beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are
closely divided by joints and bedding planes and are thus readily
broken up into blocks by mechanical agencies.
OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant
or where the attack for any reason is less severe. Such remnant
masses, if large, are known as outliers. When
Note the rain furrows on the slope at the foot of the monuments.
In the foreground are seen fragments of petrified trunks of trees,
composed of silica and extremely resistant to the weather. On the
removal of the rock layers in which these fragments were imbedded
they are left to strew the surface in the same way as are the
residual flints of southern Missouri. flat-topped, because of the
protection of a resistant horizontal capping layer, they are
termed mesas,--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are
finally consumed.
Monuments are smaller masses and may be but partially detached
from the cliff face. In the breaking down of sheets of horizontal
strata, outliers grow smaller and smaller and are reduced to
massive rectangular monuments resembling castles (Fig. 17). The
rock castle falls into ruin, leaving here and there an isolated
tower; the tower crumbles to a lonely pillar, soon to be
overthrown. The various and often picturesque shapes of monuments
depend on the kind of rock, the attitude of the strata, and the
agent by which they are chiefly carved. Thus pillars may have a
capital formed of a resistant stratum. Monuments may be undercut
and come to rest on narrow pedestals, wherever they weather more
rapidly near the ground, either because of the greater moisture
there, or--in arid climates--because worn at their base by
drifting sands.
Stony clays disintegrating under the rain often contain bowlders
which protect the softer material beneath from the vertical blows
of raindrops, and thus come to stand on pedestals of some height.
One may sometimes see on the ground beneath dripping eaves pebbles
left in the same way, protecting tiny pedestals of sand.
MOUNTAIN PEAKS AND RIDGES. Most mountains have been carved out of
great broadly uplifted folds and blocks of the earth's crust.
Running water and glacier ice have cut these folds and blocks into
masses divided by deep valleys; but it is by the weather, for the
most part, that the masses thus separated have been sculptured to
the present forms of the individual peaks and ridges.
Frost and heat and cold sculpture high mountains to sharp,
tusklike peaks and ragged, serrate crests, where their waste is
readily removed.
The Matterhorn of the Alps is a famous example of a mountain peak
whose carving by the frost and other agents is in active progress.
On its face "scarcely a rock anywhere is firmly attached," and the
fall of loosened stones is incessant. Mountain climbers who have
camped at its base tell how huge rocks from time to time come
leaping down its precipices, followed by trains of dislodged
smaller fragments and rock dust; and how at night one may trace
the course of the bowlders by the sparks which they strike from
the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles
the Matterhorn in form and has been carved by the same agencies.
"The Needles" of Arizona are examples of sharp mountain peaks in a
warm arid region sculptured chiefly by temperature changes.
Chemical decay, especially when carried on beneath a cover of
waste and vegetation, favors the production of rounded knobs and
dome-shaped mountains.
THE WEATHER CURVE. We have seen that weathering reduces the
angular block quarried by the frost to a rounded bowlder by
chipping off its corners and smoothing away its edges. In much the
same way weathering at last reduces to rounded hills the earth
blocks cut by streams or formed in any other way. High mountains
may at first be sculptured by the weather to savage peaks (Fig.
181), but toward the end of their life history they wear down to
rounded hills (Fig. 182). The weather curve, which may be seen on
the summits of low hills (Fig. 21), is convex upward.
In Figure 22, representing a cubic block of stone whose faces are
a yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner a; by one situated in the
middle of an edge b; by one in the center of a side c? How much
faster will a and b weather than c, and what will be the effect on
the shape of the block?
THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the
sake of clearness it is necessary to describe the work of each
geological agent separately. We must not forget, however, that in
Nature no agent works independently and alone; that every result
is the outcome of a long chain of causes. Thus, in order that the
mountain peak may be carved by the agents of disintegration, the
waste must be rapidly removed,--a work done by many agents,
including some which we are yet to study; and in order that the
waste may be removed as fast as formed, the region must first have
been raised well above the level of the sea, so that the agents of
transportation could do their work effectively. The sculpture of
the rocks is accomplished only by the cooperation of many forces.
The constant removal of waste from the surface by creep and wash
and carriage by streams is of the highest importance, because it
allows the destruction of the land by means of weathering to go on
as long as any land remains above sea level. If waste were not
removed, it would grow to be so thick as to protect the rock
beneath from further weathering, and the processes of destruction
which we have studied would be brought to an end. The very
presence of the mantle of waste over the land proves that on the
whole rocks weather more rapidly than their waste is removed. The
destruction of the land is going on as fast as the waste can be
carried away.
We have now learned to see in the mantle of waste the record of
the destructive action of the agencies of weathering on the rocks
of the land surface. Similar records we shall find buried deeply
among the rocks of the crust in old soils and in rocks pitted and
decayed, telling of old land surfaces long wasted by the weather.
Ever since the dry land appeared these agencies have been as now
quietly and unceasingly at work upon it, and have ever been the
chief means of the destruction of its rocks. The vast bulk of the
stratified rocks of the earth's crust is made up almost wholly of
the waste thus worn from ancient lands.
In studying the various geological agencies we must remember the
almost inconceivable times in which they work. The slowest process
when multiplied by the immense time in which it is carried on
produces great results. The geologist looks upon the land forms of
the earth's surface as monuments which record the slow action of
weathering and other agents during the ages of the past. The
mountain peak, the rounded hill, the wide plain which lies where
hills and mountains once stood, tell clearly of the great results
which slow processes will reach when given long time in which to
do their work. We should accustom ourselves also to think of the
results which weathering will sooner or later bring to pass. The
tombstone and the bowlder of the field, which each year lose from
their surfaces a few crystalline grains, must in time be wholly
destroyed. The hill whose rocks are slowly rotting underneath a
cover of waste must become lower and lower as the centuries and
millenniums come and go, and will finally disappear. Even the
mountains are crumbling away continually, and therefore are but
fleeting features of the landscape.
CHAPTER II
THE WORK OF GROUND WATER
LAND WATERS. We have seen how large is the part that water plays
at and near the surface of the land in the processes of weathering
and in the slow movement of waste down all slopes to the stream
ways. We now take up the work of water as it descends beneath the
ground,--a corrosive agent still, and carrying in solution as its
load the invisible waste of rocks derived from their soluble
parts.
Land waters have their immediate source in the rainfall. By the
heat of the sun water is evaporated from the reservoir of the
ocean and from moist surfaces everywhere. Mingled as vapor with
the air, it is carried by the winds over sea and land, and
condensed it returns to the earth as rain or snow. That part of
the rainfall which descends on the ocean does not concern us, but
that which falls on the land accomplishes, as it returns to the
sea, the most important work of all surface geological agencies.
The rainfall may be divided into three parts: the first DRIES UP,
being discharged into the air by evaporation either directly from
the soil or through vegetation; the second RUNS OFF over the
surface to flood the streams; the third SOAKS IN the ground and is
henceforth known as GROUND or UNDERGROUND WATER.
THE DESCENT OF GROUND WATER. Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying
rock. All rocks of the upper crust of the earth are more or less
porous, and all drink in water. IMPERVIOUS ROCKS, such as granite,
clay, and shale, have pores so minute that the water which they
take in is held fast within them by capillary attraction, and none
drains through. PERVIOUS ROCKS, on the other hand, such as many
sandstones, have pore spaces so large that water filters through
them more or less freely. Besides its seepage through the pores of
pervious rocks, water passes to lower levels through the joints
and cracks by which all rocks, near the surface are broken.
Even the closest-grained granite has a pore space of 1 in 400,
while sandstone may have a pore space of 1 in 4. Sand is so porous
that it may absorb a third of its volume of water, and a loose
loam even as much as one half.
THE GROUND-WATER SURFACE is the name given the upper surface of
ground water, the level below which all rocks are saturated. In
dry seasons the ground-water surface sinks. For ground water is
constantly seeping downward under gravity, it is evaporated in the
waste and its moisture is carried upward by capillarity and the
roots of plants to the surface to be evaporated in the air. In wet
seasons these constant losses are more than made good by fresh
supplies from that part of the rainfall which soaks into the
ground, and the ground-water surface rises.
In moist climates the ground-water surface (Fig. 24) lies, as a
rule, within a few feet of the land surface and conforms to it in
a general way, although with slopes of less inclination than those
of the hills and valleys. In dry climates permanent ground water
may be found only at depths of hundreds of feet. Ground water is
held at its height by the fact that its circulation is constantly
impeded by capillarity and friction. If it were as free to drain
away as are surface streams, it would sink soon after a rain to
the level of the deepest valleys of the region.
WELLS AND SPRINGS. Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells.
Where valleys cut this surface permanent streams are formed, the
water either oozing forth along ill-defined areas or issuing at
definite points called springs, where it is concentrated by the
structure of the rocks. A level tract where the ground-water
surface coincides with the surface of the ground is a swamp or
marsh.
By studying a spring one may learn much of the ways and work of
ground water. Spring water differs from that of the stream into
which it flows in several respects. If we test the spring with a
thermometer during successive months, we shall find that its
temperature remains much the same the year round. In summer it is
markedly cooler than the stream; in winter it is warmer and
remains unfrozen while the latter perhaps is locked in ice. This
means that its underground path must lie at such a distance from
the surface that it is little affected by summer's heat and
winter's cold.
While the stream is often turbid with surface waste washed into it
by rains, the spring remains clear; its water has been filtered
during its slow movement through many small underground passages
and the pores of rocks. Commonly the spring differs from the
stream in that it carries a far larger load of dissolved rock.
Chemical analysis proves that streams contain various minerals in
solution, but these are usually in quantities so small that they
are not perceptible to the taste or feel. But the water of springs
is often well charged with soluble minerals; in its slow, long
journey underground it has searched out the soluble parts of the
rocks through which it seeps and has dissolved as much of them as
it could. When spring water is boiled away, the invisible load
which it has carried is left behind, and in composition is found
to be practically identical with that of the soluble ingredients
of the country rock. Although to some extent the soluble waste of
rocks is washed down surface slopes by the rain, by far the larger
part is carried downward by ground water and is delivered to
streams by springs.
In limestone regions springs are charged with calcium carbonate
(the carbonate of lime), and where the limestone is magnesian they
contain magnesium carbonate also. Such waters are "hard"; when
used in washing, the minerals which they contain combine with the
fatty acids of soap to form insoluble curdy compounds. When
springs rise from rocks containing gypsum they are hard with
calcium sulphate. In granite regions they contain more or less
soda and potash from the decay of feldspar.
The flow of springs varies much less during the different seasons
of the year than does that of surface streams. So slow is the
movement of ground water through the rocks that even during long
droughts large amounts remain stored above the levels of surface
drainage.
MOVEMENTS OF GROUND WATER. Ground water is in constant movement
toward its outlets. Its rate varies according to many conditions,
but always is extremely slow. Even through loose sands beneath the
beds of rivers it sometimes does not exceed a fifth of a mile a
year.
In any region two zones of flow may be distinguished. The UPPER
ZONE OF FLOW extends from the ground-water surface downward
through the waste mantle and any permeable rocks on which the
mantle rests, as far as the first impermeable layer, where the
descending movement of the water is stopped. The DEEP ZONES OF
FLOW occupy any pervious rocks which may be found below the
impervious layer which lies nearest to the surface. The upper zone
is a vast sheet of water saturating the soil and rocks and slowly
seeping downward through their pores and interstices along the
slopes to the valleys, where in part it discharges in springs and
often unites also in a wide underflowing stream which supports and
feeds the river (Fig. 24).
A city in a region of copious rains, built on the narrow flood
plain of a river, overlooked by hills, depends for its water
supply on driven wells, within the city limits, sunk in the sand a
few yards from the edge of the stream. Are these wells fed by
water from the river percolating through the sand, or by ground
water on its way to the stream and possibly contaminated with the
sewage of the town?
At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to
go dry? It may be possible to get data from different wells and to
draw a diagram showing the ground-water surface as compared with
the surface of the ground.
FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in
pervious strata which are overlain by some impervious stratum.
Such layers are often carried by their dip to great depths, and
water may circulate in them to far below the level of the surface
streams and even of the sea. When a fissure crosses a water-
bearing stratum, or AQUIFIER, water is forced upward by the
pressure of the weight of the water contained in the higher parts
of the stratum, and may reach the surface as a fissure spring. A
boring which taps such an aquifer is known as an artesian well, a
name derived from a province in France where wells of this kind
have been long in use. The rise of the water in artesian wells,
and in fissure springs also, depends on the following conditions
illustrated in Figure 29. The aquifer dips toward the region of
the wells from higher ground, where it outcrops and receives its
water. It is inclosed between an impervious layer above and water-
tight or water-logged layers beneath. The weight of the column of
water thus inclosed in the aquifer causes water to rise in the
well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.
Which will supply the larger region with artesian wells, an
aquifer whose dip is steep or one whose dip is gentle? Which of
the two aquifers, their thickness being equal, will have the
larger outcrop and therefore be able to draw upon the larger
amount of water from the rainfall? Illustrate with diagrams.
THE ZONE OF SOLUTION. Near the surface, where the circulation of
ground water is most active, it oxidizes, corrodes, and dissolves
the rocks through which it passes. It leaches soils and subsoils
of their lime and other soluble minerals upon which plants depend
for their food. It takes away the soluble cements of rocks; it
widens fissures and joints and opens winding passages along the
bedding planes; it may even remove whole beds of soluble rocks,
such as rock salt, limestone, or gypsum. The work of ground water
in producing landslides has already been noticed. The zone in
which the work of ground water is thus for the most part
destructive we may call the zone of solution.
CAVES. In massive limestone rocks, ground water dissolves channels
which sometimes form large caves (Fig. 30). The necessary
conditions for the excavation of caves of great size are well
shown in central Kentucky, where an upland is built throughout of
thick horizontal beds of limestone. The absence of layers of
insoluble or impervious rock in its structure allows a free
circulation of ground water within it by the way of all natural
openings in the rock. These water ways have been gradually
enlarged by solution and wear until the upland is honeycombed with
caves. Five hundred open caverns are known in one county.
Mammoth Cave, the largest of these caverns, consists of a
labyrinth of chambers and winding galleries whose total length is
said to be as much as thirty miles. One passage four miles long
has an average width of about sixty feet and an average height of
forty feet. One of the great halls is three hundred feet in width
and is overhung by a solid arch of limestone one hundred feet
above the floor. Galleries at different levels are connected by
well-like pits, some of which measure two hundred and twenty-five
feet from top to bottom. Through some of the lowest of these
tunnels flows Echo River, still at work dissolving and wearing
away the rock while on its dark way to appear at the surface as a
great spring.
NATURAL BRIDGES. As a cavern enlarges and the surface of the land
above it is lowered by weathering, the roof at last breaks down
and the cave becomes an open ravine. A portion of the roof may for
a while remain, forming a "natural bridge."
SINK HOLES. In limestone regions channels under ground may become
so well developed that the water of rains rapidly drains away
through them. Ground water stands low and wells must be sunk deep
to find it. Little or no surface water is left to form brooks.
Thus across the limestone upland of central Kentucky one meets but
three surface streams in a hundred miles. Between their valleys
surface water finds its way underground by means of sink holes.
These are pits, commonly funnel shaped, formed by the enlargement
of crevice or joint by percolating water, or by the breakdown of
some portion of the roof of a cave. By clogging of the outlet a
sink hole may come to be filled by a pond.
Central Florida is a limestone region with its drainage largely
subterranean and in part below the level even of the sea. Sink
holes are common, and many of them are occupied by lakelets. Great
springs mark the point of issue of underground streams, while some
rise from beneath the sea. Silver Spring, one of the largest,
discharges from a basin eight hundred feet wide and thirty feet
deep a little river navigable for small steamers to its source.
About the spring there are no surface streams for sixty miles.
THE KARST. Along the eastern coast of the Adriatic, as far south
as Montenegro, lies a belt of limestone mountains singularly worn
and honeycombed by the solvent action of water. Where forests have
been cut from the mountain sides and the red soil has washed away,
the surface of the white limestone forms a pathless desert of rock
where each square rod has been corroded into an intricate branch
work of shallow furrows and sharp ridges. Great sink holes, some
of them six hundred feet deep and more, pockmark the surface of
the land. The drainage is chiefly subterranean. Surface streams
are rare and a portion of their courses is often under ground.
Fragmentary valleys come suddenly to an end at walls of rock where
the rivers which occupy the valleys plunge into dark tunnels to
reappear some miles away. Ground water stands so far below the
surface that it cannot be reached by wells, and the inhabitants
depend on rain water stored for household uses. The finest cavern
of Europe, the Adelsberg Grotto, is in this region. Karst, the
name of a part of this country, is now used to designate any
region or landscape thus sculptured by the chemical action of
surface and ground water. We must remember that Karst regions are
rare, and striking as is the work of their subterranean streams,
it is far less important than the work done by the sheets of
underground water slowly seeping through all subsoils and porous
rocks in other regions.
Even when gathered into definite channels, ground water does not
have the erosive power of surface streams, since it carries with
it little or no rock waste. Regions whose underground drainage is
so perfect that the development of surface streams has been
retarded or prevented escape to a large extent the leveling action
of surface running waters, and may therefore stand higher than the
surrounding country. The hill honeycombed by Luray Cavern,
Virginia, has been attributed to this cause.
CAVERN DEPOSITS. Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles
from the roof of caverns, the lime carbonate which it has taken
into solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called STALACTITES.
As the drops splash on the floor there are built up in the same
way thicker masses called STALAGMITES, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often
seals with rock the earth which accumulates in caverns, together
with whatever relics of cave dwellers, either animals or men, it
may contain.
Can you explain why slender stalactites formed by the drip of
single drops are often hollow pipes?
THE ZONE OF CEMENTATION. With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At
such depths it deposits these minerals in the pores of rocks,
cementing their grains together, and in crevices and fissures,
forming mineral veins. Thus below the zone of solution where the
work of water is to dissolve, lies the zone of cementation where
its work is chemical deposit. A part of the invisible load of
waste is thus transferred from rocks near the surface to those at
greater depths.
As the land surface is gradually lowered by weathering and the
work of rain and streams, rocks which have lain deep within the
zone of cementation are brought within the zone of solution. Thus
there are exposed to view limestones, whose cracks were filled
with calcite (crystallized carbonate of lime), with quartz or
other minerals, and sandstones whose grains were well cemented
many feet below the surface.
CAVITY FILLING. Small cavities in the rocks are often found more
or less completely filled with minerals deposited from solution by
water in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of
evaporating brine, but in the latter instance the solution is not
renewed as in the case of cavities in the rocks. A cavity thus
lined with inward-pointing crystals is called a GEODE.
CONCRETIONS. Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays
concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of REPLACEMENT. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock. Clays
and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or
shell, frequently forms the nucleus around which the concretion
grows.
Why are building stones more easily worked when "green" than after
their quarry water has dried out?
DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where
ground water is drawn by capillarity to the surface and there
evaporates, it leaves as surface incrustations the minerals held
in solution. White limy incrustations of this nature cover
considerable tracts in northern Mexico. Evaporating beneath the
surface, ground water may deposit a limy cement in beds of loose
sand and gravel. Such firmly cemented layers are not uncommon in
western Kansas and Nebraska, where they are known as "mortar
beds."
THERMAL SPRINGS. While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average
annual temperature of the region, and are then known as thermal
springs. In regions of present or recent volcanic activity, such
as the Yellowstone National Park, we may believe that the heat of
thermal springs is derived from uncooled lavas, perhaps not far
below the surface. But when hot springs occur at a distance of
hundreds of miles from any volcano, as in the case of the hot
springs of Bath, England, it is probable that their waters have
risen from the heated rocks of the earth's interior. The springs
of Bath have a temperature of 120 degrees F., 70 degrees above the
average annual temperature of the place. If we assume that the
rate of increase in the earth's internal heat is here the average
rate, 1 degree F. to every sixty feet of descent, we may conclude
that the springs of Bath rise from at least a depth of forty-two
hundred feet.
Water may descend to depths from which it can never be brought
back by hydrostatic pressure. It is absorbed by highly heated
rocks deep below the surface. From time to time some of this deep-
seated water may be returned to open air in the steam of volcanic
eruptions.
SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to
the surface highly charged with minerals in solution, on exposure
to the air it is commonly compelled to lay down much of its
invisible load in chemical deposits about the spring. These are
thrown down from solution either because of cooling, evaporation,
the loss of carbon dioxide, or the work of algae.
Many springs have been charged under pressure with carbon dioxide
from subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through
which they pass. On reaching the surface the pressure is relieved,
the gas escapes, and the lime carbonate is thrown down in deposits
called TRAVERTINE. The gas is sometimes withdrawn and the deposit
produced in large part by the action of algae and other humble
forms of plant life.
At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of
travertine are now building, chiefly by means of algae which cover
the bottoms, rims, and sides of the basins and deposit lime
carbonate upon them in successive sheets. The rock, snow-white
where dry, is coated with red and orange gelatinous mats where the
algae thrive in the over-flowing waters.
Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs
which formed these ancient deposits discharged near what was then
the bottom of the valley, and that as the valley has been deepened
by the river the ground water of the region has found lower and
lower points of issue.
In many parts of the country calcareous springs occur which coat
with lime carbonate mosses, twigs, and other objects over which
their waters flow. Such are popularly known as petrifying springs,
although they merely incrust the objects and do not convert them
into stone.
Silica is soluble in alkaline waters, especially when these are
hot. Hot springs rising through alkaline siliceous rocks, such as
lavas, often deposit silica in a white spongy formation known as
SILICEOUS SINTER, both by evaporation and by the action of algae
which secrete silica from the waters. It is in this way that the
cones and mounds of the geysers in the Yellowstone National Park
and in Iceland have been formed.
Where water oozes from the earth one may sometimes see a rusty
deposit on the ground, and perhaps an iridescent scum upon the
water. The scum is often mistaken for oil, but at a touch it
cracks and breaks, as oil would not do. It is a film of hydrated
iron oxide, or LIMONITE, and the spring is an iron, or chalybeate,
spring. Compounds of iron have been taken into solution by ground
water from soil and rocks, and are now changed to the insoluble
oxide on exposure to the oxygen of the air.
In wet ground iron compounds leached by ground water from the soil
often collect in reddish deposits a few feet below the surface,
where their downward progress is arrested by some impervious clay.
At the bottom of bogs and shallow lakes iron ores sometimes
accumulate to a depth of several feet.
Decaying organic matter plays a large part in these changes. In
its presence the insoluble iron oxides which give color to most
red and yellow rocks are decomposed, leaving the rocks of a gray
or bluish color, and the soluble iron compounds which result are
readily leached out,--effects seen where red or yellow clays have
been bleached about some decaying tree root.
The iron thus dissolved is laid down as limonite when oxidized, as
about a chalybeate spring; but out of contact with the air and in
the presence of carbon dioxide supplied by decaying vegetation, as
in a peat bog, it may be deposited as iron carbonate, or SIDERITE.
TOTAL AMOUNT OF UNDERGROUND WATERS. In order to realize the vast
work in solution and cementation which underground waters are now
doing and have done in all geological ages, we must gain some
conception of their amount. At a certain depth, estimated at about
six miles, the weight of the crust becomes greater than the rocks
can bear, and all cavities and pores in them must be completely
closed by the enormous pressure which they sustain. Below a depth
of even three or four miles it is believed that ground water
cannot circulate. Estimating the average pore spaces of the
different rocks of the earth's crust above this depth, and the
average per cents of their pore spaces occupied by water, it has
been recently computed that the total amount of ground water is
equal to a sheet of water one hundred feet deep, covering the
entire surface of the earth.
CHAPTER III
RIVERS AND VALLEYS
THE RUN-OFF. We have traced the history of that portion of the
rainfall which soaks into the ground; let us now return to that
part which washes along the surface and is known as the RUN-OFF.
Fed by rains and melting snows, the run-off gathers into courses,
perhaps but faintly marked at first, which join more definite and
deeply cut channels, as twigs their stems. In a humid climate the
larger ravines through which the run-off flows soon descend below
the ground-water surface. Here springs discharge along the sides
of the little valleys and permanent streams begin. The water
supplied by the run-off here joins that part of the rainfall which
had soaked into the soil, and both now proceed together by way of
the stream to the sea.
RIVER FLOODS. Streams vary greatly in volume during the year. At
stages of flood they fill their immediate banks, or overrun them
and inundate any low lands adjacent to the channel; at stages of
low water they diminish to but a fraction of their volume when at
flood.
At times of flood, rivers are fed chiefly by the run-off; at times
of low water, largely or even wholly by springs.
How, then, will the water of streams differ at these times in
turbidity and in the relative amount of solids carried in
solution?
In parts of England streams have been known to continue flowing
after eighteen months of local drought, so great is the volume of
water which in humid climates is stored in the rocks above the
drainage level, and so slowly is it given off in springs.
In Illinois and the states adjacent, rivers remain low in winter
and a "spring freshet" follows the melting of the winter's snows.
A "June rise" is produced by the heavy rains of early summer. Low
water follows in July and August, and streams are again swollen to
a moderate degree under the rains of autumn.
THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the
drainage area, the texture of the rocks, and other factors. With
an annual rainfall of fifty inches in an open country, about fifty
per cent is discharged; while with a rainfall of twenty inches
only fifteen per cent is discharged, part of the remainder being
evaporated and part passing underground beyond the drainage area.
Thus the Ohio discharges thirty per cent of the rainfall of its
basin, while the Missouri carries away but fifteen per cent. A
number of the streams of the semi-arid lands of the West do not
discharge more than five per cent of the rainfall.
Other things being equal, which will afford the larger proportion
of run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen
or unfrozen ground? Will there be a larger proportion of run-off
after long rains or after a season of drought? after long and
gentle rains, or after the same amount of precipitation in a
violent rain? during the months of growing vegetation, from June
to August, or during the autumn months?
DESERT STREAMS. In arid regions the ground-water surface lies so
low that for the most part stream ways do not intersect it.
Streams therefore are not fed by springs, but instead lose volume
as their waters soak into the thirsty rocks over which they flow.
They contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they
wither at times of drought to a mere trickle of water, to a chain
of pools, or go wholly dry, while at long intervals rains fill
their dusty beds with sudden raging torrents. Desert rivers
therefore periodically shorten and lengthen their courses,
withering back at times of drought for scores of miles, or even
for a hundred miles from the point reached by their waters during
seasons of rain.
THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams TRANSPORT
the waste of the land; they wear, or ERODE, their channels both on
bed and banks; and they DEPOSIT portions of their load from time
to time along their courses, finally laying it down in the sea.
Most of the work of streams is done at times of flood.
TRANSPORTATION
THE INVISIBLE LOAD OF STREAMS. Of the waste which a river
transports we may consider first the invisible load which it
carries in solution, supplied chiefly by springs but also in part
by the run-off and from the solution of the rocks of its bed. More
than half the dissolved solids in the water of the average river
consists of the carbonates of lime and magnesia; other substances
are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate
(Epsom salts), sodium chloride (common salt), and even silica, the
least soluble of the common rock-making minerals. The amount of
this invisible load is surprisingly large. The Mississippi, for
example, transports each year 113,000,000 tons of dissolved rock
to the Gulf.
THE VISIBLE LOAD OF STREAMS. This consists of the silt which the
stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood
one may note the muddy water, its silt being kept from settling by
the rolling, eddying currents; and often by placing his ear close
to the bottom of a boat one may hear the clatter of pebbles as
they are hurried along. In mountain torrents the rumble of
bowlders as they clash together may be heard some distance away.
The amount of the load which a stream can transport depends on its
velocity. A current of two thirds of a mile per hour can move fine
sand, while one of four miles per hour sweeps along pebbles as
large as hen's eggs. The transporting power of a stream varies as
the sixth power of its velocity. If its velocity is multiplied by
two, its transporting power is multiplied by the sixth power of
two: it can now move stones sixty-four times as large as it could
before.
Stones weigh from two to three times as much as water, and in
water lose the weight of the volume of water which they displace.
What proportion, then, of their weight in air do stones lose when
submerged?
MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream
by its velocity per second, giving the discharge per second in
cubic feet. The amount of silt to a cubic foot of water is found
by filtering samples of the water taken from different parts of
the stream and at different times in the year, and drying and
weighing the residues. The average amount of silt to the cubic
foot of water, multiplied by the number of cubic feet of water
discharged per year, gives the total load carried in suspension
during that time. Adding to this the estimated amount of sand and
gravel rolled along the bed, which in many swift rivers greatly
exceeds the lighter material held in suspension, and adding also
the total amount of dissolved solids, we reach the exceedingly
important result of the total load of waste discharged by the
river. Dividing the volume of this load by the area of the river
basin gives another result of the greatest geological interest,--
the rate at which the region is being lowered by the combined
action of weathering and erosion, or the rate of denudation.
THE RATE OF DENUDATION OF RIVER BASINS. This rate varies widely.
The Mississippi basin may be taken as a representative land
surface because of the varieties of surface, altitude and slope,
climate, and underlying rocks which are included in its great
extent. Careful measurements show that the Mississippi basin is
now being lowered at a rate of one four-thousandth of a foot a
year, or one foot in four thousand years. Taking this as the
average rate of denudation for the land surfaces of the globe,
estimates have been made of the length of time required at this
rate to wash and wear the continents to the level of the sea. As
the average elevation of the lands of the globe is reckoned at
2411 feet, this result would occur in nine or ten million years,
if the present rate of denudation should remain unchanged. But
even if no movements of the earth's crust should lift or depress
the continents, the rate of wear and the removal of waste from
their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their
slopes become more gentle. The length of time required to wear
them away is therefore far in excess of that just stated.
The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to
the depth of four feet. At what rate is the Potomac basin being
lowered from this cause alone?
It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years.
Why so much faster than the Potomac and the Mississippi?
HOW STREAMS GET THEIR LOADS. The load of streams is derived from a
number of sources, the larger part being supplied by the
weathering of valley slopes. We have noticed how the mantle of
waste creeps and washes to the stream ways. Watching the run-off
during a rain, as it hurries muddy with waste along the gutter or
washes down the hillside, we may see the beginning of the route by
which the larger part of their load is delivered to rivers.
Streams also secure some of their load by wearing it from their
beds and banks,--a process called erosion.
EROSION
Streams erode their beds chiefly by means of their bottom load,--
the stones of various sizes and the sand and even the fine mud
which they sweep along. With these tools they smooth, grind, and
rasp the rock of their beds, using them in much the fashion of
sandpaper or a file.
WEATHERING OF RIVER BEDS. The erosion of stream beds is greatly
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and
cold, joints are opened, rocks are pried loose and broken up and
made ready to be swept away by the stream at time of flood.
POTHOLES. In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which
gather in it are whirled round and round, and, acting like the bit
of an auger, bore out a cylindrical pit called a pothole. Potholes
sometimes reach a depth of a score of feet. Where they are
numerous they aid materially in deepening the channel, as the
walls between them are worn away and they coalesce.
WATERFALLS. One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges
stones from the face of the ledge over which it pours, and often
undermines it by excavating a deep pit at its base. Slice after
slice is thus thrown down from the front of the cliff, and the
cataract cuts its way upstream leaving a gorge behind it.
NIAGARA FALLS. The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of
the region. Some thirteen miles from the outlet it plunges over a
ledge one hundred and seventy feet high into the head of a narrow
gorge which extends for seven miles to the escarpment of the
upland in which the gorge is cut. The strata which compose the
upland dip gently upstream and consist at top of a massive
limestone, at the Falls about eighty feet thick, and below of soft
and easily weathered shale. Beneath the Falls the underlying shale
is cut and washed away by the descending water and retreats also
because of weathering, while the overhanging limestone breaks down
in huge blocks from time to time.
Niagara is divided by Goat Island into the Horseshoe Falls and the
American Falls. The former is supplied by the main current of the
river, and from the semicircular sweep of its rim a sheet of water
in places at least fifteen or twenty feet deep plunges into a pool
a little less than two hundred feet in depth. Here the force of
the falling water is sufficient to move about the fallen blocks of
limestone and use them in the excavation of the shale of the bed.
At the American Falls the lesser branch of the river, which flows
along the American side of Goat Island, pours over the side of the
gorge and breaks upon a high talus of limestone blocks which its
smaller volume of water is unable to grind to pieces and remove.
A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while
the American Falls retreated at the rate of 0.64 feet in the same
period. We cannot doubt that the same agency which is now
lengthening the gorge at this rapid rate has cut it back its
entire length of seven miles.
While Niagara Falls have been cutting back a gorge seven miles
long and from two hundred to three hundred feet deep, the river
above the Falls has eroded its bed scarcely below the level of the
upland on which it flows. Like all streams which are the outlets
of lakes, the Niagara flows out of Lake Erie clear of sediment, as
from a settling basin, and carries no tools with which to abrade
its bed. We may infer from this instance how slight is the erosive
power of clear water on hard rock.
Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat
Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST,
how long is it since the Niagara River fell over the edge of the
escarpment where now is the mouth of the present gorge?
The profile of the bed of the Niagara along the gorge (Fig. 39)
shows alternating deeps and shallows which cannot be accounted
for, except in a single instance, by the relative hardness of the
rocks of the river bed. The deeps do not exceed that at the foot
of the Horseshoe Falls at the present time. When the gorge was
being cut along the shallows, how did the Falls compare in
excavating power, in force, and volume with the Niagara of to-day?
How did the rate of recession at those times compare with the
present rate? Is the assumption made above that the rate of
recession has been uniform correct?
The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-
five feet. From what data could such an estimate be made?
Suggest a reason why the Horseshoe Falls are convex upstream.
At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will
be the fate of the Falls left behind when the other has passed
beyond the head of the island?
The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-
jointed rocks? on horizontal strata or on strata steeply inclined?
DEPOSITION
While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to
many delays en route. Now and again it is laid aside, to be picked
up later and carried some distance farther on its way. One of the
most striking features of the river therefore is the waste
accumulated along its course, in bars and islands in the channel,
beneath its bed, and in flood plains along its banks. All this
alluvium, to use a general term for river deposits, with which the
valley is cumbered is really en route to the sea; it is only
temporarily laid aside to resume its journey later on. Constantly
the river is destroying and rebuilding its alluvial deposits, here
cutting and there depositing along its banks, here eroding and
there building a bar, here excavating its bed and there filling it
up, and at all times carrying the material picked up at one point
some distance on downstream before depositing it at another.
These deposits are laid down by slackening currents where the
velocity of the stream is checked, as on the inner side of curves,
and where the slope of the bed is diminished, and in the lee of
islands, bridge piers and projecting points of land. How slight is
the check required to cause a current to drop a large part of its
load may be inferred from the law of the relation of the
transporting power to the velocity. If the velocity is decreased
one half, the current can move fragments but one sixty-fourth the
size of those which it could move before, and must drop all those
of larger size.
Will a river deposit more at low water or at flood? when rising or
when falling?
STRATIFICATION. River deposits are stratified, as may be seen in
any fresh cut in banks or bars. The waste of which they are built
has been sorted and deposited in layers, one above another; some
of finer and some of coarser material. The sorting action of
running water depends on the fact that its transporting power
varies with the velocity. A current whose diminishing velocity
compels it to drop coarse gravel, for example, is still able to
move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower
currents may deposit on the gravel bed layers of sand, and, still
later, slack water may leave on these a layer of mud. In case of
materials lighter than water the transporting power does not
depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.
CROSS BEDDING. A section of a bar exposed at low water may show
that it is formed of layers of sand, or coarser stuff, inclined
downstream as steeply often as the angle of repose of the
material. From a boat anchored over the lower end of a submerged
sand bar we may observe the way in which this structure, called
cross bedding, is produced. Sand is continually pushed over the
edge of the bar at b (Fig. 42) and comes to rest in successive
layers on the sloping surface. At the same time the bar may be
worn away at the upper end, a, and thus slowly advance down
stream. While the deposit is thus cross bedded, it constitutes as
a whole a stratum whose upper and lower surfaces are about
horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.
WATER WEAR. The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are WATER
WORN, or rounded, except when near their source. Rolling along the
bottom they have been worn round by impact and friction as they
rubbed against one another and the rocky bed of the stream.
Experiments have shown that angular fragments of granite lose
nearly half their weight and become well rounded after traveling
fifteen miles in rotating cylinders partly filled with water.
Marbles are cheaply made in Germany out of small limestone cubes
set revolving in a current of water between a rotating bed of
stone and a block of oak, the process requiring but about fifteen
minutes. It has been found that in the upper reaches of mountain
streams a descent of less than a mile is sufficient to round
pebbles of granite.
LAND FORMS DUE TO RIVER EROSION
RIVER VALLEYS. In their courses to the sea, rivers follow valleys
of various forms, some shallow and some deep, some narrow and some
wide. Since rivers are known to erode their beds and banks, it is
a fair presumption that, aided by the weather, they have excavated
the valleys in which they flow.
Moreover, a bird's-eye view or a map of a region shows the
significant fact that the valleys of a system unite with one
another in a branch work, as twigs meet their stems and the
branches of a tree its trunk. Each valley, from that of the
smallest rivulet to that of the master stream, is proportionate to
the size of the stream which occupies it. With a few explainable
exceptions the valleys of tributaries join that of the trunk
stream at a level; there is no sudden descent or break in the bed
at the point of juncture. These are the natural consequences which
must follow if the land has long been worked upon by streams, and
no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been
formed by the river systems which drain them, aided by the work of
the weather; they are not gaping fissures in the earth's crust, as
early observers imagined, but are the furrows which running water
has drawn upon the land.
As valleys are made by the slow wear of streams and the action of
the weather, they pass in their development through successive
stages, each of which has its own characteristic features. We may
therefore classify rivers and valleys according to the stage which
they have reached in their life history from infancy to old age.
YOUNG RIVER VALLEYS
INFANCY. The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba
was so recently covered by the waters of an extinct lake, known as
Lake Agassiz, that the surface remains much as it was left when
the lake was drained away. The flat floor, spread smooth with
lake-laid silts, is still a plain, to the eye as level as the sea.
Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not
exceeding sixty feet in depth, their gradients differing little
from the general slopes of the region. The trunk streams have but
few tributaries; the river system, like a sapling with few limbs,
is still undeveloped. Along the banks of the trunk streams short
gullies are slowly lengthening headwards, like growing twigs which
are sometime to become large branches.
The flat interstream areas are as yet but little scored by
drainage lines, and in wet weather water lingers in ponds in any
initial depressions on the plain.
CONTOURS. In order to read the topographic maps of the text-book
and the laboratory the student should know that contours are lines
drawn on maps to represent relief, all points on any given contour
being of equal height above sea level. The CONTOUR INTERVAL is the
uniform vertical distance between two adjacent contours and varies
on different maps.
To express regions of faint relief a contour interval of ten or
twenty feet is commonly selected; while in mountainous regions a
contour interval of two hundred and fifty, five hundred, or even
one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.
Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or
on a spread of glacial drift left by the retreat of continental
ice sheets, such as covers much of Canada and the northeastern
parts of the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground
water stands high, and, exuding in the undrained initial
depressions, forms marshes and lakes.
LAKES. Lakes are perhaps the most obvious of these fleeting
features of infancy. They are short-lived, for their destruction
is soon accomplished by several means. As a river system advances
toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the
undrained depressions of the region. Lakes having outlets are
drained away as their basin rims are cut down by the outflowing
streams,--a slow process where the rim is of hard rock, but a
rapid one where it is of soft material such as glacial drift.
Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the
shore and strew the debris worn from it over the lake bed. Shallow
lakes are often filled with organic matter from decaying
vegetation.
Does the outflowing stream, from a lake carry sediment? How does
this fact affect its erosive power on hard rock? on loose
material?
Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters
of the lake for a length of twenty miles with the waste brought
down from the high Alps. For this distance there extends up the
Rhone Valley an alluvial plain, which has grown lakeward at the
rate of a mile and a half since Roman times, as proved by the
distance inland at which a Roman port now stands.
How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the
great dam across the Colorado River at Austin, Texas, sediments
thirty-nine feet deep gathered in seven years.
Lake Mendota, one of the many beautiful lakes of southern
Wisconsin, is rapidly cutting back the soft glacial drift of its
shores by means of the abrasion of its waves. While the shallow
basin is thus broadened, it is also being filled with the waste;
and the time is brought nearer when it will be so shoaled that
vegetation can complete the work of its effacement.
Along the margin of a shallow lake mosses, water lilies, grasses,
and other water-loving plants grow luxuriantly. As their decaying
remains accumulate on the bottom, the ring of marsh broadens
inwards, the lake narrows gradually to a small pond set in the
midst of a wide bog, and finally disappears. All stages in this
process of extinction may be seen among the countless lakelets
which occupy sags in the recent sheets of glacial drift in the
northern states; and more numerous than the lakes which still
remain are those already thus filled with carbonaceous matter
derived from the carbon dioxide of the atmosphere. Such fossil
lakes are marked by swamps or level meadows underlain with muck.
THE ADVANCE TO MATURITY. The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins
of its area, are gradually effaced. By the furrowing action of the
rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire
area, and not an acre is left on which the fallen raindrop does
not find already cut for it an uninterrupted downward path which
leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is
now wholly destroyed; the region is all reduced to valley slopes.
THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds
with the initial surface of the region on which the stream begins
to flow, although its way may lead through basins and down steep
descents. The successive profiles to which it reduces its bed are
illustrated in Figure 51. As the gradient, or rate of descent of
its bed, is lowered, the velocity of the river is decreased until
its lessening energy is wholly consumed in carrying its load and
it can no longer erode its bed. The river is now AT GRADE, and its
capacity is just equal to its load. If now its load is increased
the stream deposits, and thus builds up, or AGGRADES, its bed. On
the other hand, if its load is diminished it has energy to spare,
and resuming its work of erosion, DEGRADES its bed. In either case
the stream continues aggrading or degrading until a new gradient
is found where the velocity is just sufficient to move the load,
and here again it reaches grade.
V-VALLEYS. Vigorous rivers well armed with waste make short work
of cutting their beds to grade, and thus erode narrow, steep-sided
gorges only wide enough at the base to accommodate the stream. The
steepness of the valley slopes depends on the relative rates at
which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream
may saw out a gorge whose sides are nearly or even quite vertical,
but as a rule young valleys whose streams have not yet reached
grade are V-shaped; their sides flare at the top because here the
rocks have longest been opened up to the action of the weather.
Some of the deepest canyons may be found where a rising land mass,
either mountain range or plateau, has long maintained by its
continued uplift the rivers of the region above grade.
In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you
suggest a reason?
THE GRAND CANYON OF THE COLORADO RIVER IN ARIZONA. The Colorado
River trenches the high plateau of northern Arizona with a
colossal canyon two hundred and eighteen miles long and more than
a mile in greatest depth. The rocks in which the canyon is cut are
for the most part flat-lying, massive beds of limestones and
sandstones, with some shales, beneath which in places harder
crystalline rocks are disclosed. Where the canyon is deepest its
walls have been profoundly dissected. Lateral ravines have widened
into immense amphitheaters, leaving between them long ridges of
mountain height, buttressed and rebuttressed with flanking spurs
and carved into majestic architectural forms. From the extremity
of one of these promontories it is two miles or more across the
gulf to the point of the one opposite, and the heads of the
amphitheaters are thirteen miles apart.
The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the
river, a swift, powerful stream a few hundred feet wide, turbid
with reddish silt, by means of which it continually rasps its
rocky bed as it hurries on. The Colorado is still deepening its
gorge. In the Grand Canyon its gradient is seven and one half feet
to the mile, but, as in all ungraded rivers, the descent is far
from uniform. Graded reaches in soft rock alternate with steeper
declivities in hard rock, forming rapids such as, for example, a
stretch of ten miles where the fall averages twenty-one feet to
the mile. Because of these dangerous rapids the few exploring
parties who have traversed the Colorado canyon have done so at the
hazard of their lives.
The canyon has been shaped by several agencies. Its depth is due
to the river which has sawed its way far toward the base of a
lofty rising plateau. Acting alone this would have produced a
slitlike gorge little wider than the breadth of the stream. The
impressive width of the canyon and the magnificent architectural
masses which fill it are owing to two causes.: Running water has
gulched the walls and weathering has everywhere attacked and
driven them back. The horizontal harder beds stand out in long
lines of vertical cliffs, often hundreds of feet in height, at
whose feet talus slopes conceal the outcrop of the weaker strata.
As the upper cliffs have been sapped and driven back by the
weather, broad platforms are left at their bases and the sides of
the canyon descend to the river by gigantic steps. Far up and down
the canyon the eye traces these horizontal layers, like the
flutings of an elaborate molding, distinguishing each by its
contour as well as by its color and thickness.
The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no
more than well begun. It has not yet wholly reached grade, and the
great task of the river and its tributaries--the task of leveling
the lofty plateau to a low plain and of transporting it grain by
grain to the sea--still lies almost entirely in the future.
WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to
the initial surface over which it began its course; still more
commonly are they developed in the rock mass through which it is
cutting its valley. Thus, wherever a stream leaves harder rocks to
flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or
rapid, results.
At time of flood young tributaries with steeper courses than that
of the trunk stream may bring down stones and finer waste, which
the gentler current cannot move along, and throw them as a dam
across its way. The rapids thus formed are also ephemeral, for as
the gradient of the tributaries is lowered the main stream becomes
able to handle the smaller and finer load which they discharge.
A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in
the erosion of their beds because of their smaller volume, and
thus join it by plunging over the side of its gorge. But as the
river approaches grade and slackens its down cutting, the
tributaries sooner or later overtake it, and effacing their falls,
unite with it on a level.
Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have
been obliterated already.
LOCAL BASELEVELS. That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks,
ungraded outcrops of hard rocks remaining as barriers which give
rise to rapids or falls. Until these barriers are worn away they
constitute local baselevels, below which level the stream, up
valley from them, cannot cut. They are eroded to grade one after
another, beginning with the least strong, or the one nearest the
mouth of the stream. In a similar way the surface of a lake in a
river's course constitutes for all inflowing streams a local
baselevel, which disappears when the basin is filled or drained.
MATURE AND OLD RIVERS
Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great
task of wearing down the land mass which it drains and carrying it
particle by particle to the sea. The relief of the land is now at
its greatest; for the main channels have been sunk to grade, while
the divides remain but little worn below their initial altitudes.
Ground water now stands low. The run-off washes directly to the
streams, with the least delay and loss by evaporation in ponds and
marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley
slopes is now largest and sheds to the streams a heavier load of
waste than ever before. At maturity the river system is doing its
greatest amount of work both in erosion and in the carriage of
water and of waste to the sea.
LATERAL EROSION. On reaching grade a river ceases to scour its
bed, and it does not again begin to do so until some change in
load or volume enables it to find grade at a lower level. On the
other hand, a stream erodes its banks at all stages in its
history, and with graded rivers this process, called lateral
erosion, or PLANATION, is specially important. The current of a
stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and
continually wears and saps its banks. On the inner side deposition
takes place in the more shallow and slower-moving water. The inner
bank of bends is thus built out while the outer bank is worn away.
By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is
thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle
slopes. While widening their valleys streams maintain a constant
width of channel, so that a wide-floored valley does not signify
that it ever was occupied by a river of equal width.
THE GRADIENT. The gradients of graded rivers differ widely. A
large river with a light load reaches grade on a faint slope,
while a smaller stream heavily burdened with waste requires a
steep slope to give it velocity sufficient to move the load.
The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the
Great Plains and surcharged with the waste brought down both by
its branches in the mountains and by those whose tracks lie over
the soft rocks of the plains. It is compelled to maintain a
gradient of eight feet to the mile in western Nebraska. The Ohio
reaches grade with a slope of less than four inches to the mile
from Cincinnati to its mouth, and the powerful Mississippi washes
along its load with a fall of but three inches per mile from Cairo
to the Gulf.
Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream
supplied with gravel or one with silt?
Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the
load should be increased in amount or in coarseness?
THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed
over the gentler slopes of the lowering hills. With smaller loads
to carry, the rivers now deepen their valleys and find grade with
fainter declivities nearer the level of the sea. This limit of the
level of the sea beneath which they cannot erode is known as
baselevel. [Footnote: The term "baselevel" is also used to
designate the close approximation to sea level to which streams
are able to subdue the land.] As streams grow old they approach
more and more closely to baselevel, although they are never able
to attain it. Some slight slope is needed that water may flow and
waste be transported over the land. Meanwhile the relief of the
land has ever lessened. The master streams and their main
tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion
of their innumerable branches and the wear of the weather the
divides everywhere are lowered and subdued to more and more gentle
slopes. Mountains and high plateaus are thus reduced to rolling
hills, and at last to plains, surmounted only by such hills as may
still be unreduced to the common level, because of the harder
rocks of which they are composed or because of their distance from
the main erosion channels. Such regions of faint relief, worn down
to near base level by subaerial agencies, are known as PENEPLAINS
(almost plains). Any residual masses which rise above them are
called MONADNOCKS, from the name of a conical peak of New
Hampshire which overlooks the now uplifted peneplain of southern
New England.
In its old age a region becomes mantled with thick sheets of fine
and weathered waste, slowly moving over the faint slopes toward
the water ways and unbroken by ledges of bare rock. In other
words, the waste mantle also is now graded, and as waterfalls have
been effaced in the river beds, so now any ledges in the wide
streams of waste are worn away and covered beneath smooth slopes
of fine soil. Ground water stands high and may exude in areas of
swamp. In youth the land mass was roughhewn and cut deep by stream
erosion. In old age the faint reliefs of the land dissolve away,
chiefly under the action of the weather, beneath their cloak of
waste.
THE CYCLE OF EROSION. The successive stages through which a land
mass passes while it is being leveled to the sea constitute
together a cycle of erosion. Each stage of the cycle from infancy
to old age leaves, as we have seen, its characteristic records in
the forms sculptured on the land, such as the shapes of valleys
and the contours of hills and plains. The geologist is thus able
to determine by the land forms of any region the stage in the
erosion cycle to which it now belongs, and knowing what are the
earlier stages of the cycle, to read something of the geological
history of the region.
INTERRUPTED CYCLES. So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual
movements of the earth's crust, by which a region is either
depressed or elevated relative to sea level.
The DEPRESSION of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their
beds and carry their loads to a degree corresponding to the amount
of the depression, and by lessening the amount of work they have
to do. The slackened river currents deposit their waste in Hood
plains which increase in height as the subsidence continues. The
lower courses of the rivers are invaded by the sea and become
estuaries, while the lower tributaries are cut off from the trunk
stream.
ELEVATION, on the other hand, increases the activity of all
agencies of weathering, erosion, and transportation, restores the
region to its youth, and inaugurates a new cycle of erosion.
Streams are given a steeper gradient, greater velocity, and
increased energy to carry their loads and wear their beds. They
cut through the alluvium of their flood plains, leaving it on
either bank as successive terraces, and intrench themselves in the
underlying rock. In their older and wider valleys they cut narrow,
steep-walled inner gorges, in which they flow swiftly over rocky
floors, broken here and there by falls and rapids where a harder
layer of rock has been discovered. Winding streams on plains may
thus incise their meanders in solid rock as the plains are
gradually uplifted. Streams which are thus restored to their youth
are said to be REVIVED.
As streams cut deeper and the valley slopes are steepened, the
mantle of waste of the region undergoing elevation is set in more
rapid movement. It is now removed particle by particle faster than
it forms. As the waste mantle thins, weathering attacks the rocks
of the region more energetically until an equilibrium is reached
again; the rocks waste rapidly and their waste is as rapidly
removed.
DISSECTED PENEPLAINS. When a rise of the land brings one cycle to
an end and begins another, the characteristic land forms of each
cycle are found together and the topography of the region is
composite until the second cycle is so far advanced that the land
forms of the first cycle are entirely destroyed. The contrast
between the land surfaces of the later and the earlier cycles is
most striking when the earlier had advanced to age and the later
is still in youth. Thus many peneplains which have been elevated
and dissected have been recognized by the remnants of their
ancient erosion surfaces, and the length of time which has elapsed
since their uplift has been measured by the stage to which the new
cycle has advanced.
THE PIEDMONT BELT. As an example of an ancient peneplain uplifted
and dissected we may cite the Piedmont Belt, a broad upland lying
between the Appalachian Mountains and the Atlantic coastal plain.
The surface of the Piedmont is gently rolling. The divides, which
are often smooth areas of considerable width, rise to a common
plane, and from them one sees in every direction an even sky line
except where in places some lone hill or ridge may lift itself
above the general level (Fig. 62). The surface is an ancient one,
for the mantle of residual waste lies deep upon it, soils are
reddened by long oxidation, and the rocks are rotted to a depth of
scores of feet.
At present, however, the waste mantle is not forming so rapidly as
it is being removed. The streams of the upland are actively
engaged in its destruction. They flow swiftly in narrow, rock-
walled valleys over rocky beds. This contrast between the young
streams and the aged surface which they are now so vigorously
dissecting can only be explained by the theory that the region
once stood lower than at present and has recently been upraised.
If now we imagine the valleys refilled with the waste which the
streams have swept away, and the upland lowered, we restore the
Piedmont region to the condition in which it stood before its
uplift and dissection,--a gently rolling plain, surmounted here
and there by isolated hills and ridges.
The surface of the ancient Piedmont plain, as it may be restored
from the remnants of it found on the divides, is not in accordance
with the structures of the country rocks. Where these are exposed
to view they are seen to be far from horizontal. On the walls of
river gorges they dip steeply and in various directions and the
streams flow over their upturned edges. As shown in Figure 67, the
rocks of the Piedmont have been folded and broken and tilted.
It is not reasonable to believe that when the rocks of the
Piedmont were thus folded and otherwise deformed the surface of
the region was a plain. The upturned layers have not always
stopped abruptly at the even surface of the Piedmont plain which
now cuts across them. They are the bases of great folds and tilted
blocks which must once have risen high in air. The complex and
disorderly structures of the Piedmont rocks are those seen in
great mountain ranges, and there is every reason to believe that
these rocks after their deformation rose to mountain height.
The ancient Piedmont plain cuts across these upturned rocks as
independently of their structure as the even surface of the sawed
stump of some great tree is independent of the direction of its
fibers. Hence the Piedmont plain as it was before its uplift was
not a coastal plain formed of strata spread in horizontal sheets
beneath the sea and then uplifted; nor was it a structural plain,
due to the resistance to erosion of some hard, flat-lying layer of
rock. Even surfaces developed on rocks of discordant structure,
such as the Piedmont shows, are produced by long denudation, and
we may consider the Piedmont as a peneplain formed by the wearing
down of mountain ranges, and recently uplifted.
THE LAURENTIAN PENEPLAIN. This is the name given to a denuded
surface on very ancient rocks which extends from the Arctic Ocean
to the St. Lawrence River and Lake Superior, with small areas also
in northern Wisconsin and New York. Throughout this U-shaped area,
which incloses Hudson Bay within its arms, the country rocks have
the complicated and contorted structures which characterize
mountain ranges. But the surface of the area is by no means
mountainous. The sky line when viewed from the divides is unbroken
by mountain peaks or rugged hills. The surface of the arm west of
Hudson Bay is gently undulating and that of the eastern arm has
been roughened to low-rolling hills and dissected in places by
such deep river gorges as those of the Ottawa and Saguenay. This
immense area may be regarded as an ancient peneplain truncating
the bases of long-vanished mountains and dissected after
elevation.
In the examples cited the uplift has been a broad one and to
comparatively little height. Where peneplains have been uplifted
to great height and have since been well dissected, and where they
have been upfolded and broken and uptilted, their recognition
becomes more difficult. Yet recent observers have found evidences
of ancient lowland surfaces of erosion on the summits of the
Allegheny ridges, the Cascade Mountains (Fig. 69), and the western
slope of the Sierra Nevadas.
THE SOUTHERN APPALACHIAN REGION. We have here an example of an
area the latter part of whose geological history may be deciphered
by means of its land forms. The generalized section of Figure 70,
which passes from west to east across a portion of the region in
eastern Tennessee, shows on the west a part of the broad
Cumberland plateau. On the east is a roughened upland platform,
from which rise in the distance the peaks of the Great Smoky
Mountains. The plateau, consisting of strata but little changed
from their original flat-lying attitude, and the platform,
developed on rocks of disordered structure made crystalline by
heat and pressure, both stand at the common level of the line AB.
They are separated by the Appalachian valley, forty miles wide,
cut in strata which have been folded and broken into long narrow
blocks. The valley is traversed lengthwise by long, low ridges,
the outcropping edges of the harder strata, which rise to about
the same level,--that of the line cd. Between these ridges stretch
valley lowlands at the level ef excavated in the weaker rocks,
while somewhat below them lie the channels of the present streams
now busily engaged in deepening their beds.
THE VALLEY LOWLANDS. Were they planed by graded or ungraded
streams? Have the present streams reached grade? Why did the
streams cease widening the floors of the valley lowlands? How long
since? When will they begin anew the work of lateral planation?
What effect will this have on the ridges if the present cycle of
erosion continues long uninterrupted?
THE RIDGES OF THE APPALACHIAN VALLEY. Why do they stand above the
valley lowlands? Why do their summits lie in about the same plane?
Refilling the valleys intervening between these ridges with the
material removed by the streams, what is the nature of the surface
thus restored? Does this surface cd accord with the rock
structures on which' it has been developed? How may it have been
made? At what height did the land stand then, compared with its
present height? What elevations stood above the surface cd? Why?
What name may you use to designate them? How does the length of
time needed to develop the surface cd compare with that needed to
develop the valley lowlands?
THE PLATFORM AND PLATEAU. Why do they stand at a common level ab?
Of what surface may they be remnants? Is it accordant with the
rock structure? How was it produced? What unconsumed masses
overlooked it? Did the rocks of the Appalachian valley stand above
this surface when it was produced? Did they then stand below it?
Compare the time needed to develop this surface with that needed
to develop cd. Which surface is the older?
How many cycles of erosion are represented here? Give the erosion
history of the region by cycles, beginning with the oldest, the
work done in each and the work left undone, what brought each
cycle to a close, and how long relatively it continued.
CHAPTER IV
RIVER DEPOSITS
The characteristic features of river deposits and the forms which
they assume may be treated under three heads: (1) valley deposits,
(2) basin deposits, and (3) deltas.
VALLEY DEPOSITS
FLOOD PLAINS are the surfaces of the alluvial deposits which
streams build along their courses at times of flood. A swift
current then sweeps along the channel, while a shallow sheet of
water moves slowly over the flood plain, spreading upon it a thin
layer of sediment. It has been estimated that each inundation of
the Nile leaves a layer of fertilizing silt three hundredths of an
inch thick over the flood plain of Egypt.
Flood plains may consist of a thin spread of alluvium over the
flat rock floor of a valley which is being widened by the lateral
erosion of a graded stream (Fig. 60). Flood-plain deposits of
great thickness may be built by aggrading rivers even in valleys
whose rock floors have never been thus widened.
A cross section of a flood plain shows that it is highest next the
river, sloping gradually thence to the valley sides. These wide
natural embankments are due to the fact that the river deposit is
heavier near the bank, where the velocity of the silt-laden
channel current is first checked by contact with the slower-moving
overflow.
Thus banked off from the stream, the outer portions of a flood
plain are often ill-drained and swampy, and here vegetal deposits,
such as peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the Mississippi or
the Missouri (Fig. 77), shows that the courses of the tributaries
on entering it are deflected downstream. Why?
The aggrading streams by which flood plains are constructed
gradually build their immediate banks and beds to higher and
higher levels, and therefore find it easy at times of great floods
to break their natural embankments and take new courses over the
plain. In this way they aggrade each portion of it in turn by
means of their shifting channels,
BRAIDED CHANNELS. A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep
gradient and often flows down it in a fairly direct course and
through a network of braided channels. From time to time a channel
becomes choked with waste, and the water no longer finding room in
it breaks out and cuts and builds itself a new way which reunites
down valley with the other channels. Thus there becomes
established a network of ever-changing channels inclosing low
islands of sand and gravel.
TERRACES. While aggrading streams thus tend to shift their
channels, degrading streams, on the contrary, become more and more
deeply intrenched in their valleys. It often occurs that a stream,
after having built a flood plain, ceases to aggrade its bed
because of a lessened load or for other reasons, such as an uplift
of the region, and begins instead to degrade it. It leaves the
original flood plain out of reach of even the highest floods. When
again it reaches grade at a lower level it produces a new flood
plain by lateral erosion in the older deposits, remnants of which
stand as terraces on one or both sides of the valley. In this way
a valley may be lined with a succession of terraces at different
levels, each level representing an abandoned flood plain.
MEANDERS. Valleys aggraded with fine waste form well-nigh level
plains over which streams wind from side to side of a direct
course in symmetric bends known as meanders, from the name of a
winding river of Asia Minor. The giant Mississippi has developed
meanders with a radius of one and one half miles, but a little
creek may display on its meadow as perfect curves only a rod or so
in radius. On the flood plain of either river or creek we may find
examples of the successive stages in the development of the
meander, from its beginning in the slight initial bend sufficient
to deflect the current against the outer side. Eroding here and
depositing on the inner side of the bend, it gradually reaches
first the open bend whose width and length are not far from equal,
and later that of the horseshoe meander whose diameter transverse
to the course of the stream is much greater than that parallel
with it. Little by little the neck of land projecting into the
bend is narrowed, until at last it is cut through and a "cut-off"
is established. The old channel is now silted up at both ends and
becomes a crescentic lagoon, or oxbow lake, which fills gradually
to an arc-shaped shallow depression.
FLOOD PLAINS CHARACTERISTIC OF MATURE RIVERS. On reaching grade a
stream planes a flat floor for its continually widening valley.
Ever cutting on the outer bank of its curves, it deposits on the
inner bank scroll-like flood-plain patches. For a while the valley
bluffs do not give its growing meanders room to develop to their
normal size, but as planation goes on, the bluffs are driven back
to the full width of the meander belt and still later to a width
which gives room for broad stretches of flood plain on either
side.
Usually a river first attains grade near its mouth, and here first
sinks its bed to near baselevel. Extending its graded course
upstream by cutting away barrier after barrier, it comes to have a
widened and mature valley over its lower course, while its young
headwaters are still busily eroding their beds. Its ungraded
branches may thus bring down to its lower course more waste than
it is competent to carry on to the sea, and here it aggrades its
bed and builds a flood plain in order to gain a steeper gradient
and velocity enough to transport its load.
As maturity is past and the relief of the land is lessened, a
smaller and smaller load of waste is delivered to the river. It
now has energy to spare and again degrades its valley, excavating
its former flood plains and leaving them in terraces on either
side, and at last in its old age sweeping them away.
ALLUVIAL CONES AND FANS. In hilly and mountainous countries one
often sees on a valley side a conical or fan-shaped deposit of
waste at the mouth of a lateral stream. The cause is obvious: the
young branch has not been able as yet to wear its bed to accordant
level with the already deepened valley of the master stream. It
therefore builds its bed to grade at the point of juncture by
depositing here its load of waste,--a load too heavy to be carried
along the more gentle profile of the trunk valley.
Where rivers descend from a mountainous region upon the plain they
may build alluvial fans of exceedingly gentle slope. Thus the
rivers of the western side of the Sierra Nevada Mountains have
spread fans with a radius of as much as forty miles and a slope
too slight to be detected without instruments, where they leave
the rock-cut canyons in the mountains and descend upon the broad
central valley of California.
As a river flows over its fan it commonly divides into a
branchwork of shifting channels called DISTRIBUTARIES, since they
lead off the water from the main stream. In this way each part of
the fan is aggraded and its symmetric form is preserved.
PIEDMONT PLAINS. Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains.
These are especially characteristic of arid lands, where the
streams wither as they flow out upon the thirsty lowlands and are
therefore compelled to lay down a large portion of their load. In
humid climates mountain-born streams are usually competent to
carry their loads of waste on to the sea, and have energy to spare
to cut the lower mountain slopes into foothills. In arid regions
foothills are commonly absent and the ranges rise, as from
pedestals, above broad, sloping plains of stream-laid waste.
THE HIGH PLAINS. The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste
which stretches forward from the base of the mountains for
hundreds of miles and in places is five hundred feet thick (Fig.
80). That the deposit was made in ancient times on land and not in
the sea is proved by the remains which it contains of land animals
and plants of species now extinct. That it was laid by rivers and
not by fresh-water lakes is shown by its structure. Wide stretches
of flat-lying, clays and sands are interrupted by long, narrow
belts of gravel which mark the channels of the ancient streams.
Gravels, and sands are often cross bedded, and their well worn
pebbles may be identified with the rocks of the mountains. After
building this sheet of waste the streams ceased to aggrade and
began the work of destruction. Large uneroded remnants, their
surfaces flat as a floor, remain as the High Plains of western
Kansas and Nebraska.
RIVER DEPOSITS IN SUBSIDING TROUGHS. To a geologist the most
important river deposits are those which gather in areas of
gradual subsidence; they are often of vast extent and immense
thickness, and such deposits of past geological ages have not
infrequently been preserved, with all their records of the times
in which they were built, by being carried below the level of the
sea, to be brought to light by a later uplift. On the other hand,
river deposits which remain above baselevels of erosion are swept
away comparatively soon.
THE GREAT VALLEY OF CALIFORNIA is a monotonously level plain of
great fertility, four hundred miles in length and fifty miles in
average width, built of waste swept down by streams from the
mountain ranges which inclose it,--the Sierra Nevada on the east
and the Coast Range on the west. On the waste slopes at the foot
of the bordering hills coarse gravels and even bowlders are left,
while over the interior the slow-flowing streams at times of
flood spread wide sheets of silt. Organic deposits are now forming
by the decay of vegetation in swampy tule (reed) lands and in
shallow lakes which occupy depressions left by the aggrading
streams.
Deep borings show that this great trough is filled to a depth of
at least two thousand feet below sea level with recent
unconsolidated sands and silts containing logs of wood and fresh-
water shells. These are land deposits, and the absence of any
marine deposits among them proves that the region has not been
invaded by the sea since the accumulation began. It has therefore
been slowly subsiding and its streams, although continually
carried below grade, have yet been able to aggrade the surface as
rapidly as the region sank, and have maintained it, as at present,
slightly above sea level.
THE INDO-GANGETIC PLAIN, spread by the Brahmaputra, the Ganges,
and the Indus river systems, stretches for sixteen hundred miles
along the southern base of the Himalaya Mountains and occupies an
area of three hundred thousand square miles (Fig.342). It consists
of the flood plains of the master streams and the confluent fans
of the tributaries which issue from the mountains on the north.
Large areas are subject to overflow each season of flood, and
still larger tracts mark abandoned flood plains below which the
rivers have now cut their beds. The plain is built of far-
stretching beds of clay, penetrated by streaks of sand, and also
of gravel near the mountains. Beds of impure peat occur in it, and
it contains fresh-water shells and the bones of land animals of
species now living in northern India. At Lucknow an artesian well
was sunk to one thousand feet below sea level without reaching the
bottom of these river-laid sands and silts, proving a slow
subsidence with which the aggrading rivers have kept pace.
WARPED VALLEYS. It is not necessary that an area should sink below
sea level in order to be filled with stream-swept waste. High
valleys among growing mountain ranges may suffer warping, or may
be blockaded by rising mountain folds athwart them. Where the
deformation is rapid enough, the river may be ponded and the
valley filled with lake-laid sediments. Even when the river is
able to maintain its right of way it may yet have its declivity so
lessened that it is compelled to aggrade its course continually,
filling the valley with river deposits which may grow to an
enormous thickness.
Behind the outer ranges of the Himalaya Mountains lie several
waste-filled valleys, the largest of which are Kashmir and Nepal,
the former being an alluvial plain about as large as the state of
Delaware. The rivers which drain these plains have already cut
down their outlet gorges sufficiently to begin the task of the
removal of the broad accumulations which they have brought in from
the surrounding mountains. Their present flood plains lie as much
as some hundreds of feet below wide alluvial terraces which mark
their former levels. Indeed, the horizontal beds of the Hundes
Valley have been trenched to the depth of nearly three thousand
feet by the Sutlej River. These deposits are recent or subrecent,
for there have been found at various levels the remains of land
plants and land and fresh-water shells, and in some the bones of
such animals as the hyena and the goat, of species or of genera
now living. Such soft deposits cannot be expected to endure
through any considerable length of future time the rapid erosion
to which their great height above the level of the sea will
subject them.
CHARACTERISTICS OF RIVER DEPOSITS. The examples just cited teach
clearly the characteristic features of extensive river deposits.
These deposits consist of broad, flat-lying sheets of clay and
fine sand left by the overflow at time of flood, and traversed
here and there by long, narrow strips of coarse, cross-bedded
sands and gravels thrown down by the swifter currents of the
shifting channels. Occasional beds of muck mark the sites of
shallow lakelets or fresh-water swamps. The various strata also
contain some remains of the countless myriads of animals and
plants which live upon the surface of the plain as it is in
process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land
animals as suffer drowning at times of flood or are mired in
swampy places, logs of wood, and the stems and leaves of plants
are examples of the variety of the remains of land and fresh-water
organisms which are entombed in river deposits and sealed away as
a record of the life of the time, and as proof that the deposits
were laid by streams and not beneath the sea.
BASIN DEPOSITS
DEPOSITS IN DRY BASINS. On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of
central Asia, stream-swept waste accumulates indefinitely. The
rivers of the surrounding mountains, fed by the rains and melting
snows of these comparatively moist elevations, dry and soak away
as they come down upon the arid plains. They are compelled to lay
aside their entire load of waste eroded from the mountain valleys,
in fans which grow to enormous size, reaching in some instances
thousands of feet in thickness.
The monotonous levels of Turkestan include vast alluvial tracts
now in process of building by the floods of the frequently
shifting channels of the Oxus and other rivers of the region. For
about seven hundred miles from its mouth in Aral Lake the Oxus
receives no tributaries, since even the larger branches of its
system are lost in a network of distributaries and choked with
desert sands before they reach their master stream. These
aggrading rivers, which have channels but no valleys, spread their
muddy floods--which in the case of the Oxus sometimes equal the
average volume of the Mississippi--far and wide over the plain,
washing the bases of the desert dunes.
PLAYAS. In arid interior basins the central depressions may be
occupied by playas,--plains of fine mud washed forward from the
margins. In the wet season the playa is covered with a thin sheet
of muddy water, a playa lake, supplied usually by some stream at
flood. In the dry season the lake evaporates, the river which fed
it retreats, and there is left to view a hard, smooth, level floor
of sun-baked and sun-cracked yellow clay utterly devoid of
vegetation.
In the Black Rock desert of Nevada a playa lake spreads over an
area fifty miles long and twenty miles wide. In summer it
disappears; the Quinn River, which feeds it, shrinks back one
hundred miles toward its source, leaving an absolutely barren
floor of clay, level as the sea.
LAKE DEPOSITS. Regarding lakes as parts of river systems, we may
now notice the characteristic features of the deposits in lake
basins. Soundings in lakes of considerable size and depth show
that their bottoms are being covered with tine clays. Sand and
gravel are found along; their margins, being brought in by streams
and worn by waves from the shore, but there are no tidal or other
strong currents to sweep coarse waste out from shore to any
considerable distance. Where fine clays are now found on the land
in even, horizontal layers containing the remains of fresh-water
animals and plants, uncut by channels tilled with cross-bedded
gravels and sands and bordered by beach deposits of coarse waste,
we may safely infer the existence of ancient lakes.
MARL. Marl is a soft, whitish deposit of carbonate of lime,
mingled often with more or less of clay, accumulated in small
lakes whose feeding springs are charged with carbonate of lime and
into which little waste is washed from the land. Such lakelets are
not infrequent on the surface of the younger drift sheets of
Michigan and northern Indiana, where their beds of marl--sometimes
as much as forty feet thick--are utilized in the manufacture of
Portland cement. The deposit results from the decay of certain
aquatic plants which secrete lime carbonate from the water, from
the decomposition of the calcareous shells of tiny mollusks which
live in countless numbers on the lake floor, and in some cases
apparently from chemical precipitation.
PEAT. We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants
of the surface of the bog lies a spongy mass composed of dead
vegetable tissue, which passes downward gradually into PEAT,--a
dense, dark brown carbonaceous deposit in which, to the unaided
eye, little or no trace of vegetable structure remains. When
dried, peat forms a fuel of some value and is used either cut into
slabs and dried or pressed into bricks by machinery.
When vegetation decays in open air the carbon of its tissues,
taken from the atmosphere by the leaves, is oxidized and returned
to it in its original form of carbon dioxide. But decomposing in
the presence of water, as in a bog, where the oxygen of the air is
excluded, the carbonaceous matter of plants accumulates in
deposits of peat.
Peat bogs are numerous in regions lately abandoned by glacier ice,
where river systems are so immature that the initial depressions
left in the sheet of drift spread over the country have not yet
been drained. One tenth of the surface of Ireland is said to be
covered with peat, and small bogs abound in the drift-covered area
of New England and the states lying as far west as the Missouri
River. In Massachusetts alone it has been reckoned that there are
fifteen billion cubic feet of peat, the largest bog occupying
several thousand acres.
Much larger swamps occur on the young coastal plain of the
Atlantic from New Jersey to Florida. The Dismal Swamp, for
example, in Virginia and North Carolina is forty miles across. It
is covered with a dense growth of water-loving trees such as the
cypress and black gum. The center of the swamp is occupied by Lake
Drummond, a shallow lake seven miles in diameter, with banks of
pure-peat, and still narrowing from the encroachment of vegetation
along its borders.
SALT LAKES. In arid climates a lake rarely receives sufficient
inflow to enable it to rise to the basin rim and find an outlet.
Before this height is reached its surface becomes large enough to
discharge by evaporation into the dry air the amount of water that
is supplied by streams. As such a lake has no outlet, the minerals
in solution brought into it by its streams cannot escape from the
basin. The lake water becomes more and more heavily charged with
such substances as common salt and the sulphates and carbonates of
lime, of soda, and of potash, and these are thrown down from
solution one after another as the point of saturation for each
mineral is reached. Carbonate of lime, the least soluble and often
the most abundant mineral brought in, is the first to be
precipitated. As concentration goes on, gypsum, which is insoluble
in a strong brine, is deposited, and afterwards common salt. As
the saltness of the lake varies with the seasons and with climatic
changes, gypsum and salt are laid in alternate beds and are
interleaved with sedimentary clays spread from the waste brought
in by streams at times of flood. Few forms of life can live in
bodies of salt water so concentrated that chemical deposits take
place, and hence the beds of salt, gypsum, and silt of such lakes
are quite barren of the remains of life. Similar deposits are
precipitated by the concentration of sea water in lagoons and arms
of the sea cut off from the ocean.
LAKES BONNEVILLE AND LAHONTAN. These names are given to extinct
lakes which once occupied large areas in the Great Basin, the
former in Utah, the latter in northwestern Nevada. Their records
remain in old horizontal beach lines which they drew along their
mountainous shores at the different levels at which they stood,
and in the deposits of their beds. At its highest stage Lake
Bonneville, then one thousand feet deep, overflowed to the north
and was a fresh-water lake. As it shrank below the outlet it
became more and more salty, and the Great Salt Lake, its withered
residue, is now depositing salt along its shores. In its strong
brine lime carbonate is insoluble, and that brought in by streams
is thrown down at once in the form of travertine.
Lake Lahontan never had an outlet. The first chemical deposits to
be made along its shores were deposits of travertine, in places
eighty feet thick. Its floor is spread with fine clays, which must
have been laid in deep, still water, and which are charged with
the salts absorbed by them as the briny water of the lake dried
away. These sedimentary clays are in two divisions, the upper and
lower, each being about one hundred feet thick. They are separated
by heavy deposits of well-rounded, cross-bedded gravels and sands,
similar to those spread at the present time by the intermittent
streams of arid regions. A similar record is shown in the old
floors of Lake Bonneville. What conclusions do you draw from these
facts as to the history of these ancient lakes?
DELTAS
In the river deposits which are left above sea level particles of
waste are allowed to linger only for a time. From alluvial fans
and flood plains they are constantly being taken up and swept
farther on downstream. Although these land forms may long persist,
the particles which compose them are ever changing. We may
therefore think of the alluvial deposits of a valley as a stream
of waste fed by the waste mantle as it creeps and washes down the
valley sides, and slowly moving onwards to the sea.
In basins waste finds a longer rest, but sooner or later lakes and
dry basins are drained or filled, and their deposits, if above sea
level, resume their journey to their final goal. It is only when
carried below the level of the sea that they are indefinitely
preserved.
On reaching this terminus, rivers deliver their load to the ocean.
In some cases the ocean is able to take it up by means of strong
tidal and other currents, and to dispose of it in ways which we
shall study later. But often the load is so large, or the tides
are so weak, that much of the waste which the river brings in
settles at its mouth, there building up a deposit called the
DELTA, from the Greek letter of that name, whose shape it
sometimes resembles.
Deltas and alluvial fans have many common characteristics. Both
owe their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline,
the apex pointing upstream; and both are traversed by
distributaries which build up all parts in turn.
In a delta we may distinguish deposits of two distinct kinds,--
the submarine and the subaerial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves
and tides over the sea bottom adjacent to the river's mouth. The
origin of these deposits is recorded in the remains of marine
animals and plants which they contain.
As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaerial deposits
altogether similar to those of the flood plain, of which indeed
the subaerial delta is the prolongation. Here extended deposits of
peat may accumulate in swamps, and the remains of land and fresh-
water animals and plants swept down by the stream are imbedded in
the silts laid at times of flood.
Borings made in the deltas of great rivers such as the
Mississippi, the Ganges, and the Nile, show that the subaerial
portion often reaches a surprising thickness. Layers of peat, old
soils, and forest grounds with the stumps of trees are discovered
hundreds of feet below sea level. In the Nile delta some eight
layers of coarse gravel were found interbedded with river silts,
and in the Ganges delta at Calcutta a boring nearly five hundred
feet in depth stopped in such a layer.
The Mississippi has built a delta of twelve thousand three hundred
square miles, and is pushing the natural embankments of its chief
distributaries into the Gulf at a maximum rate of a mile in
sixteen years. Muddy shoals surround its front, shallow lakes,
e.g. lakes Pontchartrain and Borgne, are formed between the
growing delta and the old shore line, and elongate lakes and
swamps are inclosed between the natural embankments of the
distributaries.
The delta of the Indus River, India, lies so low along shore that
a broad tract of country is overflowed by the highest tides. The
submarine portion of the delta has been built to near sea level
over so wide a belt offshore that in many places large vessels
cannot come even within sight of land because of the shallow
water.
A former arm of the sea, the Rann of Cutch, adjoining the delta on
the east has been silted up and is now an immense barren flat of
sandy mud two hundred miles in length and one hundred miles in
greatest breadth. Each summer it is flooded with salt water when
the sea is brought in by strong southwesterly monsoon winds, and
the climate during the remainder of the year is hot and dry. By
the evaporation of sea water the soil is thus left so salty that
no vegetation can grow upon it, and in places beds of salt several
feet in thickness have accumulated. Under like conditions salt
beds of great thickness have been formed in the past and are now
found buried among the deposits of ancient deltas.
SUBSIDENCE OF GREAT DELTAS. As a rule great deltas are slowly
sinking. In some instances upbuilding by river deposits has gone
on as rapidly as the region has subsided. The entire thickness of
the Ganges delta, for example, so far as it has been sounded,
consists of deposits laid in open air. In other cases interbedded
limestones and other sedimentary rocks containing marine fossils
prove that at times subsidence has gained on the upbuilding and
the delta has been covered with the sea.
It is by gradual depression that delta deposits attain enormous
thickness, and, being lowered beneath the level of the sea, are
safely preserved from erosion until a movement of the earth's
crust in the opposite direction lifts them to form part of the
land. We shall read later in the hard rocks of our continent the
records of such ancient deltas, and we shall not be surprised to
find them as thick as are those now building at the mouths of
great rivers.
LAKE DELTAS. Deltas are also formed where streams lose their
velocity on entering the still waters of lakes. The shore lines of
extinct lakes, such as Lake Agassiz and Lakes Bonneville and
Lahontan, may be traced by the heavy deposits at the mouths of
their tributary streams.
We have seen that the work of streams is to drain the lands of the
water poured upon them by the rainfall, to wear them down, and to
carry their waste away to the sea, there to be rebuilt by other
agents into sedimentary rocks. The ancient strata of which the
continents are largely made are composed chiefly of material thus
worn from still more ancient lands--lands with their hills and
valleys like those of to-day--and carried by their rivers to the
ocean. In all geological times, as at the present, the work of
streams has been to destroy the lands, and in so doing to furnish
to the ocean the materials from which the lands of future ages
were to be made. Before we consider how the waste of the land
brought in by streams is rebuilt upon the ocean floor, we must
proceed to study the work of two agents, glacier ice and the wind,
which cooperate with rivers in the denudation of the land.
CHAPTER V
THE WORK OF GLACIERS
THE DRIFT. The surface of northeastern North America, as far south
as the Ohio and Missouri rivers, is generally covered by the
drift,--a formation which is quite unlike any which we have so far
studied. A section of it, such as that illustrated in Figure 87,
shows that for the most part it is unstratified, consisting of
clay, sand, pebbles, and even large bowlders, all mingled pell-
mell together. The agent which laid the drift is one which can
carry a load of material of all sizes, from the largest bowlder to
the finest clay, and deposit it without sorting.
The stones of the drift are of many kinds. The region from which
it was gathered may well have been large in order to supply these
many different varieties of rocks. Pebbles and bowlders have been
left far from their original homes, as may be seen in southern
Iowa, where the drift contains nuggets of copper brought from the
region about Lake Superior. The agent which laid the drift is one
able to gather its load over a large area and carry it a long way.
The pebbles of the drift are unlike those rounded by running water
or by waves. They are marked with scratches. Some are angular,
many have had their edges blunted, while others have been ground
flat and smooth on one or more sides, like gems which have been
faceted by being held firmly against the lapidary's wheel. In many
places the upper surface of the country rock beneath the drift has
been swept clean of residual clays and other waste. All rock
rotten has been planed away, and the ledges of sound rock to which
the surface has been cut down have been rubbed smooth and
scratched with long, straight, parallel lines. The agent which
laid the drift can hold sand and pebbles firmly in its grasp and
can grind them against the rock beneath, thus planing it down and
scoring it, while faceting the pebbles also.
Neither water nor wind can do these things. Indeed, nothing like
the drift is being formed by any process now at work anywhere in
the eastern United States. To find the agent which has laid this
extensive formation we must go to a region of different climatic
conditions.
THE INLAND ICE OF GREENLAND. Greenland is about fifteen hundred
miles long and nearly seven hundred miles in greatest width. With
the exception of a narrow fringe of mountainous coast land, it is
completely buried beneath a sheet of ice, in shape like a vast
white shield, whose convex surface rises to a height of nine
thousand feet above the sea. The few explorers who have crossed
the ice cap found it a trackless desert destitute of all life save
such lowly forms as the microscopic plant which produces the so-
called "red snow." On the smooth plain of the interior no rock
waste relieves the snow's dazzling whiteness; no streams of
running water are seen; the silence is broken only by howling
storm winds and the rustle of the surface snow which they drive
before them. Sounding with long poles, explorers find that below
the powdery snow of the latest snowfall lie successive layers of
earlier snows, which grow more and more compact downward, and at
last have altered to impenetrable ice. The ice cap formed by the
accumulated snows of uncounted centuries may well be more than a
mile in depth. Ice thus formed by the compacting of snow is
distinguished when in motion as GLACIER ICE.
The inland ice of Greenland moves. It flows with imperceptible
slowness under its own weight, like, a mass of some viscous or
plastic substance, such as pitch or molasses candy, in all
directions outward toward the sea. Near the edge it has so thinned
that mountain peaks are laid bare, these islands in the sea of ice
being known as NUNATAKS. Down the valleys of the coastal belt it
drains in separate streams of ice, or GLACIERS. The largest of
these reach the sea at the head of inlets, and are therefore
called TIDE GLACIERS. Their fronts stand so deep in sea water that
there is visible seldom more than three hundred feet of the wall
of ice, which in many glaciers must be two thousand and more feet
high. From the sea walls of tide glaciers great fragments break
off and float away as icebergs. Thus snows which fell in the
interior of this northern land, perhaps many thousands of years
ago, are carried in the form of icebergs to melt at last in the
North Atlantic.
Greenland, then, is being modeled over the vast extent of its
interior not by streams of running water, as are regions in warm
and humid climates, nor by currents of air, as are deserts to a
large extent, but by a sheet of flowing ice. What the ice sheet is
doing in the interior we may infer from a study of the separate
glaciers into which it breaks at its edge.
THE SMALLER GREENLAND GLACIERS. Many of the smaller glaciers of
Greenland do not reach the sea, but deploy on plains of sand and
gravel. The edges of these ice tongues are often as abrupt as if
sliced away with a knife (Fig. 92), and their structure is thus
readily seen. They are stratified, their layers representing in
part the successive snowfalls of the interior of the country. The
upper layers are commonly white and free from stones; but the
lower layers, to the height of a hundred feet or more, are dark
with debris which is being slowly carried on. So thickly studded
with stones is the base of the ice that it is sometimes difficult
to distinguish it from the rock waste which has been slowly
dragged beneath the glacier or left about its edges. The waste
beneath and about the glacier is unsorted. The stones are of many
kinds, and numbers of them have been ground to flat faces. Where
the front of the ice has retreated the rock surface is seen to be
planed and scored in places by the stones frozen fast in the sole
of the glacier.
We have now found in glacier ice an agent able to produce the
drift of North America. The ice sheet of Greenland is now doing
what we have seen was done in the recent past in our own land. It
is carrying for long distances rocks of many kinds gathered, we
may infer, over a large extent of country. It is laying down its
load without assortment in unstratified deposits. It grinds down
and scores the rock over which it moves, and in the process many
of the pebbles of its load are themselves also ground smooth and
scratched. Since this work can be done by no other agent, we must
conclude that the northeastern part of our own continent was
covered in the recent past by glacier ice, as Greenland is to-day.
VALLEY GLACIERS
The work of glacier ice can be most conveniently studied in the
separate ice streams which creep down mountain valleys in many
regions such as Alaska, the western mountains of the United States
and Canada, the Himalayas, and the Alps. As the glaciers of the
Alps have been studied longer and more thoroughly than any others,
we shall describe them in some detail as examples of valley
glaciers in all parts of the world.
CONDITIONS OF GLACIER FORMATION. The condition of the great
accumulation of snow to which glaciers are due--that more or less
of each winter's snow should be left over unmelted and
unevaporated to the next--is fully met in the Alps. There is
abundant moisture brought by the winds from neighboring seas. The
currents of moist air driven up the mountain slopes are cooled by
their own expansion as they rise, and the moisture which they
contain is condensed at a temperature at or below 32 degrees F.,
and therefore is precipitated in the form of snow. The summers are
cool and their heat does not suffice to completely melt the heavy
snow of the preceding winter. On the Alps the SNOW LINE--the lower
limit of permanent snow--is drawn at about eight thousand five
hundred feet above sea level. Above the snow line on the slopes
and crests, where these are not too steep, the snow lies the year
round and gathers in valley heads to a depth of hundreds of feet.
This is but a small fraction of the thickness to which snow would
be piled on the Alps were it not constantly being drained away.
Below the snow fields which mantle the heights the mountain
valleys are occupied by glaciers which extend as much as a
vertical mile below the snow line. The presence in the midst of
forests and meadows and cultivated fields of these tongues of ice,
ever melting and yet from year to year losing none of their bulk,
proves that their loss is made good in the only possible way. They
are fed by snow fields above, whose surplus of snow they drain
away in the form of ice. The presence of glaciers below the snow
line is a clear proof that, rigid and motionless as they appear,
glaciers really are in constant motion down valley.
THE NEVE FIELD. The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow. Great
peaks tower above it, and snowy slopes rise on either side on the
flanks of mountain spurs. From these heights fierce winds drift
the snows into the amphitheater, and avalanches pour in their
torrents of snow and waste. The snow of the amphitheater is like
that of drifts in late winter after many successive thaws and
freezings. It is made of hard grains and pellets and is called
NEVE. Beneath the surface of the neve field and at its outlet the
granular neve has been compacted to a mass of porous crystalline
ice. Snow has been changed to neve, and neve to glacial ice, both
by pressure, which drives the air from the interspaces of the
snowflakes, and also by successive meltings and freezings, much as
a snowball is packed in the warm hand and becomes frozen to a ball
of ice.
THE BERGSCHRUND. The neve is in slow motion. It breaks itself
loose from the thinner snows about it, too shallow to share its
motion, and from the rock rim which surrounds it, forming a deep
fissure called the bergschrund, sometimes a score and more feet
wide.
SIZE OF GLACIERS. The ice streams of the Alps vary in size
according to the amount of precipitation and the area of the neve
fields which they drain. The largest of Alpine glaciers, the
Aletsch, is nearly ten miles long and has an average width of
about a mile. The thickness of some of the glaciers of the Alps is
as much as a thousand feet. Giant glaciers more than twice the
length of the longest in the Alps occur on the south slope of the
Himalaya Mountains, which receive frequent precipitations of snow
from moist winds from the Indian Ocean. The best known of the many
immense glaciers of Alaska, the Muir, has an area of about eight
hundred square miles (Fig. 95).
GLACIER MOTION. The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the
enormous pressure of their weight and because of less marginal
resistance, move faster than small ones. The Muir advances at the
rate of seven feet a day, and some of the larger tide glaciers of
Greenland are reported to move at the exceptional rate of fifty
feet and more in the same time. Glaciers move faster by day than
by night, and in summer than in winter. Other laws of glacier
motion may be discovered by a study of Figures 96 and 97. It is
important to remember that glaciers do not slide bodily over their
beds, but urged by gravity move slowly down valley in somewhat the
same way as would a stream of thick mud. Although small pieces of
ice are brittle, the large mass of granular ice which composes a
glacier acts as a viscous substance.
CREVASSES. Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions
under which the ice cracks and opens in great fissures called
crevasses. At an abrupt descent in the bed the ice is shattered
into great fragments, which unite again below the icefall.
Crevasses are opened on lines at right angles to the direction of
the tension. TRANSVERSE CREVASSES are due to a convexity in the
bed which stretches the ice lengthwise (Fig. 99). MARGINAL
CREVASSES are directed upstream and inwards; RADIAL CREVASSES are
found where the ice stream deploys from some narrow valley and
spreads upon some more open space. What is the direction of the
tension which causes each and to what is it due?
LATERAL AND MEDIAL MORAINES. The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center
are called the medial moraines. The one on either margin is a
lateral moraine, and is clearly formed of waste which has fallen
on the edge of the ice from the valley slopes. A medial moraine
cannot be formed in this way, since no rock fragments can fall so
far out from the sides. But following it up the glacial stream,
one finds that a medial moraine takes its beginning at the
junction of the glacier and some tributary and is formed by the
union of their two adjacent lateral moraines. Each branch thus
adds a medial moraine, and by counting the number of medial
moraines of a trunk stream one may learn of how many branches it
is composed.
Surface moraines appear in the lower course of the glacier as
ridges, which may reach the exceptional height of one hundred
feet. The bulk of such a ridge is ice. It has been protected from
the sun by the veneer of moraine stuff; while the glacier surface
on either side has melted down at least the distance of the height
of the ridge. In summer the lowering of the glacial surface by
melting goes on rapidly. In Swiss glaciers it has been estimated
that the average lowering of the surface by melting and
evaporation amounts to ten feet a year. As a moraine ridge grows
higher and more steep by the lowering of the surface of the
surrounding ice, the stones of its cover tend to slip down its
sides. Thus moraines broaden, until near the terminus of a glacier
they may coalesce in a wide field of stony waste.
ENGLACIAL DRIFT. This name is applied to whatever debris is
carried within the glacier. It consists of rock waste fallen on
the neve and there buried by accumulations of snow, and of that
engulfed in the glacier where crevasses have opened beneath a
surface moraine. As the surface of the glacier is lowered by
melting, more or less englacial drift is brought again to open
air, and near the terminus it may help to bury the ice from view
beneath a sheet of debris.
THE GROUND MORAINE. The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been
torn and worn from the glacier's bed and banks. While the stones
of the surface moraines remain as angular as when they lodged on
the ice, many of those of the ground moraine have been blunted on
the edges and faceted and scratched by being ground against one
another and the rocky bed.
In glaciers such as those of Greenland, whose basal layers are
well loaded with drift and whose surface layers are nearly clean,
different layers have different rates of motion, according to the
amount of drift with which they are clogged. One layer glides over
another, and the stones inset in each are ground and smoothed and
scratched. Usually the sides of glaciated pebbles are more worn
than the ends, and the scratches upon them run with the longer
axis of the stone. Why?
THE TERMINAL MORAINE. As a glacier is in constant motion, it
brings to its end all of its load except such parts of the ground
moraine as may find permanent lodgment beneath the ice. Where the
glacier front remains for some time at one place, there is formed
an accumulation of drift known as the terminal moraine. In valley
glaciers it is shaped by the ice front to a crescent whose convex
side is downstream. Some of the pebbles of the terminal moraine
are angular, and some are faceted and scored, the latter having
come by the hard road of the ground moraine. The material of the
dump is for the most part unsorted, though the water of the
melting ice may find opportunity to leave patches of stratified
sands and gravels in the midst of the unstratified mass of drift,
and the finer material is in places washed away.
GLACIER DRAINAGE. The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as VALLEY DRIFT.
The sand brought out by streams from beneath a glacier differs
from river sand in that it consists of freshly broken angular
grains. Why?
The stream derives its water chiefly from the surface melting of
the glacier. As the ice is touched by the rays of the morning sun
in summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells
cut by their whirling waters where some crevasse has begun to open
across their path. These wells lead into chambers and tunnels by
which sooner or later their waters find way to the rock floor of
the valley and there unite in a subglacial stream.
THE LOWER LIMIT OF GLACIERS. The glaciers of a region do not by
any means end at a uniform height above sea level. Each terminates
where its supply is balanced by melting. Those therefore which are
fed by the largest and deepest neves and those also which are best
protected from the sun by a northward exposure or by the depth of
their inclosing valleys flow to lower levels than those whose
supply is less and whose exposure to the sun is greater.
A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years
causes them to wither and melt back. The variation in glaciers is
now carefully observed in many parts of the world. The Muir
glacier has retreated two miles in twenty years. The glaciers of
the Swiss Alps are now for the most part melting back, although a
well-known glacier of the eastern Alps, the Vernagt, advanced five
hundred feet in the year 1900, and was then plowing up its
terminal moraine.
How soon would you expect a glacier to advance after its neve
fields have been swollen with unusually heavy snows, as compared
with the time needed for the flood of a large river to reach its
mouth after heavy rains upon its headwaters?
On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig.
108). These "glacier tables" commonly slope more or less strongly
to the south, and thus may be used to indicate roughly the points
of the compass. Can you explain their formation and the direction
of their slope? On the other hand, a small and thin stone, or a
patch of dust, lying on the ice, tends to sink a few inches into
it. Why?
In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?
Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove
this fact?
What effect would you expect the laws of glacier motion to have on
the slant of the sides of transverse crevasses?
A trunk glacier has four medial moraines. Of how many tributaries
is it composed? Illustrate by diagram.
State all the evidences which you have found that glaciers move.
If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?
PIEDMONT GLACIERS
THE MALASPINA GLACIER. Piedmont (foot of the mountain) glaciers
are, as the name implies, ice fields formed at the foot of
mountains by the confluence of valley glaciers. The Malaspina
glacier of Alaska, the typical glacier of this kind, is seventy
miles wide and stretches for thirty miles from the foot of the
Mount Saint Elias range to the shore of the Pacific Ocean. The
valley glaciers which unite and spread to form this lake of ice
lie above the snow line and their moraines are concealed beneath
neve. The central area of the Malaspina is also free from debris;
but on the outer edge large quantities of englacial drift are
exposed by surface melting and form a belt of morainic waste a few
feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand
feet in depth. The glacier here is practically stagnant, and lakes
a few hundred yards across, which could not exist were the ice in
motion and broken with crevasses, gather on their beds sorted
waste from the moraine. The streams which drain the glacier have
cut their courses in englacial and subglacial tunnels; none flow
for any distance on the surface. The largest, the Yahtse River,
issues from a high archway in the ice,--a muddy torrent one
hundred feet wide and twenty feet deep, loaded with sand and
stones which it deposits in a broad outwash plain (Fig. 110).
Where the ice has retreated from the sea there is left a hummocky
drift sheet with hollows filled with lakelets. These deposits help
to explain similar hummocky regions of drift and similar plains of
coarse, water-laid material often found in the drift-covered area
of the northeastern United States.
THE GEOLOGICAL WORK OF GLACIER ICE
The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its
channel occupies but a small portion of the valley. The glacier is
slow and big; its rate of motion may be less than a millionth of
that of running water over the same declivity, and its bulk is
proportionately large and fills the valley to great depth.
Moreover, glacier ice is a solid body plastic under slowly applied
stresses, while the water of rivers is a nimble fluid.
TRANSPORTATION. Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand;
while streams push and roll much of their load along their beds,
and their power of transporting waste depends solely upon their
velocity. The amount of the surface load of glaciers is limited
only by the amount of waste received from the mountain slopes
above them. The moving floor of ice stretched high across a valley
sweeps along as lateral moraines much of the waste which a
mountain stream would let accumulate in talus and alluvial cones.
While a valley glacier carries much of its load on top, an ice
sheet, such as that of Greenland, is free from surface debris,
except where moraines trail away from some nunatak. If at its edge
it breaks into separate glaciers which drain down mountain
valleys, these tongues of ice will carry the selvages of waste
common to valley glaciers. Both ice sheets and valley glaciers
drag on large quantities of rock waste in their ground moraines.
Stones transported by glaciers are sometimes called erratics. Such
are the bowlders of the drift of our northern states. Erratics may
be set down in an insecure position on the melting of the ice.
DEPOSIT. Little need be added here to what has already been said
of ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged
beneath the glacier as ground moraine is often an extremely dense,
stony clay, having been compacted under the pressure of the
overriding ice.
EROSION. A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.
The rock bed over which a glacier has moved is seen in places to
have been abraded, or ground away, to smooth surfaces which are
marked by long, straight, parallel scorings aligned with the line
of movement of the ice and varying in size from hair lines and
coarse scratches to exceptional furrows several feet deep. Clearly
this work has been accomplished by means of the sharp sand, the
pebbles, and the larger stones with which the base of the glacier
is inset, and which it holds in a firm grasp as running water
cannot. Hard and fine-grained rocks, such as granite and
quartzite, are often not only ground down to a smooth surface but
are also highly polished by means of fine rock flour worn from the
glacier bed.
In other places the bed of the glacier is rough and torn. The
rocks have been disrupted and their fragments have been carried
away,--a process known as PLUCKING. Moving under immense pressure
the ice shatters the rock, breaks off projections, presses into
crevices and wedges the rocks apart, dislodges the blocks into
which the rock is divided by joints and bedding planes, and
freezing fast to the fragments drags them on. In this work the
freezing and thawing of subglacial waters in any cracks and
crevices of the rock no doubt play an important part. Plucking
occurs especially where the bed rock is weak because of close
jointing. The product of plucking is bowlders, while the product
of abrasion is fine rock flour and sand.
Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?
ROCHES MOUTONNEES AND ROUNDED HILLS. The prominences left between
the hollows due to plucking are commonly ground down and rounded
on the stoss side,--the side from which the ice advances,--and
sometimes on the opposite, the lee side, as well. In this way the
bed rock often comes to have a billowy surface known as roches
moutonnees (sheep rocks). Hills overridden by an ice sheet often
have similarly rounded contours on the stoss side, while on the
lee side they may be craggy, either because of plucking or because
here they have been less worn from their initial profile.
THE DIRECTION OF GLACIER MOVEMENT. The direction of the flow of
vanished glaciers and ice sheets is recorded both in the
differences just mentioned in the profiles of overridden hills and
also in the minute details of the glacier trail.
Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities,
on the other hand, have their edges worn on the lee side and left
sharp upon the stoss.
Surfaces worn and torn in the ways which we have mentioned are
said to be glaciated. But it must not be supposed that a glacier
everywhere glaciates its bed. Although in places it acts as a rasp
or as a pick, in others, and especially where its pressure is
least, as near the terminus, it moves over its bed in the manner
of a sled. Instances are known where glaciers have advanced over
deposits of sand and gravel without disturbing them to any notable
degree. Like a river, a glacier does not everywhere erode. In
places it leaves its bed undisturbed and in places aggrades it by
deposits of the ground moraine.
CIRQUES. Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet
waters reflect the sheltering ramparts of rugged rock which tower
far above them. Such mountain hollows are termed CIRQUES. As a
powerful spring wears back a recess in the valley side where it
discharges, so the fountain head of a glacier gradually wears back
a cirque. In its slow movement the neve field broadly scours its
bed to a flat or basined floor. Meanwhile the sides of the valley
head are steepened and driven back to precipitous walls. For in
winter the crevasse of the bergschrund which surrounds the neve
field is filled with snow and the neve is frozen fast to the rocky
sides of the valley. In early summer the neve tears itself free,
dislodging and removing any loosened blocks, and the open fissure
of the bergschrund allows frost and other agencies of weathering
to attack the unprotected rock. As cirques are thus formed and
enlarged the peaks beneath which they lie are sharpened, and the
mountain crests are scalloped and cut back from either side to
knife-edged ridges.
In the western mountains of the United States many cirques, now
empty of neve and glacier ice, and known locally as "basins,"
testify to the fact that in recent times the snow line stood
beneath the levels of their floors, and thus far below its present
altitude.
GLACIER TROUGHS. The channel worn to accommodate the big and
clumsy glacier differs markedly from the river valley cut as with
a saw by the narrow and flexible stream and widened by the weather
and the wash of rains. The valley glacier may easily be from one
thousand to three thousand feet deep and from one to three miles
wide. Such a ponderous bulk of slowly moving ice does not readily
adapt itself to sharp turns and a narrow bed. By scouring and
plucking all resisting edges it develops a fitting channel with a
wide, flat floor, and steep, smooth sides, above which are seen
the weathered slopes of stream-worn mountain valleys. Since the
trunk glacier requires a deeper channel than do its branches, the
bed of a branch glacier enters the main trough at some distance
above the floor of the latter, although the surface of the two ice
streams may be accordant. Glacier troughs can be studied best
where large glaciers have recently melted completely away, as is
the case in many valleys of the mountains of the western United
States and of central and northern Europe (Fig. 114). The typical
glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little
broken by projecting spurs and lateral ravines. It is as if a V-
valley cut by a river had afterwards been gouged deeper with a
gigantic chisel, widening the floor to the width of the chisel
blade, cutting back the spurs, and smoothing and steepening the
sides. A river valley could only be as wide-floored as this after
it had long been worn down to grade.
The floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a
thousand feet in height, over which the stream that now drains the
valley tumbles in waterfalls. Reaches between the steps are often
basined. Lakelets may occupy hollows excavated in solid rock, and
other lakes may be held behind terminal moraines left as dams
across the valley at pauses in the retreat of the glacier.
FJORDS are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow
bay with steep rock walls and basined floor. Fjords are found only
in regions which have suffered glaciation, such as Norway and
Alaska.
HANGING VALLEYS. These are lateral valleys which open on their
main valley some distance above its floor. They are conspicuous
features of glacier troughs from which the ice has vanished; for
the trunk glacier in widening and deepening its channel cut its
bed below the bottoms of the lateral valleys.
Since the mouths of hanging valleys are suspended on the walls of
the glacier trough, their streams are compelled to plunge down its
steep, high sides in waterfalls. Some of the loftiest and most
beautiful waterfalls of the world leap from hanging valleys,--
among them the celebrated Staubbach of the Lauterbrunnen valley of
Switzerland, and those of the fjords of Norway and Alaska.
Hanging valleys are found also in river gorges where the smaller
tributaries have not been able to keep pace with a strong master
stream in cutting down their beds. In this case, however, they are
a mark of extreme youth; for, as the trunk stream approaches grade
and its velocity and power to erode its bed decrease, the side
streams soon cut back their falls and wear their beds at their
mouths to a common level with that of the main river. The Grand
Canyon of the Colorado must be reckoned a young valley. At its
base it narrows to scarcely more than the width of the river, and
yet its tributaries, except the very smallest, enter it at a
common level.
Why could not a wide-floored valley, such as a glacier trough,
with hanging valleys opening upon it, be produced in the normal
development of a river valley?
THE TROUGHS OF YOUNG AND OF MATURE GLACIERS. The features of a
glacier trough depend much on the length of time the preexisting
valley was occupied with ice. During the infancy of a glacier, we
may believe, the spurs of the valley which it fills are but little
blunted and its bed is but little broken by steps. In youth the
glacier develops icefalls, as a river in youth develops
waterfalls, and its bed becomes terraced with great stairs. The
mature glacier, like the mature river, has effaced its falls and
smoothed its bed to grade. It has also worn back the projecting
spurs of its valley, making itself a wide channel with smooth
sides. The bed of a mature glacier may form a long basin, since it
abrades most in its upper and middle course, where its weight and
motion are the greatest. Near the terminus, where weight and
motion are the least, it erodes least, and may instead deposit a
sheet of ground moraine, much as a river builds a flood plain in
the same part of its course as it approaches maturity. The bed of
a mature glacier thus tends to take the form of a long, relatively
narrow basin, across whose lower end may be stretched the dam of
the terminal moraine. On the disappearance of the ice the basin is
rilled with a long, narrow lake, such as Lake Chelan in Washington
and many of the lakes in the Highlands of Scotland.
Piedmont glaciers apparently erode but little. Beneath their lake-
like expanse of sluggish or stagnant ice a broad sheet of ground
moraine is probably being deposited.
Cirques and glaciated valleys rapidly lose their characteristic
forms after the ice has withdrawn. The weather destroys all
smoothed, polished, and scored surfaces which are not protected
beneath glacial deposits. The oversteepened sides of the trough
are graded by landslips, by talus slopes, and by alluvial cones.
Morainic heaps of drift are dissected and carried away. Hanging
valleys and the irregular bed of the trough are both worn down to
grade by the streams which now occupy them. The length of time
since the retreat of the ice from a mountain valley may thus be
estimated by the degree to which the destruction of the
characteristic features of the glacier trough has been carried.
In Figure 104 what characteristics of a glacier trough do you
notice? What inference do you draw as to the former thickness of
the glacier?
Name all the evidences you would expect to find to prove the fact
that in the recent geological past the valleys of the Alps
contained far larger glaciers than at present, and that on the
north of the Alps the ice streams united in a piedmont glacier
which extended across the plains of Switzerland to the sides of
the Jura Mountains.
THE RELATIVE IMPORTANCE OF GLACIERS AND OF RIVERS. Powerful as
glaciers are, and marked as are the land forms which they produce,
it is easy to exaggerate their geological importance as compared
with rivers. Under present climatic conditions they are confined
to lofty mountains or polar lands. Polar ice sheets are permanent
only so long as the lands remain on which they rest. Mountain
glaciers can stay only the brief time during which the ranges
continue young and high. As lofty mountains, such as the Selkirks
and the Alps, are lowered by frost and glacier ice, the snowfall
will decrease, the line of permanent snow will rise, and as the
mountain hollows in which snow may gather are worn beneath the
snow line, the glaciers must disappear. Under present climatic
conditions the work of glaciers is therefore both local and of
short duration.
Even the glacial epoch, during which vast ice sheets deposited
drift over northeastern North America, must have been brief as
well as recent, for many lofty mountains, such as the Rockies and
the Alps, still bear the marks of great glaciers which then filled
their valleys. Had the glacial epoch been long, as the earth
counts time, these mountains would have been worn low by ice; had
the epoch been remote, the marks of glaciation would already have
been largely destroyed by other agencies.
On the other hand, rivers are well-nigh universally at work over
the land surfaces of the globe, and ever since the dry land
appeared they have been constantly engaged in leveling the
continents and in delivering to the seas the waste which there is
built into the stratified rocks.
ICEBERGS. Tide glaciers, such as those of Greenland and Alaska,
are able to excavate their beds to a considerable distance below
sea level. From their fronts the buoyancy of sea water raises and
breaks away great masses of ice which float out to sea as
icebergs. Only about one seventh of a mass of glacier ice floats
above the surface, and a berg three hundred feet high may be
estimated to have been detached from a glacier not less than two
thousand feet thick where it met the sea.
Icebergs transport on their long journeys whatever drift they may
have carried when part of the glacier, and scatter it, as they
melt, over the ocean floor. In this way pebbles torn by the inland
ice from the rocks of the interior of Greenland and glaciated
during their carriage in the ground moraine are dropped at last
among the oozes of the bottom of the North Atlantic.
CHAPTER VI
THE WORK OF THE WIND
We are now to study the geological work of the currents of the
atmosphere, and to learn how they erode, and transport and deposit
waste as they sweep over the land. Illustrations of the wind's
work are at hand in dry weather on any windy day.
Clouds of dust are raised from the street and driven along by the
gale. Here the roadway is swept bare; and there, in sheltered
places, the dust settles in little windrows. The erosive power of
waste-laden currents of air is suggested as the sharp grains of
flying sand sting one's face or clatter against the window. In the
country one sometimes sees the dust whirled in clouds from dry,
plowed fields in spring and left in the lee of fences in small
drifts resembling in form those of snow in winter.
THE ESSENTIAL CONDITIONS for the wind's conspicuous work are
illustrated in these simple examples; they are aridity and the
absence of vegetation. In humid climates these conditions are only
rarely and locally met; for the most part a thick growth of
vegetation protects the moist soil from the wind with a cover of
leaves and stems and a mattress of interlacing roots. But in arid
regions either vegetation is wholly lacking, or scant growths are
found huddled in detached clumps, leaving interspaces of
unprotected ground (Fig. 119). Here, too, the mantle of waste,
which is formed chiefly under the action of temperature changes,
remains dry and loose for long periods. Little or no moisture is
present to cause its particles to cohere, and they are therefore
readily lifted and drifted by the wind.
TRANSPORTATION BY THE WIND
In the desert the finer waste is continually swept to and fro by
the ever-shifting wind. Even in quiet weather the air heated by
contact with the hot sands rises in whirls, and the dust is lifted
in stately columns, sometimes as much as one thousand feet in
height, which march slowly across the plain. In storms the sand is
driven along the ground in a continuous sheet, while the air is
tilled with dust. Explorers tell of sand storms in the deserts of
central Asia and Africa, in which the air grows murky and
suffocating. Even at midday it may become dark as night, and
nothing can be heard except the roar of the blast and the whir of
myriads of grains of sand as they fly past the ear.
Sand storms are by no means uncommon in the arid regions of the
western United States. In a recent year, six were reported from
Yuma, Arizona. Trains on transcontinental railways are
occasionally blockaded by drifting sand, and the dust sifts into
closed passenger coaches, covering the seats and floors. After
such a storm thirteen car loads of sand were removed from the
platform of a station on a western railway.
DUST FALLS. Dust launched by upward-whirling winds on the swift
currents of the upper air is often blown for hundreds of miles
beyond the arid region from which it was taken. Dust falls from
western storms are not unknown even as far east as the Great
Lakes. In 1896 a "black snow" fell in Chicago, and in another dust
storm in the same decade the amount of dust carried in the air
over Rock Island, Ill., was estimated at more than one thousand
tons to the cubic mile.
In March, 1901, a cyclonic storm carried vast quantities of dust
from the Sahara northward across the Mediterranean to fall over
southern and central Europe. On March 8th dust storms raged in
southern Algeria; two days later the dust fell in Italy; and on
the 11th it had reached central Germany and Denmark. It is
estimated that in these few days one million eight hundred
thousand tons of waste were carried from northern Africa and
deposited on European soil.
We may see from these examples the importance of the wind as an
agent of transportation, and how vast in the aggregate are the
loads which it carries. There are striking differences between air
and water as carriers of waste. Rivers flow in fixed and narrow
channels to definite goals. The channelless streams of the air
sweep across broad areas, and, shifting about continually, carry
their loads back and forth, now in one direction and now in
another.
WIND DEPOSITS
The mantle of waste of deserts is rapidly sorted by the wind. The
coarser rubbish, too heavy to be lifted into the air, is left to
strew wide tracts with residual gravels (Fig. 120). The sand
derived from the disintegration of desert rocks gathers in vast
fields. About one eighth of the surface of the Sahara is said to
be thus covered with drifting sand. In desert mountains, as those
of Sinai, it lies like fields of snow in the high valleys below
the sharp peaks. On more level tracts it accumulates in seas of
sand, sometimes, as in the deserts of Arabia, two hundred and more
feet deep.
DUNES. The sand thus accumulated by the wind is heaped in wavelike
hills called dunes. In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is laid in
longitudinal dunes, i.e. in stripes running parallel with the
direction of the wind; but commonly dunes lie, like ripple marks,
transverse to the wind current. On the windward side they show a
long, gentle slope, up which grains of sand can readily be moved;
while to the lee their slope is frequently as great as the angle
of repose (Fig. 122). Dunes whose sands are not fixed by
vegetation travel slowly with the wind; for their material is ever
shifted forward as the grains are driven up the windward slope
and, falling over the crest, are deposited in slanting layers in
the quiet of the lee.
Like river deposits, wind-blown sands are stratified, since they
are laid by currents of air varying in intensity, and therefore
in transporting power, which carry now finer and now coarser
materials and lay them down where their velocity is checked (Fig.
123). Since the wind varies in direction, the strata dip in
various directions. They also dip at various angles, according to
the inclination of the surface on which they were laid.
Dunes occur not only in arid regions, but also wherever loose sand
lies unprotected by vegetation from the wind. From the beaches of
sea and lake shores the wind drives inland the surface sand left
dry between tides and after storms, piling it in dunes which may
invade forests and fields and bury villages beneath their slowly
advancing waves. On flood plains during summer droughts river
deposits are often worked over by the wind; the sand is heaped in
hummocks and much of the fine silt is caught and held by the
forests and grassy fields of the bordering hills.
The sand of shore dunes differs little in composition and the
shape of its grains from that of the beach from which it was
derived. But in deserts, by the long wear of grain on grain as
they are blown hither and thither by the wind, all soft minerals
are ground to powder and the sand comes to consist almost wholly
of smooth round grams of hard quartz.
Some marine sandstones, such as the St. Peter sandstone of the
upper Mississippi valley, are composed so entirely of polished
spherules of quartz that it has been believed by some that their
grains were long blown about in ancient deserts before they were
deposited in the sea.
DUST DEPOSITS. As desert sands are composed almost wholly of
quartz, we may ask what has become of the softer minerals of which
the rocks whose disintegration has supplied the sand were in part,
and often in large part, composed. The softer minerals have been
ground to powder, and little by little the quartz sand also is
worn by attrition to fine dust. Yet dust deposits are scant and
few in great deserts such as the Sahara. The finer waste is blown
beyond its limits and laid in adjacent oceans, where it adds to
the muds and oozes of their floors, and on bordering steppes and
forest lands, where it is bound fast by vegetation and slowly
accumulates in deposits of unstratified loose yellow earth. The
fine waste of the Sahara has been identified in dredgings from the
bottom of the Atlantic Ocean, taken hundreds of miles from the
coast of Africa.
LOESS. In northern China an area as large as France is deeply
covered with a yellow pulverulent earth called loess (German,
loose), which many consider a dust deposit blown from the great
Mongolian desert lying to the west. Loess mantles the recently
uplifted mountains to the height of eight thousand feet and
descends on the plains nearly to sea level. Its texture and lack
of stratification give it a vertical cleavage; hence it stands in
steep cliffs on the sides of the deep and narrow trenches which
have been cut in it by streams.
On loess hillsides in China are thousands of villages whose
eavelike dwellings have been excavated in this soft, yet firm, dry
loam. While dust falls are common at the present time in this
region, the loess is now being rapidly denuded by streams, and its
yellow silt gives name to the muddy Hwang-ho (Yellow River), and
to the Yellow Sea, whose waters it discolors for scores of miles
from shore.
Wind deposits both of dust and of sand may be expected to contain
the remains of land shells, bits of wood, and bones of land
animals, testifying to the fact that they were accumulated in open
air and not in the sea or in bodies of fresh water.
WIND EROSION
Sand-laden currents of air abrade and smooth and polish exposed
rock surfaces, acting in much the same way as does the jet of
steam fed with sharp sand, which is used in the manufacture of
ground glass. Indeed, in a single storm at Cape Cod a plate glass
of a lighthouse was so ground by flying sand that its transparency
was destroyed and its removal made necessary.
Telegraph poles and wires whetted by wind-blown sands are
destroyed within a few years. In rocks of unequal resistance the
harder parts are left in relief, while the softer are etched away.
Thus in the pass of San Bernardino, Cal., through which strong
winds stream from the west, crystals of garnet are left projecting
on delicate rock fingers from the softer rock in which they were
imbedded.
Wind-carved pebbles are characteristically planed, the facets
meeting along a summit ridge or at a point like that of a pyramid.
We may suppose that these facets were ground by prevalent winds
from certain directions, or that from time to time the stone was
undermined and rolled over as the sand beneath it was blown away
on the windward side, thus exposing fresh surfaces to the driving
sand. Such wind-carved pebbles are sometimes found in ancient
rocks and may be accepted as evidence that the sands of which the
rocks are composed were blown about by the wind.
DEFLATION. In the denudation of an arid region, wind erosion is
comparatively ineffective as compared with deflation (Latin, de,
from; flare, to blow),--a term by which is meant the constant
removal of waste by the wind, leaving the rocks bare to the
continuous attack of the weather. In moist climates denudation is
continually impeded by the mantle of waste and its cover of
vegetation, and the land surface can be lowered no faster than the
waste is removed by running water. Deep residual soils come to
protect all regions of moderate slope, concealing from view the
rock structure, and the various forms of the land are due more to
the agencies of erosion and transportation than to differences in
the resistance of the underlying rocks.
But in arid regions the mantle is rapidly removed, even from well-
nigh level plains and plateaus, by the sweep of the wind and the
wash of occasional rains. The geological structure of these
regions of naked rock can be read as far as the eye can see, and
it is to this structure that the forms of the land are there
largely due. In a land mass of horizontal strata, for example, any
softer surface rocks wear down to some underlying, resistant
stratum, and this for a while forms the surface of a level plateau
(Fig. 129). The edges of the capping layer, together with those of
any softer layers beneath it, wear back in steep cliffs, dissected
by the valleys of wet-weather streams and often swept bare to the
base by the wind. As they are little protected by talus, which
commonly is removed about as fast as formed, these escarpments and
the walls of the valleys retreat indefinitely, exposing some hard
stratum beneath which forms the floor of a widening terrace.
The high plateaus of northern Arizona and southern Utah, north of
the Grand Canyon of the Colorado River, are composed of stratified
rocks more than ten thousand feet thick and of very gentle
inclination northward. From the broad plat form in which the
canyon has been cut rises a series of gigantic stairs, which are
often more than one thousand feet high and a score or more of
miles in breadth. The retreating escarpments, the cliffs of the
mesas and buttes which they have left behind as outliers, and the
walls of the ravines are carved into noble architectural forms--
into cathedrals, pyramids, amphitheaters, towers, arches, and
colonnades--by the processes of weathering aided by deflation. It
is thus by the help of the action of the wind that great plateaus
in arid regions are dissected and at last are smoothed away to
waterless plains, either composed of naked rock, or strewed with
residual gravels, or covered with drifting residual sand.
The specific gravity of air is 1/823 that of water. How does this
fact affect the weight of the material which each can carry at the
same velocity?
If the rainfall should lessen in your own state to from five to
ten inches a year, what changes would take place in the vegetation
of the country? in the soil? in the streams? in the erosion of
valleys? in the agencies chiefly at work in denuding the land?
In what way can a wind-carved pebble be distinguished from a
river-worn pebble? from a glaciated pebble?
CHAPTER VII
THE SEA AND ITS SHORES
We have already seen that the ocean is the goal at which the waste
of the land arrives. The mantle of rock waste, creeping down
slopes, is washed to the sea by streams, together with the
material which the streams have worn from their beds and that
dissolved by underground waters. In arid regions the winds sweep
waste either into bordering oceans or into more humid regions
where rivers take it up and carry it on to the sea. Glaciers
deliver the load of their moraines either directly to the sea or
leave it for streams to transport to the same goal. All deposits
made on the land, such as the flood plains of rivers, the silts of
lake beds, dune sands, and sheets of glacial drift, mark but
pauses in the process which is to bring all the materials of the
land now above sea level to rest upon the ocean bed.
But the sea is also at work along all its shores as an agent of
destruction, and we must first take up its work in erosion before
we consider how it transports and deposits the waste of the land.
SEA EROSION
THE SEA CLIFF AND THE ROCK BENCH. On many coasts the land fronts
the ocean in a line of cliffs. To the edge of the cliffs there
lead down valleys and ridges, carved by running water, which, if
extended, would meet the water surface some way out from shore.
Evidently they are now abruptly cut short at the present shore
line because the land has been cut back.
Along the foot of the cliff lies a gently shelving bench of rock,
more or less thickly veneered with sand and shingle. At low tide
its inner margin is laid bare, but at high tide it is covered
wholly, and the sea washes the base of the cliffs. A notch, of
which the SEA CLIFF and the ROCK BENCH are the two sides, has been
cut along the shore.
WAVES. The position of the rock bench, with its inner margin
slightly above low tide, shows that it has been cut by some agent
which acts like a horizontal saw set at about sea level. This
agent is clearly the surface agitation of the water; it is the
wind-raised wave.
As a wave comes up the shelving bench the crest topples forward
and the wave "breaks," striking a blow whose force is measured by
the momentum of all its tons of falling water. On the coast of
Scotland the force of the blows struck by the waves of the
heaviest storms has sometimes exceeded three tons to the square
foot. But even a calm sea constantly chafes the shore. It heaves
in gentle undulations known as the ground swell, the result of
storms perhaps a thousand miles distant, and breaks on the shore
in surf.
The blows of the waves are not struck with clear water only, else
they would have little effect on cliffs of solid rock. Storm waves
arm themselves with the sand and gravel, the cobbles, and even the
large bowlders which lie at the base of the cliff, and beat
against it with these hammers of stone.
Where a precipice descends sheer into deep water, waves swash up
and down the face of the rocks but cannot break and strike
effective blows. They therefore erode but little until the talus
fallen from the cliff is gradually built up beneath the sea to the
level at which the waves drag bottom upon it and break.
Compare the ways in which different agents abrade. The wind
lightly brushes sand and dust over exposed surfaces of rock.
Running water sweeps fragments of various sizes along its
channels, holding them with a loose hand. Glacial ice grinds the
stones of its ground moraine against the underlying rock with the
pressure of its enormous weight. The wave hurls fragments of rock
against the sea cliff, bruising and battering it by the blow. It
also rasps the bench as it drags sand and gravel to and fro upon
it.
WEATHERING OF SEA CLIFFS. The sea cliff furnishes the weapons for
its own destruction. They are broken from it not only by the wave
but also by the weather. Indeed the sea cliff weathers more
rapidly, as a rule, than do rock ledges inland. It is abundantly
wet with spray. Along its base the ground water of the neighboring
land finds its natural outlet in springs which under mine it.
Moreover, it is unprotected by any shield of talus. Fragments of
rock as they fall from its face are battered to pieces by the
waves and swept out to sea. The cliff is thus left exposed to the
attack of the weather, and its retreat would be comparatively
rapid for this reason alone.
Sea cliffs seldom overhang, but commonly, as in Figure 134, slope
seaward, showing that the upper portion has retreated at a more
rapid rate than has the base. Which do you infer is on the whole
the more destructive agent, weathering or the wave?
Draw a section of a sea cliff cut in well jointed rocks whose
joints dip toward the land. Draw a diagram of a sea cliff where
the joints dip toward the sea.
SEA CAVES. The wave does not merely batter the face of the cliff.
Like a skillful quarryman it inserts wedges in all natural
fissures, such as joints, and uses explosive forces. As a wave
flaps against a crevice it compresses the air within with the
sudden stroke; as it falls back the air as suddenly expands. On
lighthouses heavily barred doors have been burst outward by the
explosive force of the air within, as it was released from
pressure when a partial vacuum was formed by the refluence of the
wave. Where a crevice is filled with water the entire force of the
blow of the wave is transmitted by hydraulic pressure to the sides
of the fissure. Thus storm waves little by little pry and suck the
rock loose, and in this way, and by the blows which they strike
with the stones of the beach, they quarry out about a joint, or
wherever the rock may be weak, a recess known as a SEA CAVE,
provided that the rock above is coherent enough to form a roof.
Otherwise an open chasm results.
BLOWHOLES AND SEA ARCHES. As a sea cave is drilled back into the
rock, it may encounter a joint or crevice opened to the surface by
percolating water. The shock of the waves soon enlarges this to a
blowhole, which one may find on the breezy upland, perhaps a
hundred yards and more back from the cliff's edge. In quiet
weather the blowhole is a deep well; in storm it plays a fountain
as the waves drive through the long tunnel below and spout their
spray high in air in successive jets. As the roof of the cave thus
breaks down in the rear, there may remain in front for a while a
sea arch, similar to the natural bridges of land caverns.
STACKS AND WAVE-CUT ISLANDS. As the sea drives its tunnels and
open drifts into the cliff, it breaks through behind the
intervening portions and leaves them isolated as stacks, much as
monuments are detached from inland escarpments by the weather; and
as the sea cliff retreats, these remnant masses may be left behind
as rocky islets. Thus the rock bench is often set with stacks,
islets in all stages of destruction, and sunken reefs, all wrecks
of the land testifying to its retreat before the incessant attack
of the waves.
COVES. Where zones of soft or closely jointed rock outcrop along a
shore, or where minor water courses conic down to the sea and aid
in erosion, the shore is worn back in curved reentrants called
coves; while the more resistant rocks on either hand are left
projecting as headlands (Fig. 139). After coves are cut back a
short distance by the waves, the headlands come to protect them,
as with breakwaters, and prevent their indefinite retreat. The
shore takes a curve of equilibrium, along which the hard rock of
the exposed headland and the weak rock of the protected cove wear
back at an equal rate.
RATE OF RECESSION. The rate at which a shore recedes depends on
several factors. In soft or incoherent rocks exposed to violent
storms the retreat is so rapid as to be easily measured. The coast
of Yorkshire, England, whose cliffs are cut in glacial drift,
loses seven feet a year on the average, and since the Norman
conquest a strip a mile wide, with farmsteads and villages and
historic seaports, has been devoured by the sea. The sandy south
shore of Martha's Vineyard wears back three feet a year. But hard
rocks retreat so slowly that their recession has seldom been
measured by the records of history.
SHORE DRIFT
BOWLDER AND PEBBLE BEACHES. About as fast as formed the waste of
the sea cliff is swept both along the shore and out to sea. The
road of waste along shore is the BEACH. We may also define the
beach as the exposed edge of the sheet of sediment formed by the
carriage of land waste out to sea. At the foot of sea cliffs,
where the waves are pounding hardest, one commonly finds the rock
bench strewn on its inner margin with large stones, dislodged by
the waves and by the weather and some-what worn on their corners
and edges. From this BOWLDER BEACH the smaller fragments of waste
from the cliff and the fragments into which the bowlders are at
last broken drift on to more sheltered places and there accumulate
in a PEBBLE BEACH, made of pebbles well rounded by the wear which
they have suffered. Such beaches form a mill whose raw material is
constantly supplied by the cliff. The breakers of storms set it in
motion to a depth of several feet, grinding the pebbles together
with a clatter to be heard above the roar of the surf. In such a
rock crusher the life of a pebble is short. Where ships have
stranded on our Atlantic coast with cargoes of hard-burned brick
or of coal, a year of time and a drift of five miles along the
shore have proved enough to wear brick and coal to powder. At no
great distance from their source, therefore, pebble beaches give
place to beaches of sand, which occupy the more sheltered reaches
of the shore.
SAND BEACHES. The angular sand grains of various minerals into
which pebbles are broken by the waves are ground together under
the beating surf and rounded, and those of the softer minerals are
crushed to powder. The process, however, is a slow one, and if we
study these sand grains under a lens we may be surprised to see
that, though their corners and edges have been blunted, they are
yet far from the spherical form of the pebbles from which they
were derived. The grains are small, and in water they have lost
about half their weiglit in air; the blows which they strike one
another are therefore weak. Besides, each grain of sand of the wet
beach is protected by a cushion of water from the blows of its
neighbors.
The shape and size of these grains and the relative proportion of
grains of the softer minerals which still remain give a rough
measure of the distance in space and time which they have traveled
from their source. The sand of many beaches, derived from the
rocks of adjacent cliffs or brought in by torrential streams from
neighboring highlands, is dark with grains of a number of minerals
softer than quartz. The white sand of other beaches, as those of
the east coast of Florida, is almost wholly composed of quartz
grains; for in its long travel down the Atlantic coast the weaker
minerals have been worn to powder and the hardest alone survive.
How does the absence of cleavage in quartz affect the durability
of quartz sand?
HOW SHORE DRIFT MIGRATES. It is under the action of waves and
currents that shore drift migrates slowly along a coast. Where
waves strike a coast obliquely they drive the waste before them
little by little along the shore. Thus on a north-south coast,
where the predominant storms are from the northeast, there will be
a migration of shore drift southwards.
All shores are swept also by currents produced by winds and tides.
These are usually far too gentle to transport of themselves the
coarse materials of which beaches are made. But while the wave
stirs the grains of sand and gravel, and for a moment lifts them
from the bottom, the current carries them a step forward on their
way. The current cannot lift and the wave cannot carry, but
together the two transport the waste along the shore. The road of
shore drift is therefore the zone of the breaking waves.
THE BAY-HEAD BEACH. As the waste derived from the wear of waves
and that brought in by streams is trailed along a coast it
assumes, under varying conditions, a number of distinct forms.
When swept into the head of a sheltered bay it constitutes the
bay-head beach. By the highest storm waves the beach is often
built higher than the ground immediately behind it, and forms a
dam inclosing a shallow pond or marsh.
THE BAY BAR. As the stream of shore drift reaches the mouth of a
bay of some size it often occurs that, instead of turning in, it
sets directly across toward the opposite headland. The waste is
carried out from shore into the deeper waters of the bay mouth;
where it is no longer supported by the breaking waves, and sinks
to the bottom. The dump is gradually built to the surface as a
stubby spur, pointing across the bay, and as it reaches the zone
of wave action current and wave can now combine to carry shore
drift along it, depositing their load continually at the point of
the spur. An embankment is thus constructed in much the same
manner as a railway fill, which, while it is building, serves as a
roadway along which the dirt from an adjacent cut is carted to be
dumped at the end. When the embankment is completed it bridges the
bay with a highway along which shore drift now moves without
interruption, and becomes a bay bar.
INCOMPLETE BAY BARS. Under certain conditions the sea cannot carry
out its intention to bridge a bay. Rivers discharging in bays
demand open way to the ocean. Strong tidal currents also are able
to keep open channels scoured by their ebb and flow. In such cases
the most that land waste can do is to build spits and shoals,
narrowing and shoaling the channel as much as possible. Incomplete
bay bars sometimes have their points recurved by currents setting
at right angles to the stream of shore drift and are then
classified as HOOKS (Fig. 142).
SAND REEFS. On low coasts where shallow water extends some
distance out, the highway of shore drift lies along a low, narrow
ridge, termed the sand reef, separated from the land by a narrow
stretch of shallow water called the LAGOON. At intervals the reef
is held open by INLETS,--gaps through which the tide flows and
ebbs, and by which the water of streams finds way to the sea.
No finer example of this kind of shore line is to be found in the
world than the coast of Texas. From near the mouth of the Rio
Grande a continuous sand reef draws its even curve for a hundred
miles to Corpus Christi Pass, and the reefs are but seldom
interrupted by inlets as far north as Galveston Harbor. On this
coast the tides are variable and exceptionally weak, being less
than one foot in height, while the amount of waste swept along the
shore is large. The lagoon is extremely shallow, and much of it is
a mud flat too shoal for even small boats. On the coast of New
Jersey strong tides are able to keep open inlets at intervals of
from two to twenty miles in spite of a heavy alongshore drift.
Sand reefs are formed where the water is so shallow near shore
that storm waves cannot run in it and therefore break some
distance out from land. Where storm waves first drag bottom they
erode and deepen the sea floor, and sweep in sediment as far as
the line where they break. Here, where they lose their force, they
drop their load and beat up the ridge which is known as the sand
reef when it reaches the surface.
SHORES OF ELEVATION AND DEPRESSION
Our studies have already brought to our notice two distinct forms
of strand lines,--one the high, rocky coast cut back to cliffs by
the attack of the waves, and the other the low, sandy coast where
the waves break usually upon the sand reef. To understand the
origin of these two types we must know that the meeting place of
sea and land is determined primarily by movements of the earth's
crust. Where a coast land emerges the--shore line moves seaward;
where it is being submerged the shore line advances on the land.
SHORES OF ELEVATION. The retreat of the sea, either because of a
local uplift of the land or for any other reason, such as the
lowering of any portion of ocean bottom, lays bare the inner
margin of the sea floor. Where the sea floor has long received the
waste of the land it has been built up to a smooth, subaqueous
plain, gently shelving from the land. Since the new shore line is
drawn across this even surface it is simple and regular, and is
bordered on the one side by shallow water gradually deepening
seaward, and on the other by low land composed of material which
has not yet thoroughly consolidated to firm rock. A sand reef is
soon beaten up by the waves, and for some time conditions will
favor its growth. The loss of sand driven into the lagoon beyond,
and of that ground to powder by the surf and carried out to sea,
is more than made up by the stream of alongshore drift, and
especially by the drag of sediments to the reef by the waves as
they deepen the sea floor on its seaward side.
Meanwhile the lagoon gradually fills with waste from the reef and
from the land. It is invaded by various grasses and reeds which
have learned to grow in salt and brackish water; the marsh, laid
bare only at low tide, is built above high tide by wind drift and
vegetable deposits, and becomes a meadow, soldering the sand reef
to the mainland.
While the lagoon has been filling, the waves have been so
deepening the sea floor off the sand reef that at last they are
able to attack it vigorously. They now wear it back, and, driving
the shore line across the lagoon or meadow, cut a line of low
cliffs on the mainland. Such a shore is that of Gascony in
southwestern France,--a low, straight, sandy shore, bordered by
dunes and unprotected by reefs from the attack of the waves of the
Bay of Biscay.
We may say, then, that on shores of elevation the presence of sand
reefs and lagoons indicates the stage of youth, while the absence
of these features and the vigorous and unimpeded attack by the sea
upon the mainland indicate the stage of maturity. Where much waste
is brought in by rivers the maturity of such a coast may be long
delayed. The waste from the land keeps the sea shallow offshore
and constantly renews the sand reef. The energy of the waves is
consumed in handling shore drift, and no energy is left for an
effective attack upon the land. Indeed, with an excessive amount
of waste brought down by streams the land may be built out and
encroach temporarily upon the sea; and not until long denudation
has lowered the land, and thus decreased the amount of waste from
it, may the waves be able to cut through the sand reef and thus
the coast reach maturity.
SHORES OF DEPRESSION
Where a coastal region is undergoing submergence the shore line
moves landward. The horizontal plane of the sea now intersects an
old land surface roughened by subaerial denudation. The shore line
is irregular and indented in proportion to the relief of the land
and the amount of the submergence which the land has suffered. It
follows up partially submerged valleys, forming bays, and bends
round the divides, leaving them to project as promontories and
peninsulas. The outlines of shores of depression are as varied as
are the forms of the land partially submerged. We give a few
typical illustrations.
The characteristics of the coast of Maine are due chiefly to the
fact that a mountainous region of hard rocks, once worn to a
peneplain, and after a subsequent elevation deeply dissected by
north-south valleys, has subsided, the depression amounting on its
southern margin to as much as six hundred feet below sea level.
Drowned valleys penetrate the land in long, narrow bays, and
rugged divides project in long, narrow land arms prolonged seaward
by islands representing the high portions of their extremities. Of
this exceedingly ragged shore there are said to be two thousand
miles from the New Brunswick boundary as far west as Portland,--a
straight-line distance of but two hundred miles. Since the time of
its greatest depression the land is known to have risen some three
hundred feet; for the bays have been shortened, and the waste with
which their floors were strewn is now in part laid bare as clay
plains about the bay heads and in narrow selvages about the
peninsulas and islands.
The coast of Dalmatia, on the Adriatic Sea, is characterized by
long land arms and chains of long and narrow islands, all parallel
to the trend of the coast. A region of parallel mountain ranges
has been depressed, and the longitudinal valleys which lie between
them are occupied by arms of the sea.
Chesapeake Bay is a branching bay due to the depression of an
ancient coastal plain which, after having emerged from the sea,
was channeled with broad, shallow valleys. The sea has invaded the
valley of the trunk stream and those of its tributaries, forming a
shallow bay whose many branches are all directed toward its axis
(Fig. 146).
Hudson Bay, and the North, the Baltic, and the Yellow seas are
examples where the sinking of the land has brought the sea in over
low plains of large extent, thus deeply indenting the continental
out-line. The rise of a few hundred feet would restore these
submerged plains to the land.
THE CYCLE OF SHORES OF DEPRESSION. In its infantile stage the
outline of a shore of depression depends almost wholly on the
previous relief of the land, and but little on erosion by the sea.
Sea cliffs and narrow benches appear where headlands and outlying
islands have been nipped by the waves. As yet, little shore waste
has been formed. The coast of Maine is an example of this stage.
In early youth all promontories have been strongly cliffed, and
under a vigorous attack of the sea the shore of open bays may be
cut back also. Sea stacks and rocky islets, caves and coves, make
the shore minutely ragged. The irregularity of the coast, due to
depression, is for a while increased by differential wave wear on
harder and softer rocks. The rock bench is still narrow. Shore
waste, though being produced in large amounts, is for the most
part swept into deeper water and buried out of sight. Examples of
this stage are the east coast of Scotland and the California coast
near San Francisco.
Later youth is characterized by a large accumulation of shore
waste. The rock bench has been cut back so that it now furnishes a
good roadway for shore drift. The stream of alongshore drift grows
larger and larger, filling the heads of the smaller bays with
beaches, building spits and hooks, and tying islands with sand
bars to the mainland. It bridges the larger bays with bay bars,
while their length is being reduced as their inclosing
promontories are cut back by the waves. Thus there comes to be a
straight, continuous, and easy road, no longer interrupted by
headlands and bays, for the transportation of waste alongshore.
The Baltic coast of Germany is in this stage.
All this while streams have been busy filling with delta deposits
the bays into which they empty. By these steps a coast gradually
advances to MATURITY, the stage when the irregularities due to
depression have been effaced, when outlying islands formed by
subsidence have been planed away, and when the shore line has been
driven back behind the former bay heads. The sea now attacks the
land most effectively along a continuous and fairly straight line
of cliffs. Although the first effect of wave wear was to increase
the irregularities of the shore, it sooner or later rectifies it,
making it simple and smooth. Northwestern France may be cited as
an upland plain, dissected and depressed, whose coast has reached
maturity.
In the OLD AGE of coasts the rock bench is cut back so far that
the waves can no longer exert their full effect upon the shore.
Their energy is dissipated in moving shore drift hither and
thither and in abrading the bench when they drag bottom upon it.
Little by little the bench is deepened by tidal currents and the
drag of waves; but this process is so slow that meanwhile the sea
cliffs melt down under the weather, and the bench becomes a broad
shoal where waves and tides gradually work over the waste from the
land to greater fineness and sweep it out to sea.
PLAINS OF MARINE ABRASION. While subaerial denudation reduces the
land to baselevel, the sea is sawing its edges to WAVE BASE, i.e.
the lowest limit of the wave's effective wear. The widened rock
bench forms when uplifted a plain of marine abrasion, which like
the peneplain bevels across strata regardless of their various
inclinations and various degrees of hardness.
How may a plain of marine abrasion be expected to differ from a
peneplain in its mantle of waste?
Compared with subaerial denudation, marine abrasion is a
comparatively feeble agent. At the rate of five feet per century--
a higher rate than obtains on the youthful rocky, coast of
Britain--it would require more than ten million years to pare a
strip one hundred miles wide from the margin of a continent, a
time sufficient, at the rate at which the Mississippi valley is
now being worn away, for subaerial denudation to lower the lands
of the globe to the level of the sea.
Slow submergence favors the cutting of a wide rock bench. The
water continually deepens upon the bench; storm waves can
therefore always ride in to the base of the cliffs and attack them
with full force; shore waste cannot impede the onset of the waves,
for it is continually washed out in deeper water below wave base.
BASAL CONGOLMERATES. As the sea marches across the land during a
slow submergence, the platform is covered with sheets of sea-laid
sediments. Lowest of these is a conglomerate,--the bowlder and
pebble beach, widened indefinitely by the retreat of the cliffs at
whose base it was formed, and preserved by the finer deposits laid
upon it in the constantly deepening water as the land subsides.
Such basal conglomerates are not uncommon among the ancient rocks
of the land, and we may know them by their rounded pebbles and
larger stones, composed of the same kind of rock as that of the
abraded and evened surface on which they lie.
CHAPTER VIII
OFFSHORE AND DEEP-SEA DEPOSITS
The alongshore deposits which we have now studied are the exposed
edge of a vast subaqueous sheet of waste which borders the
continents and extends often for as much as two or three hundred
miles from land. Soundings show that offshore deposits are laid in
belts parallel to the coast, the coarsest materials lying nearest
to the land and the finest farthest out. The pebbles and gravel
and the clean, coarse sand of beaches give place to broad
stretches of sand, which grows finer and finer until it is
succeeded by sheets of mud. Clearly there is an offshore movement
of waste by which it is sorted, the coarser being sooner dropped
and the finer being carried farther out.
OFFSHORE DEPOSITS
The debris torn by waves from rocky shores is far less in amount
than the waste of the land brought down to the sea by rivers,
being only one thirty-third as great, according to a conservative
estimate. Both mingle alongshore in all the forms of beach and bar
that have been described, and both are together slowly carried out
to sea. On the shelving ocean floor waste is agitated by various
movements of the unquiet water,--by the undertow (an outward-
running bottom current near the shore), by the ebb and flow of
tides, by ocean currents where they approach the land, and by
waves and ground swells, whose effects are sometimes felt to a
depth of six hundred feet. By all these means the waste is slowly
washed to and fro, and as it is thus ground finer and finer and
its soluble parts are more and more dissolved, it drifts farther
and farther out from land. It is by no steady and rapid movement
that waste is swept from the shore to its final resting place. Day
after day and century after century the grains of sand and
particles of mud are shifted to and fro, winnowed and spread in
layers, which are destroyed and rebuilt again and again before
they are buried safe from further disturbance.
These processes which are hidden from the eye are among the most
important of those with which our science has to do; for it is
they which have given shape to by far the largest part of the
stratified rocks of which the land is made.
THE CONTINENTAL DELTA. This fitting term has been recently
suggested for the sheet of waste slowly accumulating along the
borders of the continents. Within a narrow belt, which rarely
exceeds two or three hundred miles, except near the mouths of
muddy rivers such as the Amazon and Congo, nearly all the waste of
the continent, whether worn from its surface by the weather, by
streams, by glaciers, or by the wind, or from its edge by the
chafing of the waves, comes at last to its final resting place.
The agencies which spread the material of the continental delta
grow more and more feeble as they pass into deeper and more quiet
water away from shore. Coarse materials are therefore soon dropped
along narrow belts near land. Gravels and coarse sands lie in
thick, wedge-shaped masses which thin out seaward rapidly and give
place to sheets of finer sand.
SEA MUDS. Outermost of the sediments derived from the waste of the
continents is a wide belt of mud; for fine clays settle so slowly,
even in sea water,--whose saltness causes them to sink much faster
than they would in fresh water,--that they are wafted far before
they reach a bottom where they may remain undisturbed. Muds are
also found near shore, carpeting the floors of estuaries, and
among stretches of sandy deposits in hollows where the more quiet
water has permitted the finer silt to rest.
Sea muds are commonly bluish and consolidate to bluish shales; the
red coloring matter brought from land waste--iron oxide--is
altered to other iron compounds by decomposing organic matter in
the presence of sea water. Yellow and red muds occur where the
amount of iron oxide in the silt brought down to the sea by rivers
is too great to be reduced, or decomposed, by the organic matter
present.
Green muds and green sand owe their color to certain chemical
changes which take place where waste from the land accumulates on
the sea floor with extreme slowness. A greenish mineral called
GLAUCONITE--a silicate of iron and alumina--is then formed. Such
deposits, known as GREEN SAND, are now in process of making in
several patches off the Atlantic coast, and are found on the
coastal plain of New Jersey among the offshore deposits of earlier
geological ages.
ORGANIC DEPOSITS. Living creatures swarm along the shore and on
the shallows out from land as nowhere else in the ocean. Seaweed
often mantles the rock of the sea cliff between the levels of high
and low tide, protecting it to some degree from the blows of
waves. On the rock bench each little pool left by the ebbing tide
is an aquarium abounding in the lowly forms of marine life. Below
low-tide level occur beds of molluscous shells, such as the
oyster, with countless numbers of other humble organisms. Their
harder parts--the shells of mollusks, the white framework of
corals, the carapaces of crabs and other crustaceans, the shells
of sea urchins, the bones and teeth of fishes--are gradually
buried within the accumulating sheets of sediment, either whole
or, far more often, broken into fragments by the waves.
By means of these organic remains each layer of beach deposits and
those of the continental delta may contain a record of the life of
the time when it was laid. Such a record has been made ever since
living creatures with hard parts appeared upon the globe. We shall
find it sealed away in the stratified rocks of the continents,--
parts of ancient sea deposits now raised to form the dry land.
Thus we have in the traces of living creatures found in the rocks,
i.e. in fossils, a history of the progress of life upon the
planet.
MOLLUSCOUS SHELL DEPOSITS. The forms of marine life of importance
in rock making thrive best in clear water, where little sediment
is being laid, and where at the same time the depth is not so
great as to deprive them of needed light, heat, and of sufficient
oxygen absorbed by sea water from the air. In such clear and
comparatively shallow water there often grow countless myriads of
animals, such as mollusks and corals, whose shells and skeletons
of carbonate of lime gradually accumulate in beds of limestone.
A shell limestone made of broken fragments cemented together is
sometimes called COQUINA, a local term applied to such beds
recently uplifted from the sea along the coast of Florida (Fig.
149).
OOLITIC limestone (oon, an egg; lithos, a stone) is so named from
the likeness of the tiny spherules which compose it to the roe of
fish. Corals and shells have been pounded by the waves to
calcareous sand, and each grain has been covered with successive
concentric coatings of lime carbonate deposited about it from
solution.
The impalpable powder to which calcareous sand is ground by the
waves settles at some distance from shore in deeper and quieter
water as a limy silt, and hardens into a dense, fine-grained
limestone in which perhaps no trace of fossil is found to suggest
the fact that it is of organic origin.
From Florida Keys there extends south to the trough of Florida
Straits a limestone bank covered by from five hundred and forty to
eighteen hundred feet of water. The rocky bottom consists of
limestone now slowly building from the accumulation of the remains
of mollusks, small corals, sea urchins, worms with calcareous
tubes, and lime-secreting seaweed, which live upon its surface.
Where sponges and other silica-secreting organisms abound on
limestone banks, silica forms part of the accumulated deposit,
either in its original condition, as, for example, the spicules of
sponges, or gathered into concretions and layers of flint.
Where considerable mud is being deposited along with carbonate of
lime there is in process of making a clayey limestone or a limy
shale; where considerable sand, a sandy limestone or a limy
sandstone.
CONSOLIDATION OF OFFSHORE DEPOSITS. We cannot doubt that all these
loose sediments of the sea floor are being slowly consolidated to
solid rock. They are soaked with water which carries in solution
lime carbonate and other cementing substances. These cements are
deposited between the fragments of shells and corals, the grains
of sand and the particles of mud, binding them together into firm
rock. Where sediments have accumulated to great thickness the
lower portions tend also to consolidate under the weight of the
overlying beds. Except in the case of limestones, recent sea
deposits uplifted to form land are seldom so well cemented as are
the older strata, which have long been acted upon by underground
waters deep below the surface within the zone of cementation, and
have been exposed to view by great erosion.
RIPPLE MARKS, SUN CRACKS, ETC. The pulse of waves and tidal
currents agitates the loose material of offshore deposits,
throwing it into fine parallel ridges called ripple marks. One may
see this beautiful ribbing imprinted on beach sands uncovered by
the outgoing tide, and it is also produced where the water is of
considerable depth. While the tide is out the surface of shore
deposits may be marked by the footprints of birds and other
animals, or by the raindrops of a passing shower.
The mud of flats, thus exposed to the sun and dried, cracks in a
characteristic way. Such markings may be covered over with a thin
layer of sediment at the next flood tide and sealed away as a
lasting record of the manner and place in which the strata were
laid. In Figure 150 we have an illustration of a very ancient
ripple-marked sand consolidated to hard stone, uplifted and set on
edge by movements of the earth's crust, and exposed to open air
after long erosion.
STRATIFICATION. For the most part the sheet of sea-laid waste is
hidden from our sight. Where its edge is exposed along the shore
we may see the surface markings which have just been noticed.
Soundings also, and the observations made in shallow waters by
divers, tell something of its surface; but to learn more of its
structures we must study those ancient sediments which have been
lifted from the sea and dissected by subaerial agencies. From them
we ascertain that sea deposits are stratified. They lie in
distinct layers which often differ from one another in thickness,
in size of particles, and perhaps in color. They are parted by
bedding planes, each of which represents either a change in
material or a pause during which deposition ceased and the
material of one layer had time to settle and become somewhat
consolidated before the material of the next was laid upon
it. Stratification is thus due to intermittently acting forces,
such as the agitation of the water during storms, the flow and ebb
of the tide, and the shifting channels of tidal currents. Off the
mouths of rivers, stratification is also caused by the coarser and
more abundant material brought down at time of floods being laid
on the finer silt which is discharged during ordinary stages.
How stratified deposits are built up is well illustrated in the
flats which border estuaries, such as the Bay of Fundy. Each
advance of the tide spreads a film of mud, which dries and hardens
in the air during low water before another film is laid upon it by
the next incoming tidal flood. In this way the flats have been
covered by a clay which splits into leaves as thin as sheets of
paper.
It is in fine material, such as clays and shales and limestones,
that the thinnest and most uniform layers, as well as those of
widest extent, occur. On the other hand, coarse materials are
commonly laid in thick beds, which soon thin out seaward and give
place to deposits of finer stuff. In a general way strata are laid
in well-nigh horizontal sheets, for the surface on which they are
laid is generally of very gentle inclination. Each stratum,
however, is lenticular, or lenslike, in form, having an area where
it is thickest, and thinning out thence to its edges, where it is
overlapped by strata similar in shape.
CROSS BEDDING. There is an apparent exception to this rule where
strata whose upper and lower surfaces may be about horizontal are
made up of layers inclined at angles which may be as high as the
angle of repose. In this case each stratum grew by the addition
along its edge of successive layers of sediment, precisely as does
a sand bar in a river, the sand being pushed continuously over the
edge and coming to rest on a sloping surface. Shoals built by
strong and shifting tidal currents often show successive strata in
which the cross bedding is inclined in different directions.
THICKNESS OF SEA DEPOSITS. Remembering the vast amount of
material denuded from the land and deposited offshore, we should
expect that with the lapse of time sea deposits would have grown
to an enormous thickness. It is a suggestive fact that, as a rule,
the profile of the ocean bed is that of a soup plate,--a basin
surrounded by a flaring rim. On the CONTINENTAL SHELF, as the rim
is called, the water is seldom more than six hundred feet in depth
at the outer edge, and shallows gradually towards shore. Along the
eastern coast of the United States the continental shelf is from
fifty to one hundred and more miles in width; on the Pacific coast
it is much narrower. So far as it is due to upbuilding, a wide
continental shelf, such as that of the Atlantic coast, implies a
massive continental delta thousands of feet in thickness. The
coastal plain of the Atlantic states may be regarded as the
emerged inner margin of this shelf, and borings made along the
coast probe it to the depth of as much as three thousand feet
without finding the bottom of ancient offshore deposits.
Continental shelves may also be due in part to a submergence of
the outer margin of a continental plateau and to marine abrasion.
DEPOSITION OF SEDIMENTS AND SUBSIDENCE. The stratified rocks of
the land show in many places ancient sediments which reach a
thickness which is measured in miles, and which are yet the
product of well-nigh continuous deposition. Such strata may prove
by their fossils and by their composition and structure that they
were all laid offshore in shallow water. We must infer that,
during the vast length of time recorded by the enormous pile, the
floor of the sea along the coast was slowly sinking, and that the
trough was constantly being filled, foot by foot, as fast as it
was depressed. Such gradual, quiet movements of the earth's crust
not only modify the outline of coasts, as we have seen, but are of
far greater geological importance in that they permit the making
of immense deposits of stratified rock.
A slow subsidence continued during long time is recorded also in
the succession of the various kinds of rock that come to be
deposited in the same area. As the sea transgresses the land, i.e.
encroaches upon it, any given part of the sea bottom is brought
farther and farther from the shore. The basal conglomerate formed
by bowlder and pebble beaches comes to be covered with sheets of
sand, and these with layers of mud as the sea becomes deeper and
the shore more remote; while deposits of limestone are made when
at last no waste is brought to the place from the now distant
land, and the water is left clear for the growth of mollusks and
other lime-secreting organisms.
RATE OF DEPOSITION. As deposition in the sea corresponds to
denudation on the land, we are able to make a general estimate of
the rate at which the former process is going on. Leaving out of
account the soluble matter removed, the Mississippi is lowering
its basin at the rate of one foot in five thousand years, and we
may assume this as the average rate at which the earth's land
surface of fifty-seven million square miles is now being denuded
by the removal of its mechanical waste. But sediments from the
land are spread within a zone but two or three hundred miles in
width along the margin of the continents, a line one hundred
thousand miles long. As the area of deposition--about twenty-five
million square miles--is about one half the area of denudation,
the average rate of deposition must be twice the average rate of
denudation, i.e. about one foot in twenty-five hundred years. If
some deposits are made much more rapidly than this, others are
made much more slowly. If they were laid no faster than the
present average rate, the strata of ancient sea deposits exposed
in a quarry fifty feet deep represent a lapse of at least one
hundred and twenty-five thousand years, and those of a formation
five hundred feet thick required for their accumulation one
million two hundred and fifty thousand years.
THE SEDIMENTARY RECORD AND THE DENUDATION CYCLE. We have seen that
the successive stages in a cycle of denudation, such as that by
which a land mass of lofty mountains is worn to low plains, are
marked each by its own peculiar land forms, and that the forms of
the earlier stages are more or less completely effaced as the
cycle draws toward an end. Far more lasting records of each stage
are left in the sedimentary deposits of the continental delta.
Thus, in the youth of such a land mass as we have mentioned,
torrential streams flowing down the steep mountain sides deliver
to the adjacent sea their heavy loads of coarse waste, and thick
offshore deposits of sand and gravel (Fig. 156) record the high
elevation of the bordering land. As the land is worn to lower
levels, the amount and coarseness of the waste brought to the sea
diminishes, until the sluggish streams carry only a fine silt
which settles on the ocean floor near to land in wide sheets of
mud which harden into shale. At last, in the old age of the region
(Fig. 157), its low plains contribute little to the sea except the
soluble elements of the rocks, and in the clear waters near the
land lime-secreting organisms flourish and their remains
accumulate in beds of limestone. When long-weathered lands
mantled with deep, well-oxidized waste are uplifted by a gradual
movement of the earth's crust, and the mantle is rapidly stripped
off by the revived streams, the uprise is recorded in wide
deposits of red and yellow clays and sands upon the adjacent ocean
floor.
Where the waste brought in is more than the waves can easily
distribute, as off the mouths of turbid rivers which drain
highlands near the sea, deposits are little winnowed, and are laid
in rapidly alternating, shaly sandstones and sandy shales.
Where the highlands are of igneous rock, such as granite, and
mechanical disintegration is going on more rapidly than chemical
decay, these conditions are recorded in the nature of the deposits
laid offshore. The waste swept in by streams contains much
feldspar and other minerals softer and more soluble than quartz,
and where the waves have little opportunity to wear and winnow it,
it comes to rest in beds of sandstone in which grains of feldspar
and other soft minerals are abundant. Such feldspathic sandstones
are known as ARKOSE.
On the other hand, where the waste supplied to the sea comes
chiefly from wide, sandy, coastal plains, there are deposited off-
shore clean sandstones of well-worn grains of quartz alone. In
such coastal plains the waste of the land is stored for ages.
Again and again they are abandoned and invaded by the sea as from
time to time the land slowly emerges and is again submerged. Their
deposits are long exposed to the weather, and sorted over by the
streams, and winnowed and worked over again and again by the
waves. In the course of long ages such deposits thus become
thoroughly sorted, and the grains of all minerals softer than
quartz are ground to mud.
DEEP-SEA OOZES AND CLAYS
GLOBIGERINA OOZE. Beyond the reach of waste from the land the
bottom of the deep sea is carpeted for the most part with either
chalky ooze or a fine red clay. The surface waters of the warm
seas swarm with minute and lowly animals belonging to the order of
the Foraminifera, which secrete shells of carbonate of lime. At
death these tiny white shells fall through the sea water like
snowflakes in the air, and, slowly dissolving, seem to melt quite
away before they can reach depths greater than about three miles.
Near shore they reach bottom, but are masked by the rapid deposit
of waste derived from the land. At intermediate depths they mantle
the ocean floor with a white, soft lime deposit known as
Globigerina ooze, from a genus of the Foraminifera which
contributes largely to its formation.
RED CLAY. Below depths of from fifteen to eighteen thousand feet
the ocean bottom is sheeted with red or chocolate colored clay. It
is the insoluble residue of seashells, of the debris of submarine
volcanic eruptions, of volcanic dust wafted by the winds, and of
pieces of pumice drifted by ocean currents far from the volcanoes
from which they were hurled. The red clay builds up with such
inconceivable slowness that the teeth of sharks and the hard ear
bones of whales may be dredged in large numbers from the deep
ocean bed, where they have lain unburied for thousands of years;
and an appreciable part of the clay is also formed by the dust of
meteorites consumed in the atmosphere,--a dust which falls
everywhere on sea and land, but which elsewhere is wholly masked
by other deposits.
The dark, cold abysses of the ocean are far less affected by
change than any other portion of the surface of the lithosphere.
These vast, silent plains of ooze lie far below the reach of
storms. They know no succession of summer and winter, or of night
and day. A mantle of deep and quiet water protects them from the
agents of erosion which continually attack, furrow, and destroy
the surface of the land. While the land is the area of erosion,
the sea is the area of deposition. The sheets of sediment which
are slowly spread there tend to efface any inequalities, and to
form a smooth and featureless subaqueous plain.
With few exceptions, the stratified rocks of the land are proved
by their fossils and composition to have been laid in the sea; but
in the same way they are proved to be offshore, shallow-water
deposits, akin to those now making on continental shelves. Deep-
sea deposits are absent from the rocks of the land, and we may
therefore infer that the deep sea has never held sway where the
continents now are,--that the continents have ever been, as now,
the elevated portions of the lithosphere, and that the deep seas
of the present have ever been its most depressed portions.
THE REEF-BUILDING CORALS
In warm seas the most conspicuous of rock-making organisms are the
corals known as the reef builders. Floating in a boat over a coral
reef, as, for example, off the south coast of Florida or among the
Bahamas, one looks down through clear water on thickets of
branching coral shrubs perhaps as much as eight feet high, and
hemispherical masses three or four feet thick, all abloom with
countless minute flowerlike coral polyps, gorgeous in their colors
of yellow, orange, green, and red. In structure each tiny polyp is
little more than a fleshy sac whose mouth is surrounded with
petal-like tentacles, or feelers. From the sea water the polyps
secrete calcium carbonate and build it up into the stony framework
which supports their colonies. Boring mollusks, worms, and sponges
perforate and honeycomb this framework even while its surface is
covered with myriads of living polyps. It is thus easily broken by
the waves, and white fragments of coral trees strew the ground
beneath. Brilliantly colored fishes live in these coral groves,
and countless mollusks, sea urchins, and other forms of marine
life make here their home. With the debris from all these sources
the reef is constantly built up until it rises to low-tide level.
Higher than this the corals cannot grow, since they are killed by
a few hours' exposure to the air.
When the reef has risen to wave base, the waves abrade it on the
windward side and pile to leeward coral blocks torn from their
foundation, filling the interstices with finer fragments. Thus
they heap up along the reef low, narrow islands (Fig. 160).
Reef building is a comparatively rapid progress. It has been
estimated that off Florida a reef could be built up to the surface
from a depth of fifty feet in about fifteen hundred years.
CORAL LIMESTONES. Limestones of various kinds are due to the reef
builders. The reef rock is made of corals in place and broken
fragments of all sizes, cemented together with calcium carbonate
from solution by infiltrating waters. On the island beaches coral
sand is forming oolitic limestone, and the white coral mud with
which the sea is milky for miles about the reef in times of storm
settles and concretes into a compact limestone of finest grain.
Corals have been among the most important limestone builders of
the sea ever since they made their appearance in the early
geological ages.
The areas on which coral limestone is now forming are large. The
Great Barrier Reef of Australia, which lies off the north-eastern
coast, is twelve hundred and fifty miles long, and has a width of
from ten to ninety miles. Most of the islands of the tropics are
either skirted with coral reefs or are themselves of coral
formation.
CONDITIONS OF CORAL GROWTH. Reef-building corals cannot live
except in clear salt water less, as a rule, than one hundred and
fifty feet in depth, with a winter temperature not lower than 68
degrees F. An important condition also is an abundant food supply,
and this is best secured in the path of the warm oceanic currents.
Coral reefs may be grouped in three classes,--fringing reefs,
barrier reefs, and atolls.
FRINGING REEFS. These take their name from the fact that they are
attached as narrow fringes to the shore. An example is the reef
which forms a selvage about a mile wide along the northeastern
coast of Cuba. The outer margin, indicated by the line of white
surf, where the corals are in vigorous growth, rises from about
forty feet of water. Between this and the shore lies a stretch of
shoal across which one can wade at low water, composed of coral
sand with here and there a clump of growing coral.
BARRIER REEFS. Reefs separated from the shore by a ship channel of
quiet water, often several miles in width and sometimes as much as
three hundred feet in depth, are known as barrier reefs. The
seaward face rises abruptly from water too deep for coral growth.
Low islands are cast up by the waves upon the reef, and inlets
give place for the ebb and flow of the tides. Along the west coast
of the island of New Caledonia a barrier reef extends for four
hundred miles, and for a length of many leagues seldom approaches
within eight miles of the shore.
ATOLLS. These are ring-shaped or irregular coral islands, or
island-studded reefs, inclosing a central lagoon. The narrow zone
of land, like the rim of a great bowl sunken to the water's edge,
rises hardly more than twenty feet at most above the sea, and is
covered with a forest of trees such as the cocoanut, whose seeds
can be drifted to it uninjured from long distances. The white
beach of coral sand leads down to the growing reef, on whose outer
margin the surf is constantly breaking. The sea face of the reef
falls off abruptly, often to depths of thousands of feet, while
the lagoon varies in depth from a few feet to one hundred and
fifty or two hundred, and exceptionally measures as much as three
hundred and fifty feet.
THEORIES OF CORAL REEFS. Fringing reefs require no explanation,
since the depth of water about them is not greater than that at
which coral can grow; but barrier reefs and atolls, which may rise
from depths too great for coral growth demand a theory of their
origin.
Darwin's theory holds that barrier reefs and atolls are formed
from fringing reefs by SUBSIDENCE. The rate of sinking cannot be
greater than that of the upbuilding of the reef, since otherwise
the corals would be carried below their depth and drowned. The
process is illustrated in Figure 161, where v represents a
volcanic island in mid ocean undergoing slow depression, and ss
the sea level before the sinking began, when the island was
surrounded by a fringing reef. As the island slowly sinks, the
reef builds up with equal pace. It rears its seaward face more
steep than the island slope, and thus the intervening space
between the sinking, narrowing land and the outer margin of the
reef constantly widens. In this intervening space the corals are
more or less smothered with silt from the outer reef and from the
land, and are also deprived in large measure of the needful supply
of food and oxygen by the vigorous growth of the corals on the
outer rim. The outer rim thus becomes a barrier reef and the inner
belt of retarded growth is deepened by subsidence to a ship
channel, s's' representing sea level at this time. The final
stage, where the island has been carried completely beneath the
sea and overgrown by the contracting reef, whose outer ring now
forms an atoll, is represented by s"s".
In very many instances, however, atolls and barrier reefs may be
explained without subsidence. Thus a barrier reef may be formed by
the seaward growth of a fringing reef upon the talus of its sea
face. In Figure 162 f is a fringing reef whose outer wall rises
from about one hundred and fifty feet, the lower limit of the
reef-building species. At the foot of this submarine cliff a talus
of fallen blocks t accumulates, and as it reaches the zone of
coral growth becomes the foundation on which the reef is steadily
extended seaward. As the reef widens, the polyps of the
circumference flourish, while those of the inner belt are retarded
in their growth and at last perish. The coral rock of the inner
belt is now dissolved by sea water and scoured out by tidal
currents until it gives place to a gradually deepening ship
channel, while the outer margin is left as a barrier reef.
In much the same way atolls may be built on any shoal which lies
within the zone of coral growth. Such shoals may be produced when
volcanic islands are leveled by waves and ocean currents, and when
submarine plateaus, ridges, and peaks are built up by various
organic agencies, such as molluscous and foraminiferal shell
deposits. The reef-building corals, whose eggs are drifted widely
over the tropic seas by ocean currents, colonize such submarine
foundations wherever the conditions are favorable for their
growth. As the reef approaches the surface the corals of the inner
area are smothered by silt and starved, and their Submarine
Volcanic Peak hard parts are dissolved and scoured away; while
those of the circumference, with abundant food supply, nourish and
build the ring of the atoll. Atolls may be produced also by the
backward drift of sand from either end of a crescentic coral reef
or island, the spits uniting in the quiet water of the lee to
inclose a lagoon. In the Maldive Archipelago all gradations
between crescent-shaped islets and complete atoll rings have been
observed.
In a number of instances where coral reefs have been raised by
movements of the earth's crust, the reef formation is found to be
a thin veneer built upon a foundation of other deposits. Thus
Christmas Island, in the Indian Ocean, is a volcanic pile rising
eleven hundred feet above sea level and fifteen thousand five
hundred feet above the bottom of the sea. The summit is a plateau
surrounded by a rim of hills of reef formation, which represent
the ring of islets of an ancient atoll. Beneath the reef are thick
beds of limestone, composed largely of the remains of
foraminifers, which cover the lavas and fragraental materials of
the old submarine volcano.
Among the ancient sediments which now form the stratified rocks of
the land there occur many thin reef deposits, but none are known
of the immense thickness which modern reefs are supposed to reach
according to the theory of subsidence.
Barrier and fringing reefs are commonly interrupted off the mouths
of rivers. Why?
SUMMARY. We have seen that the ocean bed is the goal to which the
waste of the rocks of the land at last arrives. Their soluble
parts, dissolved by underground waters and carried to the sea by
rivers, are largely built up by living creatures into vast sheets
of limestone. The less soluble portions--the waste brought in by
streams and the waste of the shore--form the muds and sands of
continental deltas. All of these sea deposits consolidate and
harden, and the coherent rocks of the land are thus reconstructed
on the ocean floor. But the destination is not a final one. The
stratified rocks of the land are for the most part ancient
deposits of the sea, which have been lifted above sea level; and
we may believe that the sediments now being laid offshore are the
"dust of continents to be," and will some time emerge to form
additions to the land. We are now to study the movements of the
earth's crust which restore the sediments of the sea to the light
of day, and to whose beneficence we owe the habitable lands of the
present.
PART II
INTERNAL GEOLOGICAL AGENCIES
CHAPTER IX
MOVEMENTS OF THE EARTH'S CRUST
The geological agencies which we have so far studied--weathering,
streams, underground waters, glaciers, winds, and the ocean--all
work upon the earth from without, and all are set in motion by an
energy external to the earth, namely, the radiant energy of the
sun. All, too, have a common tendency to reduce the inequalities
of the earth's surface by leveling the lands and strewing their
waste beneath the sea.
But despite the unceasing efforts of these external agencies, they
have not destroyed the continents, which still rear their broad
plains and great plateaus and mountain ranges above the sea.
Either, then, the earth is very young and the agents of denudation
have not yet had time to do their work, or they have been opposed
successfully by other forces.
We enter now upon a department of our science which treats of
forces which work upon the earth from within, and increase the
inequalities of its surface. It is they which uplift and recreate
the lands which the agents of denudation are continually
destroying; it is they which deepen the ocean bed and thus
withdraw its waters from the shores. At times also these forces
have aided in the destruction of the lands by gradually lowering
them and bringing in the sea. Under the action of forces resident
within the earth the crust slowly rises or sinks; from time to
time it has been folded and broken; while vast quantities of
molten rock have been pressed up into it from beneath and
outpoured upon its surface. We shall take up these phenomena in
the following chapters, which treat of upheavals and depressions
of the crust, foldings and fractures of the crust, earthquakes,
volcanoes, the interior conditions of the earth, mineral veins,
and metamorphism.
OSCILLATIONS OF THE CRUST
Of the various movements of the crust due to internal agencies we
will consider first those called oscillations, which lift or
depress large areas so slowly that a long time is needed to
produce perceptible changes of level, and which leave the strata
in nearly their original horizontal attitude. These movements are
most conspicuous along coasts, where they can be referred to the
datum plane of sea level; we will therefore take our first
illustrations from rising and sinking shores.
NEW JERSEY. Along the coasts of New Jersey one may find awash at
high tide ancient shell heaps, the remains of tribal feasts of
aborigines. Meadows and old forest grounds, with the stumps still
standing, are now overflowed by the sea, and fragments of their
turf and wood are brought to shore by waves. Assuming that the sea
level remains constant, it is clear that the New Jersey coast is
now gradually sinking. The rate of submergence has been estimated
at about two feet per century.
On the other hand, the wide coastal plain of New Jersey is made of
stratified sands and clays, which, as their marine fossils show,
were outspread beneath the sea. Their present position above sea
level proves that the land now subsiding emerged in the recent
past.
The coast of New Jersey is an example of the slow and tranquil
oscillations of the earth's unstable crust now in progress along
many shores. Some are emerging from the sea, some are sinking
beneath it; and no part of the land seems to have been exempt from
these changes in the past.
EVIDENCES OF CHANGES OF LEVEL. Taking the surface of the sea as a
level of reference, we may accept as proofs of relative upheaval
whatever is now found in place above sea level and could have been
formed only at or beneath it, and as proofs of relative subsidence
whatever is now found beneath the sea and could only have been
formed above it.
Thus old strand lines with sea cliffs, wave-cut rock benches, and
beaches of wave-worn pebbles or sand, are striking proofs of
recent emergence to the amount of their present height above tide.
No less conclusive is the presence of sea-laid rocks which we may
find in the neighboring quarry or outcrop, although it may have
been long ages since they were lifted from the sea to form part of
the dry land.
Among common proofs of subsidence are roads and buildings and
other works of man, and vegetal growths and deposits, such as
forest grounds and peat beds, now submerged beneath the sea. In
the deltas of many large rivers, such as the Po, the Nile, the
Ganges, and the Mississippi, buried soils prove subsidences of
hundreds of feet; and in several cases, as in the Mississippi
delta, the depression seems to be now in progress.
Other proofs of the same movement are drowned land forms which are
modeled only in open air. Since rivers cannot cut their valleys
farther below the baselevel of the sea than the depths of their
channels, DROWNED VALLEYS are among the plainest proofs of
depression. To this class belong Narragansett, Delaware,
Chesapeake, Mobile, and San Francisco bays, and many other similar
drowned valleys along the coasts of the United States. Less
conspicuous are the SUBMARINE CHANNELS which, as soundings show,
extend from the mouths of a number of rivers some distance out to
sea. Such is the submerged channel which reaches from New York Bay
southeast to the edge of the continental shelf, and which is
supposed to have been cut by the Hudson River when this part of
the shelf was a coastal plain.
WARPING. In a region undergoing changes of level the rate of
movement commonly varies in different parts. Portions of an area
may be rising or sinking, while adjacent portions are stationary
or moving in the opposite direction. In this way a land surface
becomes WARPED. Thus, while Nova Scotia and New Brunswick are now
rising from the level of the sea, Prince Edward Island and Cape
Breton Island are sinking, and the sea now flows over the site of
the famous old town of Louisburg destroyed in 1758.
Since the close of the glacial epoch the coasts of Newfoundland
and Labrador have risen hundreds of feet, but the rate of
emergence has not been uniform. The old strand line, which stands
at five hundred and seventy-five feet above tide at St. John's,
Newfoundland, declines to two hundred and fifty feet near the
northern point of Labrador.
THE GREAT LAKES is now under-going perceptible warping. Rivers
enter the lakes from the south and west with sluggish currents and
deep channels resembling the estuaries of drowned rivers; while
those that enter from opposite directions are swift and shallow.
At the western end of Lake Erie are found submerged caves
containing stalactites, and old meadows and forest grounds are now
under water. It is thus seen that the water of the lakes is rising
along their southwestern shores, while from their north-eastern
shores it is being withdrawn. The region of the Great Lakes is
therefore warping; it is rising in the northeast as compared with
the southwest.
From old bench marks and records of lake levels it has been
estimated that the rate of warping amounts to five inches a
century for every one hundred miles. It is calculated that the
water of Lake Michigan is rising at Chicago at the rate of nine or
ten inches per century. The divide at this point between the
tributaries of the Mississippi and Lake Michigan is but eight feet
above the mean stage of the lake. If the canting of the region
continues at its present rate, in a thousand years the waters of
the lake will here overflow the divide. In three thousand five
hundred years all the lakes except Ontario will discharge by this
outlet, via the Illinois and Mississippi rivers, into the Gulf of
Mexico. The present outlet by the Niagara River will be left dry,
and the divide between the St. Lawrence and the Mississippi
systems will have shifted from Chicago to the vicinity of Buffalo.
PHYSIOGRAPHIC EFFECTS OF OSCILLATIONS. We have already mentioned
several of the most important effects of movements of elevation
and depression, such as their effects on rivers, the mantle of
waste, and the forms of coasts. Movements of elevation--including
uplifts by folding and fracture of the crust to be noticed later--
are the necessary conditions for erosion by whatever agent. They
determine the various agencies which are to be chiefly concerned m
the wear of any land,--whether streams or glaciers, weathering or
the wind,--and the degree of their efficiency. The lands must be
uplifted before they can be eroded, and since they must be eroded
before their waste can be deposited, movements of elevation are a
prerequisite condition for sedimentation also. Subsidence is a
necessary condition for deposits of great thickness, such as those
of the Great Valley of California and the Indo-Gangetic plain (p.
101), the Mississippi delta (p. 109), and the still more important
formations of the continental delta in gradually sinking troughs
(p. 183). It is not too much to say that the character and
thickness of each formation of the stratified rocks depend
primarily on these crustal movements.
Along the Baltic coast of Sweden, bench marks show that the sea is
withdrawing from the land at a rate which at the north amounts to
between three and four feet per century; Towards the south the
rate decreases. South of Stockholm, until recent years, the sea
has gained upon the land, and here in several seaboard towns
streets by the shore are still submerged. The rate of oscillation
increases also from the coast inland. On the other hand, along the
German coast of the Baltic the only historic fluctuations of sea
level are those which may be accounted for by variations due to
changes in rainfall. In 1730 Celsius explained the changes of
level of the Swedish coast as due to a lowering of the Baltic
instead of to an elevation of the land. Are the facts just stated
consistent with his theory?
At the little town of Tadousac--where the Saguenay River empties
into the St. Lawrence--there are terraces of old sea beaches, some
almost as fresh as recent railway fills, the highest standing two
hundred and thirty feet above the river. Here the Saguenay is
eight hundred and forty feet in depth, and the tide ebbs and flows
far up its stream. Was its channel cut to this depth by the river
when the land was at its present height? What oscillations are
here recorded, and to what amount?
A few miles north of Naples, Italy, the ruins of an ancient Roman
temple lie by the edge of the sea, on a narrow plain which is
overlooked in the rear by an old sea cliff (Fig. 166). Three
marble pillars are still standing. For eleven feet above their
bases these columns are uninjured, for to this height they were
protected by an accumulation of volcanic ashes; but from eleven to
nineteen feet they are closely pitted with the holes of boring
marine mollusks. From these facts trace the history of the
oscillations of the region.
FOLDINGS OF THE CRUST
The oscillations which we have just described leave the strata not
far from their original horizontal attitude. Figure 167 represents
a region in which movements of a very different nature have taken
place. Here, on either side of the valley V, we find outcrops of
layers tilted at high angles. Sections along the ridge r show that
it is composed of layers which slant inward from either side. In
places the outcropping strata stand nearly on edge, and on the
right of the valley they are quite overturned; a shale SH has come
to overlie a limestone LM although the shale is the older rock,
whose original position was beneath the limestone.
It is not reasonable to suppose that these rocks were deposited in
the attitude in which we find them now; we must believe that, like
other stratified rocks, they were outspread in nearly level sheets
upon the ocean floor. Since that time they must have been
deformed. Layers of solid rock several miles in thickness have
been crumpled and folded like soft wax in the hand, and a vast
denudation has worn away the upper portions of the folds, in part
represented in our section by dotted lines.
DIP AND STRIKE. In districts where the strata have been disturbed
it is desirable to record their attitude. This is most easily done
by taking the angle at which the strata are inclined and the
compass direction in which they slant. It is also convenient to
record the direction in which the outcrop of the strata trends
across the country.
The inclination of a bed of rocks to the horizon is its DIP. The
amount of the dip is the angle made with a horizontal plane. The
dip of a horizontal layer is zero, and that of a vertical layer is
90 degrees. The direction of the dip is taken with the compass.
Thus a geologist's notebook in describing the attitude of
outcropping strata contains many such entries as these: dip 32
degrees north, or dip 8 degrees south 20 degrees west,--meaning in
the latter case that the amount of the dip is 8 degrees and the
direction of the dip bears 20 degrees west of south.
The line of intersection of a layer with the horizontal plane is
the STRIKE. The strike always runs at right angles to the dip.
Dip and strike may be illustrated by a book set aslant on a shelf.
The dip is the acute angle made with the shelf by the side of the
book, while the strike is represented by a line running along the
book's upper edge. If the dip is north or south, the strike runs
east and west.
FOLDED STRUCTURES. An upfold, in which the strata dip away from a
line drawn along the crest and called the axis of the fold, is
known as an ANTICLINE. A downfold, where the strata dip from
either side toward the axis of the trough, is called a SYNCLINE.
There is sometimes seen a downward bend in horizontal or gently
inclined strata, by which they descend to a lower level. Such a
single flexure is a MONOCLINE.
DEGREES OF FOLDING. Folds vary in degree from broad, low swells,
which can hardly be detected, to the most highly contorted and
complicated structures. In SYMMETRIC folds the dips of the rocks
on each side the axis of the fold are equal. In UNSYMMETRICAL
folds one limb is steeper than the other, as in the anticline in
Figure 167. In OVERTURNED folds one limb is inclined beyond the
perpendicular. FAN FOLDS have been so pinched that the original
anticlines are left broader at the top than at the bottom.
In folds where the compression has been great the layers are often
found thickened at the crest and thinned along the limbs. Where
strong rocks such as heavy limestones are folded together with
weak rocks such as shales, the strong rocks are often bent into
great simple folds, while the weak rocks are minutely crumpled.
SYSTEMS OF FOLDS. As a rule, folds occur in systems. Over the
Appalachian mountain belt, for example, extending from
northeastern Pennsylvania to northern Alabama and Georgia, the
earth's crust has been thrown into a series of parallel folds
whose axes run from northeast to southwest (Fig. 175). In
Pennsylvania one may count a score or more of these earth waves,--
some but from ten to twenty miles in length, and some extending as
much as two hundred miles before they die away. On the eastern
part of this belt the folds are steeper and more numerous than on
the western side.
CAUSE AND CONDITIONS OF FOLDING. The sections which we have
studied suggest that rocks are folded by lateral pressure. While a
single, simple fold might be produced by a heave, a series of
folds, including overturns, fan folds, and folds thickened on
their crests at the expense of their limbs, could only be made in
one way,--by pressure from the side. Experiment has reproduced all
forms of folds by subjecting to lateral thrust layers of plastic
material such as wax.
Vast as the force must have been which could fold the solid rocks
of the crust as one may crumple the leaves of a magazine in the
fingers, it is only under certain conditions that it could have
produced the results which we see. Rocks are brittle, and it is
only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED,
that they can thus be folded and bent instead of being crushed to
pieces. Under these conditions, experiments prove that not only
metals such as steel, but also brittle rocks such as marble, can
be deformed and molded and made to flow like plastic clay.
ZONE OF FLOW, ZONE OF FLOW AND FRACTURE, AND ZONE OF FRACTURE. We
may believe that at depths which must be reckoned in tens of
thousands of feet the load of overlying rocks is so great that
rocks of all kinds yield by folding to lateral pressure, and flow
instead of breaking. Indeed, at such profound depths and under
such inconceivable weight no cavity can form, and any fractures
would be healed at once by the welding of grain to grain. At less
depths there exists a zone where soft rocks fold and flow under
stress, and hard rocks are fractured; while at and near the
surface hard and soft rocks alike yield by fracture to strong
pressure.
STRUCTURES DEVELOPED IN COMPRESSED ROCKS
Deformed rocks show the effects of the stresses to which they have
yielded, not only in the immense folds into which they have been
thrown but in their smallest parts as well. A hand specimen of
slate, or even a particle under the microscope, may show
plications similar in form and origin to the foldings which have
produced ranges of mountains. A tiny flake of mica in the rocks of
the Alps may be puckered by the same resistless forces which have
folded miles of solid rock to form that lofty range.
SLATY CLEAVAGE. Rocks which have yielded to pressure often split
easily in a certain direction across the bedding planes. This
cleavage is known as slaty cleavage, since it is most perfectly
developed in fine-grained, homogeneous rocks, such as slates,
which cleave to the thin, smooth-surfaced plates with which we are
familiar in the slates used in roofing and for ciphering and
blackboards. In coarse-grained rocks, pressure develops more
distant partings which separate the rocks into blocks.
Slaty cleavage cannot be due to lamination, since it commonly
crosses bedding planes at an angle, while these planes have been
often well-nigh or quite obliterated. Examining slate with a
microscope, we find that its cleavage is due to the grain of the
rock. Its particles are flattened and lie with their broad faces
in parallel planes, along which the rock naturally splits more
easily than in any other direction. The irregular grains of the
mud which has been altered to slate have been squeezed flat by a
pressure exerted at right angles to the plane of cleavage.
Cleavage is found only in folded rocks, and, as we may see in
Figure 176, the strike of the cleavage runs parallel to the strike
of the strata and the axis of the folds. The dip of the cleavage
is generally steep, hence the pressure was nearly horizontal. The
pressure which has acted at right angles to the cleavage, and to
which it is due, is the same lateral pressure which has thrown the
strata into folds.
We find additional proof that slates have undergone compression at
right angles to their cleavage in the fact that any inclusions in
them, such as nodules and fossils, have been squeezed out of shape
and have their long diameters lying in the planes of cleavage.
That pressure is competent to cause cleavage is shown by
experiment. Homogeneous material of fine grain, such as beeswax,
when subjected to heavy pressure cleaves at right angles to the
direction of the compressing force.
RATE OF FOLDING. All the facts known with regard to rock
deformation agree that it is a secular process, taking place so
slowly that, like the deepening of valleys by erosion, it escapes
the notice of the inhabitants of the region. It is only under
stresses slowly applied that rocks bend without breaking. The
folds of some of the highest mountains have risen so gradually
that strong, well-intrenched rivers which had the right of way
across the region were able to hold to their courses, and as a
circular saw cuts its way through the log which is steadily driven
against it, so these rivers sawed their gorges through the fold as
fast as it rose beneath them. Streams which thus maintain the
course which they had antecedent to a deformation of the region
are known as ANTECEDENT streams. Examples of such are the Sutlej
and other rivers of India, whose valleys trench the outer ranges
of the Himalayas and whose earlier river deposits have been
upturned by the rising ridges. On the other hand, mountain crests
are usually divides, parting the head waters of different drainage
systems. In these cases the original streams of the region have
been broken or destroyed by the uplift of the mountain mass across
their paths.
On the whole, which have worked more rapidly, processes of
deformation or of denudation?
LAND FORMS DUE TO FOLDING
As folding goes on so slowly, it is never left to form surface
features unmodified by the action of other agencies. An anticlinal
fold is attacked by erosion as soon as it begins to rise above the
original level, and the higher it is uplifted, and the stronger
are its slopes, the faster is it worn away. Even while rising, a
young upfold is often thus unroofed, and instead of appearing as a
long, Smooth, boat-shaped ridge, it commonly has had opened along
the rocks of the axis, when these are weak, a valley which is
overlooked by the infacing escarpments of the hard layers of the
sides of the fold. Under long-continued erosion, anticlines may be
degraded to valleys, while the synclines of the same system may be
left in relief as ridges.
FOLDED MOUNTAINS. The vastness of the forces which wrinkle the
crust is best realized in the presence of some lofty mountain
range. All mountains, indeed, are not the result of folding. Some,
as we shall see, are due to upwarps or to fractures of the crust;
some are piles of volcanic material; some are swellings caused by
the intrusion of molten matter beneath the surface; some are the
relicts left after the long denudation of high plateaus.
But most of the mountain ranges of the earth, and some of the
greatest, such as the Alps and the Himalayas, were originally
mountains of folding. The earth's crust has wrinkled into a fold;
or into a series of folds, forming a series of parallel ridges and
intervening valleys; or a number of folds have been mashed
together into a vast upswelling of the crust, in which the layers
have been so crumpled and twisted, overturned and crushed, that it
is exceedingly difficult to make out the original structure.
The close and intricate folds seen in great mountain ranges were
formed, as we have seen, deep below the surface, within the zone
of folding. Hence they may never have found expression in any
individual surface features. As the result of these deformations
deep under ground the surface was broadly lifted to mountain
height, and the crumpled and twisted mountain structures are now
to be seen only because erosion has swept away the heavy cover of
surface rocks under whose load they were developed.
When the structure of mountains has been deciphered it is possible
to estimate roughly the amount of horizontal compression which the
region has suffered. If the strata of the folds of the Alps were
smoothed out, they would occupy a belt seventy-four miles wider
than that to which they have been compressed, or twice their
present width. A section across the Appalachian folds in
Pennyslvania shows a compression to about two thirds the original
width; the belt has been shortened thirty-five miles in every
hundred.
Considering the thickness of their strata, the compression which
mountains have undergone accounts fully for their height, with
enough to spare for all that has been lost by denudation.
The Appalachian folds involve strata thirty thousand feet in
thickness. Assuming that the folded strata rested on an unyielding
foundation, and that what was lost in width was gained in height,
what elevation would the range have reached had not denudation
worn it as it rose?
THE LIFE HISTORY OF MOUNTAINS. While the disturbance and uplift of
mountain masses are due to deformation, their sculpture into
ridges and peaks, valleys and deep ravines, and all the forms
which meet the eye in mountain scenery, excepting in the very
youngest ranges, is due solely to erosion. We may therefore
classify mountains according to the degree to which they have been
dissected. The Juras are an example of the stage of early youth,
in which the anticlines still persist as ridges and the synclines
coincide with the valleys; this they owe as much to the slight
height of their uplift as to the recency of its date.
The Alps were upheaved at various times, the last uplift being
later than the uplift of the Juras, but to so much greater height
that erosion has already advanced them well on towards maturity.
The mountain mass has been cut to the core, revealing strange
contortions of strata which could never have found expression at
the surface. Sharp peaks, knife-edged crests, deep valleys with
ungraded slopes subject to frequent landslides, are all features
of Alpine scenery typical of a mountain range at this stage in its
life history. They represent the survival of the hardest rocks and
the strongest structures, and the destruction of the weaker in
their long struggle for existence against the agents of erosion.
Although miles of rock have been removed from such ranges as the
Alps, we need not suppose that they ever stood much, if any,
higher than at present. All this vast denudation may easily have
been accomplished while their slow upheaval was going on; in
several mountain ranges we have evidence that elevation has not
yet ceased.
Under long denudation mountains are subdued to the forms
characteristic of old age. The lofty peaks and jagged crests of
their earlier life are smoothed down to low domes and rounded
crests. The southern Appalachians and portions of the Hartz
Mountains in Germany are examples of mountains which have reached
this stage.
There are numerous regions of upland and plains in which the rocks
are found to have the same structure that we have seen in folded
mountains; they are tilted, crumpled, and overturned, and have
clearly suffered intense compression. We may infer that their
folds were once lifted to the height of mountains and have since
been wasted to low-lying lands. Such a section as that of Figure
67 illustrates how ancient mountains may be leveled to their
roots, and represents the final stage to which even the Alps and
the Himalayas must sometime arrive. Mountains, perhaps of Alpine
height, once stood about Lake Superior; a lofty range once
extended from New England and New Jersey southwestward to Georgia
along the Piedmont belt. In our study of historic geology we shall
see more clearly how short is the life of mountains as the earth
counts time, and how great ranges have been lifted, worn away, and
again upheaved into a new cycle of erosion.
THE SEDIMENTARY HISTORY OF FOLDED MOUNTAINS. We may mention here
some of the conditions which have commonly been antecedent to
great foldings of the crust.
1. Mountain ranges are made of belts of enormously and
exceptionally thick sediments. The strata of the Appalachians are
thirty thousand feet thick, while the same formations thin out to
five thousand feet in the Mississippi valley. The folds of the
Wasatch Mountains involve strata thirty thousand feet thick, which
thin to two thousand feet in the region of the Plains.
2. The sedimentary strata of which mountains are made are for the
most part the shallow-water deposits of continental deltas.
Mountain ranges have been upfolded along the margins of
continents.
3. Shallow-water deposits of the immense thickness found in
mountain ranges can be laid only in a gradually sinking area. A
profound subsidence, often to be reckoned in tens of thousands of
feet, precedes the upfolding of a mountain range.
Thus the history of mountains of folding is as follows: For long
ages the sea bottom off the coast of a continent slowly subsides,
and the great trough, as fast as it forms, is filled with
sediments, which at last come to be many thousands of feet thick.
The downward movement finally ceases. A slow but resistless
pressure sets in, and gradually, and with a long series of many
intermittent movements, the vast mass of accumulated sediments is
crumpled and uplifted into a mountain range.
FRACTURES AND DISLOCATIONS OF THE CRUST
Considering the immense stresses to which the rocks of the crust
are subjected, it is not surprising to find that they often yield
by fracture, like brittle bodies, instead of by folding and
flowing, like plastic solids. Whether rocks bend or break depends
on the character and condition of the rocks, the load of overlying
rocks which they bear, and the amount of the force and the
slowness with which it is applied.
JOINTS. At the surface, where their load is least, we find rocks
universally broken into blocks of greater or less size by partings
known as joints. Under this name are included many division planes
caused by cooling and drying; but it is now generally believed
that the larger and more regular joints, especially those which
run parallel to the dip and strike of the strata, are fractures
due to up-and-down movements and foldings and twistings of the
rocks.
Joints are used to great advantage in quarrying, and we have seen
how they are utilized by the weather in breaking up rock masses,
by rivers in widening their valleys, by the sea in driving back
its cliffs, by glaciers in plucking their beds, and how they are
enlarged in soluble rocks to form natural passageways for
underground waters. The ends of the parted strata match along both
sides of joint planes; in. joints there has been little or no
displacement of the broken rocks.
FAULTS. In Figure 184 the rocks have been both broken and
dislocated along the plane ff'. One side must have been moved up
or down past the other. Such a dislocation is called a fault. The
amount of the displacement, as measured by the vertical distance
between the ends of a parted layer, is the throw. The angle which
the fault plane makes with the vertical is the HADE. In Figure 184
the right side has gone down relatively to the left; the right is
the side of the downthrow, while the left is the side of the
upthrow. Where the fault plane is not vertical the surfaces on the
two sides may be distinguished as the HANGING WALL and the FOOT
WALL. Faults differ in throw from a fraction of an inch to many
thousands of feet.
SLICKENSIDES. If we examine the walls of a fault, we may find
further evidence of movement in the fact that the surfaces are
polished and grooved by the enormous friction which they have
suffered as they have ground one upon the other. These
appearances, called sliekensides, have sometimes been mistaken for
the results of glacial action.
NORMAL FAULTS. Faults are of two kinds,--normal faults and thrust
faults. Normal faults, of which Figure 184 is an example, hade to
the downthrow; the hanging wall has gone down. The total length of
the strata has been increased by the displacement. It seems that
the strata have been stretched and broken, and that the blocks
have readjusted themselves under the action of gravity as they
settled.
THRUST FAULTS. Thrust faults hade to the upthrow; the hanging wall
has gone up. Clearly such faults, where the strata occupy less
space than before, are due to lateral thrust. Folds and thrust
faults are closely associated. Under lateral pressure strata may
fold to a certain point and then tear apart and fault along the
surface of least resistance. Under immense pressure strata also
break by shear without folding. Thus, in Figure 185, the rigid
earth block under lateral thrust has found it easier to break
along the fault plane than to fold. Where such faults are nearly
horizontal they are distinguished as THRUST PLANES.
In all thrust faults one mass has been pushed over another, so as
to bring the underlying and older strata upon younger beds; and
when the fault planes are nearly horizontal, and especially when
the rocks have been broken into many slices which have slidden far
one upon another, the true succession of strata is extremely hard
to decipher.
In the Selkirk Mountains of Canada the basement rocks of the
region have been driven east for seven miles on a thrust plane,
over rocks which originally lay thousands of feet above them.
Along the western Appalachians, from Virginia to Georgia, the
mountain folds are broken by more than fifteen parallel thrust
planes, running from northeast to southwest, along which the older
strata have been pushed westward over the younger. The longest
continuous fault has been traced three hundred and seventy-five
miles, and the greatest horizontal displacement has been estimated
at not less than eleven miles.
CRUSH BRECCIA. Rocks often do not fault with a clean and simple
fracture, but along a zone, sometimes several yards in width, in
which they are broken to fragments. It may occur also that strata
which as a whole yield to lateral thrust by folding include beds
of brittle rocks, such as thin-layered limestones, which are
crushed to pieces by the strain. In either case the fragments when
recemented by percolating waters form a rock known as a CRUSH
BRECCIA (pronounced BRETCHA).
Breccia is a term applied to any rock formed of cemented ANGULAR
fragments. This rock may be made by the consolidation of volcanic
cinders, of angular waste at the foot of cliffs, or of fragments
of coral torn by the waves from coral reefs, as well as of strata
crushed by crustal movements.
SURFACE FEATURES DUE TO DISLOCATIONS
FAULT SCARPS. A fault of recent date may be marked at surface by a
scarp, because the face of the upthrown block has not yet been
worn to the level of the downthrow side.
After the upthrown block has been worn down to this level,
differential erosion produces fault scarps wherever weak rocks and
resistant rocks are brought in contact along the fault plane; and
the harder rocks, whether on the upthrow or the downthrow side,
emerge in a line of cliffs. Where a fault is so old that no abrupt
scarps appear, its general course is sometimes marked by the line
of division between highland and lowland or hill and plain. Great
faults have sometimes brought ancient crystalline rocks in contact
with weaker and younger sedimentary rocks, and long after erosion
has destroyed all fault scarps the harder crystallines rise in an
upland of rugged or mountainous country which meets the lowland
along the line of faulting.
The vast majority of faults give rise to no surface features. The
faulted region may be old enough to have been baseleveled, or the
rocks on both sides of the line of dislocation may be alike in
their resistance to erosion and therefore have been worn down to a
common slope. The fault may be entirely concealed by the mantle of
waste, and in such cases it can be inferred from abrupt changes in
the character or the strike and dip of the strata where they may
outcrop near it.
The plateau trenched by the Grand Canyon of the Colorado River
exhibits a series of magnificent fault scarps whose general course
is from north to south, marking the edges of the great crust
blocks into which the country has been broken. The highest part of
the plateau is a crust block ninety miles long and thirty-five
miles in maximum width, which has been hoisted to nine thousand
three hundred feet above, sea level. On the east it descends four
thousand feet by a monoclinal fold, which passes into a fault
towards the north. On the west it breaks down by a succession of
terraces faced by fault scarps. The throw of these faults varies
from seven hundred feet to more than a mile. The escarpments,
however, are due in a large degree to the erosion of weaker rock
on the downthrow side.
The Highlands of Scotland meet the Lowlands on the south with a
bold front of rugged hills along a line of dislocation which runs
across the country from sea to sea. On the one side are hills of
ancient crystalline rocks whose crumpled structures prove that
they are but the roots of once lofty mountains; on the other lies
a lowland of sandstone and other stratified rocks formed from the
waste of those long-vanished mountain ranges. Remnants of
sandstone occur in places on the north of the great fault, and are
here seen to rest on the worn and fairly even surface of the
crystallines. We may infer that these ancient mountains were
reduced along their margins to low plains, which were slowly
lowered beneath the sea to receive a cover of sedimentary rocks.
Still later came an uplift and dislocation. On the one side
erosion has since stripped off the sandstones for the most part,
but the hard crystalline rocks yet stand in bold relief. On the
other side the weak sedimentary rocks have been worn down to
lowlands.
RIFT VALLEYS. In a broken region undergoing uplift or the unequal
settling which may follow, a slice inclosed between two fissures
may sink below the level of the crust blocks on either side, thus
forming a linear depression known as a rift valley, or valley of
fracture.
One of the most striking examples of this rare type of valley is
the long trough which runs straight from the Lebanon Mountains of
Syria on the north to the Red Sea on the south, and whose central
portion is occupied by the Jordan valley and the Dead Sea. The
plateau which it gashes has been lifted more than three thousand
feet above sea level, and the bottom of the trough reaches a depth
of two thousand six hundred feet below that level in parts of the
Dead Sea. South of the Dead Sea the floor of the trough rises
somewhat above sea level, and in the Gulf of Akabah again sinks
below it. This uneven floor could be accounted for either by the
profound warping of a valley of erosion or by the unequal
depression of the floor of a rift valley. But that the trough is a
true valley of fracture is proved by the fact that on either side
it is bounded by fault scarps and monoclinal folds. The keystone
of the arch has subsided. Many geologists believe that the Jordan-
Akabah trough, the long narrow basin of the Red Sea, and the chain
of down-faulted valleys which in Africa extends from the strait of
Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain
more than thirty lakes--belong to a single system of dislocation.
Should you expect the lateral valleys of a rift valley at the time
of its formation to enter it as hanging valleys or at a common
level?
BLOCK MOUNTAINS. Dislocations take place on so grand a scale that
by the upheaval of blocks of the earth's crust or the down-
faulting of the blocks about one which is relatively stationary,
mountains known as block mountains are produced. A tilted crust
block may present a steep slope on the side upheaved and a more
gentle descent on the side depressed.
THE BASIN RANGES. The plateaus of the United States bounded by the
Rocky Mouirtains on the east, and on the west by the ranges which
front the Pacific, have been profoundly fractured and faulted. The
system of great fissures by which they are broken extends north
and south, and the long, narrow, tilted crust blocks intercepted
between the fissures give rise to the numerous north-south ranges
of the region. Some of the tilted blocks, as those of southern
Oregon, are as yet but moderately carved by erosion, and shallow
lakes lie on the waste that has been washed into the depressions
between them. We may therefore conclude that their displacement is
somewhat recent. Others, as those of Nevada, are so old that they
have been deeply dissected; their original form has been destroyed
by erosion, and the intermontane depressions are occupied by wide
plains of waste.
DISLOCATIONS AND RIVER VALLEYS. Before geologists had proved that
rivers can by their own unaided efforts cut deep canyons, it was
common to consider any narrow gorge as a gaping fissure of the
crust. This crude view has long since been set aside. A map of the
plateaus of northern Arizona shows how independent of the immense
faults of the region is the course of the Colorado River. In the
Alps the tunnels on the Saint Gotthard railway pass six times
beneath the gorge of the Reuss, but at no point do the rocks show
the slightest trace of a fault.
RATE OF DISLOCATION. So far as human experience goes, the earth
movements which we have just studied, some of which have produced
deep-sunk valleys and lofty mountain ranges, and faults whose
throw is to be measured in thousands of feet, are slow and
gradual. They are not accomplished by a single paroxysmal effort,
but by slow creep and a series of slight slips continued for vast
lengths of time.
In the Aspen mining district in Colorado faulting is now going on
at a comparatively rapid rate. Although no sudden slips take
place, the creep of the rock along certain planes of faulting
gradually bends out of shape the square-set timbers in horizontal
drifts and has closed some vertical shafts by shifting the upper
portion across the lower. Along one of the faults of this region
it is estimated that there has been a movement of at least four
hundred feet since the Glacial epoch. More conspicuous are the
instances of active faulting by means of sudden slips. In 1891
there occurred along an old fault plane in Japan a slip which
produced an earth rent traced for fifty miles (Fig. 192). The
country on one side was depressed in places twenty feet below that
on the other, and also shifted as much as thirteen feet
horizontally in the direction of the fault line.
In 1872 a slip occurred for forty miles on the great line of
dislocation which runs along the eastern base of the Sierra Nevada
Mountains. In the Owens valley, California, the throw amounted to
twenty-five feet in places, with a horizontal movement along the
fault line of as much as eighteen feet. Both this slip and that in
Japan just mentioned caused severe earthquakes.
For the sake of clearness we have described oscillations,
foldings, and fractures of the crust as separate processes, each
giving rise to its own peculiar surface features, but in nature
earth movements are by no means so simple,--they are often
implicated with one another: folds pass into faults; in a deformed
region certain rocks have bent, while others under the same
strain, but under different conditions of plasticity and load,
have broken; folded mountains have been worn to their roots, and
the peneplains to which they have been denuded have been upwarped
to mountain height and afterwards dissected,--as in the case of
the Alleghany ridges, the southern Carpathians, and other ranges,
--or, as in the case of the Sierra Nevada Mountains, have been
broken and uplifted as mountains of fracture.
Draw the following diagrams, being careful to show the direction
in which the faulted blocks have moved, by the position of the two
parts of some well-defined layer of limestone, sandstone, or
shale, which occurs on each side of the fault plane, as in Figure
184.
1. A normal fault with a hade of 15 degrees, the original fault
scarp remaining.
2. A normal fault with a hade of 50 degrees, the original fault
scarp worn away, showing cliffs caused by harder strata on the
downthrow side.
3. A thrust fault with a hade of 30 degrees, showing cliffs due to
harder strata outcropping on the downthrow.
4. A thrust fault with a hade of 80 degrees, with surface
baseleveled.
5. In a region of normal faults a coal mine is being worked along
the seam of coal AB (Fig. 193). At B it is found broken by a fault
f which hades toward A. To find the seam again, should you advise
tunneling up or down from B?
6. In a vertical shaft of a coal mine the same bed of coal is
pierced twice at different levels because of a fault. Draw a
diagram to show whether the fault is normal or a thrust.
7. Copy the diagram in Figure 194, showing how the two ridges may
be accounted for by a single resistant stratum dislocated by a
fault. Is the fault a STRIKE FAULT, i.e. one running parallel with
the strike of the strata, or a DIP FAULT, one running parallel
with the direction of the dip?
8. Draw a diagram of the block in Figure 195 as it would appear if
dislocated along the plane efg by a normal fault whose throw
equals one fourth the height of the block. Is the fault a strike
or a dip fault? Draw a second diagram showing the same block after
denudation has worn it down below the center of the upthrown side.
Note that the outcrop of the coal seam is now deceptively
repeated. This exercise may be done in blocks of wood instead of
drawings.
9. Draw diagrams showing by dotted lines the conditions both of A
and of B, Figure 196, after deformation had given the strata their
present attitude.
10. What is the attitude of the strata of this earth block, Figure
197? What has taken place along the plane bef? When did the
dislocation occur compared with the folding of the strata? With
the erosion of the valleys on the right-hand side of the mountain?
With the deposition of the sediments? Do you find any remnants of
the original surface baf produced by the dislocation? From the
left-hand side of the mountain infer what was the relief of the
region before the dislocation. Give the complete history recorded
in the diagram from the deposition of the strata to the present.
11. Which is the older fault, in Figure 198, or When did the lava
flow occur? How long a time elapsed between the formation of the
two faults as measured in the work done in the interval? How long
a time since the formation of the later fault?
12. Measure by the scale the thickness lie of the coal-bearing
strata outcropping from a to b in Figure 199. On any convenient
scale draw a similar section of strata with a dip of 30 degrees
outcropping along a horizontal line normal to the strike one
thousand feet in length, and measure the thickness of the strata
by the scale employed. The thickness may also be calculated by
trigonometry.
UNCONFORMITY
Strata deposited one upon, another in an unbroken succession are
said to be conformable. But the continuous deposition of strata is
often interrupted by movements of the earth's crust, Old sea
floors are lifted to form land and are again depressed beneath the
sea to receive a cover of sediments only after an interval during
which they were carved by subaerial erosion. An erosion surface
which thus parts older from younger strata is known as an
UNCONFORMITY, and the strata above it are said to be UNCONFORMABLE
with the rocks below, or to rest unconformably upon them. An
unconformity thus records movements of the crust and a consequent
break in the deposition of the strata. It denotes a period of land
erosion of greater or less length, which may sometimes be roughly
measured by the stage in the erosion cycle which the land surface
had attained before its burial. Unconformable strata may be
parallel, as in Figure 200, where the record includes the
deposition of strata, their emergence, the erosion of the land
surface, a submergence and the deposit of the strata, and lastly,
emergence and the erosion of the present surface.
Often the earth movements to which the uplift or depression was
due involved tilting or folding of the earlier strata, so that the
strata are now nonparallel as well as unconformable. In Figure
201, for example, the record includes deposition, uplift, and
tilting of a; erosion, depression, the deposit of b; and finally
the uplift which has brought the rocks to open air and permitted
the dissection by which the unconformity is revealed. From this
section infer that during early Silurian times the area was sea,
and thick sea muds were laid upon it. These were later altered to
hard slates by pressure and upfolded into mountains. During the
later Silurian and the Devonian the area was land and suffered
vast denudation. In the Carboniferous period it was lowered
beneath the sea and received a cover of limestone.
THE AGE OF MOUNTAINS. It is largely by means of unconformities
that we read the history of mountain making and other deformations
and movements of the crust. In Figure 203, for example, the
deformation which upfolded the range of mountains took place after
the deposit of the series of strata a of which the mountains are
composed, and before the deposit of the stratified rocks, which
rest unconformably on a and have not shared their uplift.
Most great mountain ranges, like the Sierra Nevada and the Alps,
mark lines of weakness along which the earth's crust has yielded
again and again during the long ages of geological time. The
strata deposited at various times about their flanks have been
infolded by later crumplings with the original mountain mass, and
have been repeatedly crushed, inverted, faulted, intruded with
igneous rocks, and denuded. The structure of great mountain ranges
thus becomes exceedingly complex and difficult to read. A
comparatively simple case of repeated uplift is shown in Figure
204. In the section of a portion of the Alps shown in Figure 179 a
far more complicated history may be deciphered.
UNCONFORMITIES IN THE COLORADO CANYON, ARIZONA. How geological
history may be read in unconformities is further illustrated in
Figures 207 and 208. The dark crystalline rocks a at the bottom of
the canyon are among the most ancient known, and are overlain
unconformably by a mass of tilted coarse marine sandstones b,
whose total thickness is not seen in the diagram and measures
twelve thousand feet perpendicularly to the dip. Both a and b rise
to a common level nn and upon them rest the horizontal sea-laid
strata c, in which the upper portion of the canyon has been cut.
Note that the crystalline rocks a have been crumpled and crushed.
Comparing their structure with that of folded mountains, what do
you infer as to their relief after their deformation? To which
surface were they first worn down, mm' or nm? Describe and account
for the surface mm'. How does it differ from the surface of the
crystalline rocks seen in the Torridonian Mountains, and why? This
surface mm' is one of the oldest land surfaces of which any
vestige remains.
It is a bit of fossil geography buried from view since the
earliest geological ages and recently brought to light by the
erosion of the canyon.
How did the surface mm' come to receive its cover of sandstones b?
From the thickness and coarseness of these sediments draw
inferences as to the land mass from which they were derived. Was
it rising or subsiding? high or low? Were its streams slow or
swift? Was the amount of erosion small or great?
Note the strong dip of these sandstones b. Was the surface mm'
tilted as now when the sandstones were deposited upon it? When was
it tilted? Draw a diagram showing the attitude of the rocks after
this tilting occurred, and their height relative to sea level.
The surface nn' is remarkably even, although diversified by some
low hills which rise into the bedded rocks of c, and it may be
traced for long distances up and down the canyon. Were the layers
of b and the surface mm' always thus cut short by nn' as now? What
has made the surface nn' so even? How does it come to cross the
hard crystalline rocks a and the weaker sandstones b at the same
impartial level? How did the sediments of c come to be laid upon
it? Give now the entire history recorded in the section, and in
addition that involved in the production of the platform P, shown
in Figure 130, and that of the cutting of the canyon. How does the
time involved in the cutting of the canyon compare with that
required for the production of the surfaces mm', nn', and P?
CHAPTER X
EARTHQUAKES
Any sudden movement of the rocks of the crust, as when they tear
apart when a fissure is formed or extended, or slip from time to
time along a growing fault, produces a jar called an earthquake,
which spreads in all directions from the place of disturbance.
THE CHARLESTON EARTHQUAKE. On the evening of August 31, 1886, the
city of Charleston, S.C., was shaken by one of the greatest
earthquakes which has occurred in the United States. A slight
tremor which rattled the windows was followed a few seconds later
by a roar, as of subterranean thunder, as the main shock passed
beneath the city. Houses swayed to and fro, and their heaving
floors overturned furniture and threw persons off their feet as,
dizzy and nauseated, they rushed to the doors for safety. In sixty
seconds a number of houses were completely wrecked, fourteen
thousand chimneys were toppled over, and in all the city scarcely
a building was left without serious injury. In the vicinity of
Charleston railways were twisted and trains derailed. Fissures
opened in the loose superficial deposits, and in places spouted
water mingled with sand from shallow underlying aquifers.
The point of origin, or FOCUS, of the earthquake was inferred from
subsequent investigations to be a rent in the rocks about twelve
miles beneath the surface. From the center of greatest
disturbance, which lay above the focus, a few miles northwest of
the city, the surface shock traveled outward in every direction,
with decreasing effects, at the rate of nearly two hundred miles
per minute. It was felt from Boston to Cuba, and from eastern Iowa
to the Bermudas, over a circular area whose diameter was a
thousand miles.
An earthquake is transmitted from the focus through the elastic
rocks of the crust, as a wave, or series of waves, of compression
and rarefaction, much as a sound wave is transmitted through the
elastic medium of the air. Each earth particle vibrates with
exceeding swiftness, but over a very short path. The swing of a
particle in firm rock seldom exceeds one tenth of an inch in
ordinary earthquakes, and when it reaches one half an inch and an
inch, the movement becomes dangerous and destructive.
The velocity of earthquake waves, like that of all elastic waves,
varies with the temperature and elasticity of the medium. In the
deep, hot, elastic rocks they speed faster than in the cold and
broken rocks near the surface. The deeper the point of origin and
the more violent the initial shock, the faster and farther do the
vibrations run.
Great earthquakes, caused by some sudden displacement or some
violent rending of the rocks, shake the entire planet. Their waves
run through the body of the earth at the rate of about three
hundred and fifty miles a minute, and more slowly round its
circumference, registering their arrival at opposite sides of the
globe on the exceedingly delicate instruments of modern earthquake
observatories.
GEOLOGICAL EFFECTS. Even great earthquakes seldom produce
geological effects of much importance. Landslides may be shaken
down from the sides of mountains and hills, and cracks may be
opened in the surface deposits of plains; but the transient
shiver, which may overturn cities and destroy thousands of human
lives, runs through the crust and leaves it much the same as
before.
EARTHQUAKES ATTENDING GREAT DISPLACEMENTS. Great earthquakes
frequently attend the displacement of large masses of the rocks of
the crust. In 1822 the coast of Chile was suddenly raised three or
four feet, and the rise was five or six feet a mile inland. In
1835 the same region was again upheaved from two to ten feet. In
each instance a destructive earthquake was felt for one thousand
miles along the coast.
THE GREAT CALIFORNIA EARTHQUAKE OF 1906. A sudden dislocation
occurred in 1906 along an ancient fault plane which extends for
300 miles through western California. The vertical displacement
did not exceed four feet, while the horizontal shifting reached a
maximum of twenty feet. Fences, rows of trees, and roads which
crossed the fault were broken and offset. The latitude and
longitude of all points over thousands of square miles were
changed. On each side of the fault the earth blocks moved in
opposite directions, the block on the east moving southward and
that on the west moving northward and to twice the distance. East
and west of the fault the movements lessened with increasing
distance from it.
This sudden slip set up an earthquake lasting sixty-five seconds,
followed by minor shocks recurring for many days. In places the
jar shook down the waste on steep hillsides, snapped off or
uprooted trees, and rocked houses from their foundations or threw
down their walls or chimneys. The water mains of San Francisco
were broken, and the city was thus left defenseless against a
conflagration which destroyed $500,000,000 worth of property. The
destructive effects varied with the nature of the ground.
Buildings on firm rock suffered least, while those on deep
alluvium were severely shaken by the undulations, like water
waves, into which the loose material was thrown. Well-braced steel
structures, even of the largest size, were earthquake proof, and
buildings of other materials, when honestly built and
intelligently designed to withstand earthquake shocks, usually
suffered little injury. The length of the intervals between severe
earthquakes in western California shows that a great dislocation
so relieves the stresses of the adjacent earth blocks that scores
of years may elapse before the stresses again accumulate and cause
another dislocation.
Perhaps the most violent earthquake which ever visited the United
States attended the depression, in 1812, of a region seventy-five
miles long and thirty miles wide, near New Madrid, Mo. Much of the
area was converted into swamps and some into shallow lakes, while
a region twenty miles in diameter was bulged up athwart the
channel of the Mississippi. Slight quakes are still felt in this
region from time to time, showing that the strains to which the
dislocation was due have not yet been fully relieved.
EARTHQUAKES ORIGINATING BENEATH THE SEA. Many earthquakes
originate beneath the sea, and in a number of examples they seem
to have been accompanied, as soundings indicate, by local
subsidences of the ocean bottom. There have been instances where
the displacement has been sufficient to set the entire Pacific
Ocean pulsating for many hours. In mid ocean the wave thus
produced has a height of only a few feet, while it may be two
hundred miles in width. On shores near the point of origin
destructive waves two or three score feet in height roll in, and
on coasts thousands of miles distant the expiring undulations may
be still able to record themselves on tidal gauges.
DISTRIBUTION OF EARTHQUAKES. Every half hour some considerable
area of the earth's surface is sensibly shaken by an earthquake,
but earthquakes are by no means uniformly distributed over the
globe. As we might infer from what we know as to their causes,
earthquakes are most frequent in regions now undergoing
deformation. Such are young rising mountain ranges, fault lines
where readjustments recur from time to time, and the slopes of
suboceanic depressions whose steepness suggests that subsidence
may there be in progress.
Earthquakes, often of extreme severity, frequently visit the lofty
and young ranges of the Andes, while they are little known in the
subdued old mountains of Brazil. The Highlands of Scotland are
crossed by a deep and singularly straight depression called the
Great Glen, which has been excavated along a very ancient line of
dislocation. The earthquakes which occur from time to time in this
region, such as the Inverness earthquake in 1891, are referred to
slight slips along this fault plane.
In Japan, earthquakes are very frequent. More than a thousand are
recorded every year, and twenty-nine world-shaking earthquakes
occurred in the three years ending with 1901. They originate, for
the most part, well down on the eastern flank of the earth fold
whose summit is the mountainous crest of the islands, and which
plunges steeply beneath the sea to the abyss of the Tuscarora
Deep.
MINOR CAUSES OF EARTHQUAKES. Since any concussion within the crust
sets up an earth jar, there are several minor causes of
earthquakes, such as volcanic explosions and even the collapse of
the roofs of caves. The earthquakes which attend the eruption of
volcanoes are local, even in the case of the most violent volcanic
paroxysms known. When the top of a volcano has been blown to
fragments, the accompanying earth shock has sometimes not been
felt more than twenty-five miles away.
DEPTH OF FOCUS. The focus of the Charleston earthquake, estimated
at about twelve miles below the surface, was exceptionally deep.
Volcanic earthquakes are particularly shallow, and probably no
earthquakes known have started at a greater depth than fifteen or
twenty miles. This distance is so slight compared with the earth's
radius that we may say that earthquakes are but skin-deep.
Should you expect the velocity of an earthquake to be greater in a
peneplain or in a river delta?
After an earthquake, piles on which buildings rested were found
driven into the ground, and chimneys crushed at base. From what
direction did the shock come?
Chimneys standing on the south walls of houses toppled over on the
roof. Should you infer that the shock in this case came from the
north or south?
How should you expect a shock from the east to affect pictures
hanging on the east and the west walls of a room? how the pictures
hanging on the north and the south walls?
In parts of the country, as in southwestern Wisconsin, slender
erosion pillars, or "monuments," are common. What inference could
you draw as to the occurrence in such regions of severe
earthquakes in the recent past?
CHAPTER XI
VOLCANOES
Connected with movements of the earth's crust which take place so
slowly that they can be inferred only from their effects is one of
the most rapid and impressive of all geological processes,--the
extrusion of molten rock from beneath the surface of the earth,
giving rise to all the various phenomena of volcanoes.
In a volcano, molten rock from a region deep below, which we may
call its reservoir, ascends through a pipe or fissure to the
surface. The materials erupted may be spread over vast areas, or,
as is commonly the case, may accumulate about the opening, forming
a conical pile known as the volcanic cone. It is to this cone that
popular usage refers the word VOLCANO; but the cone is simply a
conspicuous part of the volcanic mechanism whose still more
important parts, the reservoir and the pipe, are hidden from view.
Volcanic eruptions are of two types,--EFFUSIVE eruptions, in which
molten rock wells up from below and flows forth in streams of LAVA
(a comprehensive term applied to all kinds of rock emitted from
volcanoes in a molten state), and EXPLOSIVE eruptions, in which
the rock is blown out in fragments great and small by the
expansive force of steam.
ERUPTIONS OF THE EFFUSIVE TYPE
THE HAWAIIAN VOLCANOES. The Hawaiian Islands are all volcanic in
origin, and have a linear arrangement characteristic of many
volcanic groups in all parts of the world. They are strung along a
northwest-southeast line, their volcanoes standing in two parallel
rows as if reared along two adjacent lines of fracture or folding.
In the northwestern islands the volcanoes have long been extinct
and are worn low by erosion. In the southeastern island. Hawaii,
three volcanoes are still active and in process of building. Of
these Mauna Loa, the monarch of volcanoes, with a girth of two
hundred miles and a height of nearly fourteen thousand feet above
sea level, is a lava dome the slope of whose sides does not
average more than five degrees. On the summit is an elliptical
basin ten miles in circumference and several hundred feet deep.
Concentric cracks surround the rim, and from time to time the
basin is enlarged as great slices are detached from the vertical
walls and engulfed.
Such a volcanic basin, formed by the insinking of the top of the
cone, is called a CALDERA.
On the flanks of Mauna Loa, four thousand feet above sea level,
lies the caldera of Kilauea, an independent volcano whose dome has
been joined to the larger mountain by the gradual growth of the
two. In each caldera the floor, which to the eye is a plain of
black lava, is the congealed surface of a column of molten rock.
At times of an eruption lakes of boiling lava appear which may be
compared to air holes in a frozen river. Great waves surge up,
lifting tons of the fiery liquid a score of feet in air, to fall
back with a mighty plunge and roar, and occasionally the lava
rises several hundred feet in fountains of dazzling brightness.
The lava lakes may flood the floor of the basin, but in historic
times have never been known to fill it and overflow the rim.
Instead, the heavy column of lava breaks way through the sides of
the mountain and discharges in streams which flow down the
mountain slopes for a distance sometimes of as much as thirty-five
miles. With the drawing off of the lava the column in the duct of
the volcano lowers, and the floor of the caldera wholly or in part
subsides. A black and steaming abyss marks the place of the lava
lakes. After a time the lava rises in the duct, the floor is
floated higher, and the boiling lakes reappear.
The eruptions of the Hawaiian volcanoes are thus of the effusive
type. The column of lava rises, breaks through the side of the
mountain, and discharges in lava streams. There are no explosions,
and usually no earthquakes, or very slight ones, accompany the
eruptions. The lava in the calderas boils because of escaping
steam, but the vapor emitted is comparatively little, and seldom
hangs above the summits in heavy clouds. We see here in its
simplest form the most impressive and important fact in all
volcanic action, molten rock has been driven upward to the surface
from some deep-lying source.
LAVA FLOWS. As lava issues from the side of a volcano or overflows
from the summit, it flows away in a glowing stream resembling
molten iron drawn white-hot from an iron furnace. The surface of
the stream soon cools and blackens, and the hard crust of
nonconducting rock may grow thick and firm enough to form a
tunnel, within which the fluid lava may flow far before it loses
its heat to any marked degree. Such tunnels may at last be left as
caves by the draining away of the lava, and are sometimes several
miles in length.
PAHOEHOE AND AA. When the crust of highly fluid lava remains
unbroken after its first freezing, it presents a smooth, hummocky,
and ropy surface known by the Hawaiian term PAHOEHOE. On the other
hand, the crust of a viscid flow may be broken and splintered as
it is dragged along by the slowly moving mass beneath. The stream
then appears as a field of stones clanking and grinding on, with
here and there from some chink a dull red glow or a wisp of steam.
It sets to a surface called AA, of broken, sharp-edged blocks,
which is often both difficult and dangerous to traverse.
FISSURE ERUPTIONS. Some of the largest and most important outflows
of lava have not been connected with volcanic cones, but have been
discharged from fissures, flooding the country far and wide with
molten rock. Sheet after sheet of molten rock has been
successively outpoured, and there have been built up, layer upon
layer, plateaus of lava thousands of feet in thickness and many
thousands of square miles in area.
ICELAND. This island plateau has been rent from time to time by
fissures from which floods of lava have outpoured. In some
instances the lava discharges along the whole length of the
fissure, but more often only at certain points upon it. The Laki
fissure, twenty miles long, was in eruption in 1783 for seven
months. The inundation of fluid rock which poured from it is the
largest of historic record, reaching a distance of forty-seven
miles and covering two hundred and twenty square miles to an
average depth of a hundred feet. At the present time the fissure
is traced by a line of several hundred insignificant mounds of
fragmental materials which mark where the lava issued.
The distance to which the fissure eruptions of Iceland flow on
slopes extremely gentle is noteworthy. One such stream is ninety
miles in length, and another seventy miles long has a slope of
little more than one half a degree.
Where lava is emitted at one point and flows to a less distance
there is gradually built up a dome of the shape of an inverted
saucer with an immense base but comparatively low. Many LAVA DOMES
have been discovered in Iceland, although from their exceedingly
gentle slopes, often but two or three degrees, they long escaped
the notice of explorers.
The entire plateau of Iceland, a region as large as Ohio, is
composed of volcanic products,--for the most part of successive
sheets of lava whose total thickness falls little short of two
miles. The lava sheets exposed to view were outpoured in open air
and not beneath the sea; for peat bogs and old forest grounds are
interbedded with them, and the fossil plants of these vegetable
deposits prove that the plateau has long been building and is very
ancient. On the steep sea cliffs of the island, where its
structure is exhibited, the sheets of lava are seen to be cut with
many DIKES,--fissures which have been filled by molten rock,--and
there is little doubt that it was through these fissures that the
lava outwelled in successive flows which spread far and wide over
the country and gradually reared the enormous pile of the plateau.
ERUPTIONS OF THE EXPLOSIVE TYPE
In the majority of volcanoes the lava which rises in the pipe is
at least in part blown into fragments with violent explosions and
shot into the air together with vast quantities of water vapor and
various gases. The finer particles into--which the lava is
exploded are called VOLCANIC DUST or VOLCANIC ASHES, and are often
carried long distances by the wind before they settle to the
earth. The coarser fragments fall about the vent and there
accumulate in a steep, conical, volcanic mountain. As successive
explosions keep open the throat of the pipe, there remains on the
summit a cup-shaped depression called the CRATER.
STROMBOLI. To study the nature of these explosions we may visit
Stromboli, a low volcano built chiefly of fragmental materials,
which rises from the sea off the north coast of Sicily and is in
constant though moderate action.
Over the summit hangs a cloud of vapor which strikingly resembles
the column of smoke puffed from the smokestack of a locomotive, in
that it consists of globular masses, each the product of a
distinct explosion. At night the cloud of vapor is lighted with a
red glow at intervals of a few minutes, like the glow on the trail
of smoke behind the locomotive when from time to time the fire bos
is opened. Because of this intermittent light flashing thousands
of feet above the sea, Stromboli has been given the name of the
Lighthouse of the Mediterranean.
Looking down into the crater of the volcano, one sees a viscid
lava slowly seething. The agitation gradually increases. A great
bubble forms. It bursts with an explosion which causes the walls
of the crater to quiver with a miniature earthquake, and an
outrush of steam carries the fragments of the bubble aloft for a
thousand feet to fall into the crater or on the mountain side
about it. With the explosion the cooled and darkened crust of the
lava is removed, and the light of the incandescent liquid beneath
is reflected from the cloud of vapor which overhangs the cone.
At Stromboli we learn the lesson that the explosive force in
volcanoes is that of steam. The lava in the pipe is permeated with
it much as is a thick boiling porridge. The steam in boiling
porridge is unable to escape freely and gathers into bubbles which
in breaking spurt out drops of the pasty substance; in the same
way the explosion of great bubbles of steam in the viscid lava
shoots clots and fragments of it into the air.
KRAKATOA. The most violent eruption of history, that of Krakatoa,
a small volcanic island in the strait between Sumatra and Java,
occurred in the last week of August, 1883. Continuous explosions
shot a column of steam and ashes. seventeen miles in air. A black
cloud, beneath which was midnight darkness and from which fell a
rain of ashes and stones, overspread the surrounding region to a
distance of one hundred and fifty miles. Launched on the currents
of the upper air, the dust was swiftly carried westward to long
distances. Three days after the eruption it fell on the deck of a
ship sixteen hundred miles away, and in thirteen days the finest
impalpable powder from the volcano had floated round the globe.
For many months the dust hung over Europe and America as a faint
lofty haze illuminated at sunrise and sunset with brilliant
crimson. In countries nearer the eruption, as in India and Africa,
the haze for some time was so thick that it colored sun and moon
with blue, green, and copper-red tints and encircled them with
coronas.
At a distance of even a thousand miles the detonations of the
eruption sounded like the booming of heavy guns a few miles away.
In one direction they were audible for a distance as great as that
from San Francisco to Cleveland. The entire atmosphere was thrown
into undulations under which all barometers rose and fell as the
air waves thrice encircled the earth. The shock of the explosions
raised sea waves which swept round the adjacent shores at a height
of more than fifty feet, and which were perceptible halfway around
the globe.
At the close of the eruption it was found that half the mountain
had been blown away, and that where the central part of the island
had been the sea was a thousand feet deep.
MARTINIQUE AND ST. VINCENT. In 1902 two dormant volcanoes of the
West Indies, Mt. Pelee in Martinique and Soufriere in St. Vincent,
broke into eruption simultaneously. No lava was emitted, but there
were blown into the air great quantities of ashes, which mantled
the adjacent parts of the islands with a pall as of gray snow. In
early stages of the eruption lakes which occupied old craters were
discharged and swept down the ash-covered mountain valleys in
torrents of boiling mud.
On several occasions there was shot from the crater of each
volcano a thick and heavy cloud of incandescent ashes and steam,
which rushed down the mountain side like an avalanche, red with
glowing stones and scintillating with lightning flashes. Forests
and buildings in its path were leveled as by a tornado, wood was
charred and set on fire by the incandescent fragments, all
vegetation was destroyed, and to breathe the steam and hot,
suffocating dust of the cloud was death to every living creature.
On the morning of the 8th of May, 1902, the first of these
peculiar avalanches from Mt. Pelee fell on the city of St. Pierre
and instantly destroyed the lives of its thirty thousand
inhabitants.
The eruptions of many volcanoes partake of both the effusive and
the explosive types: the molten rock in the pipe is in part blown
into the air with explosions of steam, and in part is discharged
in streams of lava over the lip of the crater and from fissures in
the sides of the cone. Such are the eruptions of Vesuvius, one of
which is illustrated in Figure 219.
SUBMARINE ERUPTIONS. The many volcanic islands of the ocean and
the coral islands resting on submerged volcanic peaks prove that
eruptions have often taken place upon the ocean floor and have
there built up enormous piles of volcanic fragments and lava. The
Hawaiian volcanoes rise from a depth of eighteen thousand feet of
water and lift their heads to about thirty thousand feet above the
ocean bed. Christmas Island (see p. 194), built wholly beneath the
ocean, is a coral-capped volcanic peak, whose total height, as
measured from the bottom of the sea, is more than fifteen thousand
feet. Deep-sea soundings have revealed the presence of numerous
peaks which fail to reach sea level and which no doubt are
submarine volcanoes. A number of volcanoes on the land were
submarine in their early stages, as, for example, the vast pile of
Etna, the celebrated Sicilian volcano, which rests on stratified
volcanic fragments containing marine shells now uplifted from the
sea.
Submarine outflows of lava and deposits of volcanic fragments
become covered with sediments during the long intervals between
eruptions. Such volcanic deposits are said to be CONTEMPORANEOUS,
because they are formed during the same period as the strata among
which they are imbedded. Contemporaneous lava sheets may be
expected to bake the surface of the stratum on which they rest,
while the sediments deposited upon them are unaltered by their
heat. They are among the most permanent records of volcanic
action, far outlasting the greatest volcanic mountains built in
open air.
From upraised submarine volcanoes, such as Christmas Island, it is
learned that lava flows which are poured out upon the bottom of
the sea do not differ materially either in composition or texture
from those of the land.
VOLCANIC PRODUCTS
Vast amounts of steam are, as we have seen, emitted from
volcanoes, and comparatively small quantities of other vapors,
such as various acid and sulphurous gases. The rocks erupted from
volcanoes differ widely in chemical composition and in texture.
ACIDIC AND BASIC LAVAS. Two classes of volcanic rocks may be
distinguished,--those containing a large proportion of silica
(silicic acid, SiO2) and therefore called ACIDIC, and those
containing less silica and a larger proportion of the bases (lime,
magnesia, soda, etc.) and therefore called BASIC. The acidic
lavas, of which RHYOLITE and THRACHYTE are examples, are
comparatively light in color and weight, and are difficult to
melt. The basic lavas, of which BASALT is a type, are dark and
heavy and melt at a lower temperature.
SCORIA AND PUMICE. The texture of volcanic rocks depends in part
on the degree to which they were distended by the steam which
permeated them when in a molten state. They harden into compact
rock where the steam cannot expand. Where the steam is released
from pressure, as on the surface of a lava stream, it forms
bubbles (steam blebs) of various sizes, which give the hardened
rock a cellular structure (Fig. 220), In this way are formed the
rough slags and clinkers called SCORIA, which are found on the
surface of flows and which are also thrown out as clots of lava in
explosive eruptions.
On the surface of the seething lava in the throat of the volcano
there gathers a rock foam, which, when hurled into the air, is
cooled and falls as PUMICE,--a spongy gray rock so light that it
floats on water.
AMYGDULES. The steam blebs of lava flows are often drawn out from
a spherical to an elliptical form resembling that of an almond,
and after the rock has cooled these cavities are gradually filled
with minerals deposited from solution by underground water. From
their shape such casts are called amygdules (Greek, amygdalon, an
almond). Amygdules are commonly composed of silica. Lavas contain
both silica and the alkalies, potash and soda, and after
dissolving the alkalies, percolating water is able to take silica
also into solution. Most AGATES are banded amygdules in which the
silica has been laid in varicolored, concentric layers.
GLASSY AND STONY LAVAS. Volcanic rocks differ in texture according
also to the rate at which they have solidified. When rapidly
cooled, as on the surface of a lava flow, molten rock chills to a
glass, because the minerals of which it is composed have not had
time to separate themselves from the fused mixture and form
crystals. Under slow cooling, as in the interior of the flow, it
becomes a stony mass composed of crystals set in a glassy paste.
In thin slices of volcanic glass one may see under the microscope
the beginnings of crystal growth in filaments and needles and
feathery forms, which are the rudiments of the crystals of various
minerals.
Spherulites, which also mark the first changes of glassy lavas
toward a stony condition, are little balls within the rock,
varying from microscopic size to several inches in diameter, and
made up of radiating fibers.
Perlitic structure, common among glassy lavas, consists of
microscopic curving and interlacing cracks, due to contraction.
FLOW LINES are exhibited by volcanic rocks both to the naked eye
and under the microscope. Steam blebs, together with crystals and
their embryonic forms, are left arranged in lines and streaks by
the currents of the flowing lava as it stiffened into rock.
PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered
through it large conspicuous crystals are said to be PORPHYRITIC,
and it is especially among volcanic rocks that this structure
occurs. The ground mass of porphyries either may be glassy or may
consist in part of a felt of minute crystals; in either case it
represents the consolidation of the rock after its outpouring upon
the surface. On the other hand, the large crystals of porphyry
have slowly formed deep below the ground at an earlier date.
COLUMNAR STRUCTURE. Just as wet starch contracts on drying to
prismatic forms, so lava often contracts on cooling to a mass of
close-set, prismatic, and commonly six-sided columns, which stand
at right angles to the cooling surface. The upper portion of a
flow, on rapid cooling from the surface exposed to the air, may
contract to a confused mass of small and irregular prisms; while
the remainder forms large and beautifully regular columns, which
have grown upward by slow cooling from beneath.
FRAGMENTAL MATERIALS
Rocks weighing many tons are often thrown from a volcano at the
beginning of an outburst by the breaking up of the solidofied
floor of the crater; and during the progress of an eruption large
blocks may be torn from the throat of the volcano by the outrush
of steam. But the most important fragmental materials are those
derived from the lava itself. As lava rises in the pipe, the steam
which permeates it is released from pressure and explodes, hurling
the lava into the air in fragments of all sizes,--large pieces of
scoria, LAPILLI (fragments the size of a pea or walnut), volcanic
"sand" and volcanic "ashes." The latter resemble in appearance the
ashes of wood or coal, but they are not in any sense, like them, a
residue after combustion.
Volcanic ashes are produced in several ways: lava rising in the
volcanic duct is exploded into fine dust by the steam which
permeates it; glassy lava, hurled into the air and cooled
suddenly, is brought into a state of high strain and tension, and,
like Prince Rupert's drops, flies to pieces at the least
provocation. The clash of rising and falling projectiles also
produces some dust, a fair sample of which may be made by grating
together two pieces of pumice.
Beds of volcanic ash occur widely among recent deposits in the
western United States. In Nebraska ash beds are found in twenty
counties, and are often as white as powdered pumice. The beds grow
thicker and coarser toward the southwestern part of the state,
where their thickness sometimes reaches fifty feet. In what
direction would you look for the now extinct volcano whose
explosive eruptions are thus recorded?
TUFF. This is a convenient term designating any rock composed of
volcanic fragments. Coarse tuffs of angular fragments are called
VOLCANIC BRECIA, and when the fragments have been rounded and
sorted by water the rock is termed a VOLCANIC CONGLOMERATE. Even
when deposited in the open air, as on the slopes of a volcano,
tuffs may be rudely bedded and their fragments more or less
rounded, and unless marine shells or the remains of land plants
and animals are found as fossils in them, there is often
considerable difficulty in telling whether they were laid in water
or in air. In either case they soon become consolidated. Chemical
deposits from percolating waters fill the interstices, and the bed
of loose fragments is cemented to hard rock.
The materials of which tuffs are composed are easily recognized as
volcanic in their origin. The fragments are more or less cellular,
according to the degree to which they were distended with steam
when in a molten state, and even in the finest dust one may see
the glass or the crystals of lava from which it was derived. Tuffs
often contain VOCLANIC BOMBS,--balls of lava which took shape
while whirling in the air, and solidified before falling to the
ground.
ANCIENT VOLCANIC ROCKS. It is in these materials and structures
which we have described that volcanoes leave some of their most
enduring records. Even the volcanic rocks of the earliest geological
ages, uplifted after long burial beneath the sea and exposed to view
by deep erosion, are recognized and their history read despite the
many changes which they may have undergone. A sheet of ancient lava
may be distinguished by its composition from the sediments among
which it is imbedded. The direction of its flow lines may be noted.
The cellular and slaggy surface where the pasty lava was distended
by escaping steam is recognized by the amygdules which now fill the
ancient steam blebs. In a pile of successive sheets of lava each
flow may be distinguished and its thickness measured; for the
surface of each sheet is glassy and scoriaceous, while beneath its
upper portions the lava of each flow is more dense and stony. The
length of time which elapsed before a sheet was buried beneath the
materials of succeeding eruptions may be told by the amount of
weathering which it had undergone, the depth of ancient soil--now
baked to solid rock--upon it, and the erosion which it had suffered
in the interval.
If the flow occurred from some submarine volcano, we may recognize
the fact by the sea-laid sediments which cover it, filling the
cracks and crevices of its upper surface and containing pieces of
lava washed from it in their basal layers.
Long-buried glassy lavas devitrify, or pass to a stony condition,
under the unceasing action of underground waters; but their flow
lines and perlitic and spherulitic structures remain to tell of
their original state.
Ancient tuffs are known by the fragmental character of their
volcanic material, even though they have been altered to firm
rock. Some remains of land animals and plants may be found
imbedded to tell that the beds were laid in open air; while the
remains of marine organisms would prove as surely that the tuffs
were deposited in the sea.
In these ways ancient volcanoes have been recognized near Boston,
in southeastern Pennsylvania, about Lake Superior, and in other
regions of the United States.
THE LIFE HISTORY OF A VOLCANO
The invasion of a region by volcanic forces is attended by
movements of the crust heralded by earthquakes. A fissure or a
pipe is opened and the building of the cone or the spreading of
wide lava sheets is begun.
VOLCANIC CONES. The shape of a volcanic cone depends chiefly on
the materials erupted. Cones made of fragments may have sides as
steep as the angle of repose, which in the case of coarse scoria
is sometimes as high as thirty or forty degrees. About the base of
the mountain the finer materials erupted are spread in more gentle
slopes, and are also washed forward by rains and streams. The
normal profile is thus a symmetric cone with a flaring base.
Cones built of lava vary in form according to the liquidity of the
lava. Domes of gentle slope, as those of Hawaii, for example, are
formed of basalt, which flows to long distances before it
congeals. When superheated and emitted from many vents, this
easily melted lava builds great plateaus, such as that of Iceland.
On the other hand, lavas less fusible, or poured out at a lower
temperature, stiffen when they have flowed but a short distance,
and accumulate in a steep cone. Trachyte has been extruded in a
state so viscid that it has formed steepsided domes like that of
Sarcoui.
Most volcanoes are built, like Vesuvius, both of lava flows and of
tuffs, and sections show that the structure of the cone consists
of outward-dipping, alternating layers of lava, scoria, and ashes.
From time to time the cone is rent by the violence of explosions
and by the weight of the column of lava in the pipe. The fissures
are filled with lava and some discharge on the sides of the
mountain, building parasitic cones, while all form dikes, which
strengthen the pile with ribs of hard rock and make it more
difficult to rend.
Great catastrophes are recorded in the shape of some volcanoes
which consist of a circular rim perhaps miles in diameter,
inclosing a vast crater or a caldera within which small cones may
rise. We may infer that at some time the top of the mountain has
been blown off, or has collapsed and been engulfed because some
reservoir beneath had been emptied by long-continued eruptions.
The cone-building stage may be said to continue until eruptions of
lava and fragmental materials cease altogether. Sooner or later
the volcanic forces shift or die away, and no further eruptions
add to the pile or replace its losses by erosion during periods of
repose. Gases however are still emitted, and, as sulphur vapors
are conspicuous among them, such vents are called SOLFATARAS.
Mount Hood, in Oregon, is an example of a volcano sunk to this
stage. From a steaming rift on its side there rise sulphurous
fumes which, half a mile down the wind, will tarnish a silver
coin.
GEYSERS AND HOT SPRINGS. The hot springs of volcanic regions are
among the last vestiges of volcanic heat. Periodically eruptive
boiling springs are termed geysers. In each of the geyser regions
of the earth--the Yellowstone National Park, Iceland, and New
Zealand--the ground water of the locality is supposed to be heated
by ancient lavas that, because of the poor conductivity of the
rock, still remain hot beneath the surface.
OLD FAITHFUL, one of the many geysers of the Yellowstone National
Park, plays a fountain of boiling water a hundred feet in air;
while clouds of vapor from the escaping steam ascend to several
times that height. The eruptions take place at intervals of from
seventy to ninety minutes. In repose the geyser is a quiet pool,
occupying a craterlike depression in a conical mound some twelve
feet high. The conduit of the spring is too irregular to be
sounded. The mound is composed of porous silica deposited by the
waters of the geyser.
Geysers erupt at intervals instead of continuously boiling,
because their long, narrow, and often tortuous conduits do not
permit a free circulation of the water. After an eruption the tube
is refilled and the water again gradually becomes heated. Deep in
the tube where it is in contact with hot lavas the water sooner or
later reaches the boiling point, and bursting into steam shoots
the water above it high in air.
CARBONATED SPRINGS. After all the other signs of life have gone,
the ancient volcano may emit carbon dioxide as its dying breath.
The springs of the region may long be charged with carbon dioxide,
or carbonated, and where they rise through limestone may be
expected to deposit large quantities of travertine. We should
remember, however, that many carbonated springs, and many hot
springs, are wholly independent of volcanoes.
THE DESTRUCTION OF THE CONE. As soon as the volcanic cone ceases
to grow by eruptions the agents of erosion begin to wear it down,
and the length of time that has elapsed since the period of active
growth may be roughly measured by the degree to which the cone has
been dissected. We infer that Mount Shasta, whose conical shape is
still preserved despite the gullies one thousand feet deep which
trench its sides, is younger than Mount Hood, which erosive
agencies have carved to a pyramidal form. The pile of materials
accumulated about a volcanic vent, no matter how vast in bulk, is
at last swept entirely away. The cone of the volcano, active or
extinct, is not old as the earth counts time; volcanoes are short-
lived geological phenomena.
CRANDALL VOLCANO. This name is given to a dissected ancient
volcano in the Yellowstone National Park, which once, it is
estimated, reared its head thousands of feet above the surrounding
country and greatly exceeded in bulk either Mount Shasta or Mount
Etna. Not a line of the original mountain remains; all has been
swept away by erosion except some four thousand feet of the base
of the pile. This basal wreck now appears as a rugged region about
thirty miles in diameter, trenched by deep valleys and cut into
sharp peaks and precipitous ridges. In the center of the area is
found the nucleus (N, Fig. 237),--a mass of coarsely crystalline
rock that congealed deep in the old volcanic pipe. From it there
radiate in all directions, like the spokes of a wheel, long dikes
whose rock grows rapidly finer of grain as it leaves the vicinity
of the once heated core. The remainder of the base of the ancient
mountain is made of rudely bedded tuffs and volcanic breccia, with
occasional flows of lava, some of the fragments of the breccia
measuring as much as twenty feet in diameter. On the sides of
canyons the breccia is carved by rain erosion to fantastic
pinnacles. At different levels in the midst of these beds of tuff
and lava are many old forest grounds. The stumps and trunks of the
trees, now turned to stone, still in many cases stand upright
where once they grew on the slopes of the mountain as it was
building (Fig. 238). The great size and age of some of these trees
indicate, the lapse of time between the eruption whose lavas or
tuffs weathered to the soil on which they grew and the subsequent
eruption which buried them beneath showers of stones and ashes.
Near the edge of the area lies Death Gulch, in which carbon
dioxide is given off in such quantities that in quiet weather it
accumulates in a heavy layer along the ground and suffocates the
animals which may enter it.
CHAPTER XII
UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
It is because long-continued erosion lays bare the innermost
anatomy of an extinct volcano, and even sweeps away the entire
pile with much of the underlying strata, thus leaving the very
roots of the volcano open to view, that we are able to study
underground volcanic structures. With these we include, for
convenience, intrusions of molten rock which have been driven
upward into the crust, but which may not have succeeded in
breaking way to the surface and establishing a volcano. All these
structures are built of rock forced when in a fluid or pasty state
into some cavity which it has found or made, and we may classify
them therefore, according to the shape of the molds in which the
molten rock has congealed, as (1) dikes, (2) volcanic necks, (3)
intrusive sheets, and (4) intrusive masses.
DIKES. The sheet of once molten rock with which a fissure has been
filled is known as a dike. Dikes are formed when volcanic cones
are rent by explosions or by the weight of the lava column in the
duct, and on the dissection of the pile they appear as radiating
vertical ribs cutting across the layers of lava and tuff of which
the cone is built. In regions undergoing deformation rocks lying
deep below the ground are often broken and the fissures are filled
with molten rock from beneath, which finds no outlet to the
surface. Such dikes are common in areas of the most ancient rocks,
which have been brought to light by long erosion.
In exceptional cases dikes may reach the length of fifty or one
hundred miles. They vary in width from a fraction of a foot to
even as much as three hundred feet.
Dikes are commonly more fine of grain on the sides than in the
center, and may have a glassy and crackled surface where they meet
the inclosing rock. Can you account for this on any principle
which you have learned?
VOLCANIC NECKS. The pipe of a volcano rises from far below the
base of the cone,--from the deep reservoir from which its
eruptions are supplied. When the volcano has become extinct this
great tube remains filled with hardened lava. It forms a
cylindrical core of solid rock, except for some distance below the
ancient crater, where it may contain a mass of fragments which had
fallen back into the chimney after being hurled into the air.
As the mountain is worn down, this central column known as the
VOLCANIC NECK is left standing as a conical hill (Fig. 240). Even
when every other trace of the volcano has been swept away, erosion
will not have passed below this great stalk on which the volcano
was borne as a fiery flower whose site it remains to mark. In
volcanic regions of deep denudation volcanic necks rise solitary
and abrupt from the surrounding country as dome-shaped hills. They
are marked features in the landscape in parts of Scotland and in
the St. Lawrence valley about Montreal (Fig. 241).
INTRUSIVE SHEETS. Sheets of igneous rocks are sometimes found
interleaved with sedimentary strata, especially in regions where
the rocks have been deformed and have suffered from volcanic
action. In some instances such a sheet is seen to be
CONTEMPORANEOUS (p. 248). In other instances the sheet must be
INTRUSIVE. The overlying stratum, as well as that beneath, has
been affected by the heat of the once molten rock. We infer that
the igneous rock when in a molten state was forced between the
strata, much as a card may be pushed between the leaves of a
closed book. The liquid wedged its way between the layers, lifting
those above to make room for itself. The source of the intrusive
sheet may often be traced to some dike (known therefore as the
FEEDING DIKE), or to some mass of igneous rock.
Intrusive sheets may extend a score and more of miles, and, like
the longest surface flows, the most extensive sheets consist of
the more fusible and fluid lavas,--those of the basic class of
which basalt is an example. Intrusive sheets are usually harder
than the strata in which they lie and are therefore often left in
relief after long denudation of the region (Fig. 315).
On the west bank of the Hudson there extends from New York Bay
north for thirty miles a bold cliff several hundred feet high,--
the PALISADES OF THE HUDSON. It is the outcropping edge of a sheet
of ancient igneous rock, which rests on stratified sandstones and
is overlain by strata of the same series. Sandstones and lava
sheet together dip gently to the west arid the latter disappears
from view two miles back from the river.
It is an interesting question whether the Palisades sheet is
CONTEMPORANEOUS or INTRUSIVE. Was it outpoured on the sandstones
beneath it when they formed the floor of the sea, and covered
forthwith by the sediments of the strata above, or was it intruded
among these beds at a later date?
The latter is the case: for the overlying stratum is intensely
baked along the zone of contact. At the west edge of the sheet is
found the dike in which the lava rose to force its way far and
wide between the strata.
ELECTRIC PEAK, one of the prominent mountains of the Yellowstone
National Park, is carved out of a mass of strata into which many
sheets of molten rock have been intruded. The western summit
consists of such a sheet several hundred feet thick. Studying the
section of Figure 244, what inference do you draw as to the source
of these intrusive sheets?
INTRUSIVE MASSES
BOSSES. This name is generally applied to huge irregular masses of
coarsely crystalline igneous rock lying in the midst of other
formations. Bosses vary greatly in size and may reach scores of
miles in extent. Seldom are there any evidences found that bosses
ever had connection with the surface. On the other hand, it is
often proved that they have been driven, or have melted their way,
upward into the formations in which they lie; for they give off
dikes and intrusive sheets, and have profoundly altered the rocks
about them by their heat.
The texture of the rock of bosses proves that consolidation
proceeded slowly and at great depths, and it is only because of
vast denudation that they are now exposed to view. Bosses are
commonly harder than the rocks about them, and stand up,
therefore, as rounded hills and mountainous ridges long after the
surrounding country has worn to a low plain.
The base of bosses is indefinite or undetermined, and in this
respect they differ from laccoliths. Some bosses have broken and
faulted the overlying beds; some have forced the rocks aside and
melted them away.
The SPANISH PEAKS of southeastern Colorado were formed by the
upthrust of immense masses of igneous rock, bulging and breaking
the overlying strata. On one side of the mountains the throw of
the fault is nearly a mile, and fragments of deep-lying beds were
dragged upward by the rising masses. The adjacent rocks were
altered by heat to a distance of several thousand feet. No
evidence appears that the molten rock ever reached the surface,
and if volcanic eruptions ever took place either in lava flows or
fragmental materials, all traces of them have been effaced. The
rock of the intrusive masses is coarsely crystalline, and no doubt
solidified slowly under the pressure of vast thicknesses of
overlying rock, now mostly removed by erosion.
A magnificent system of dikes radiates from the Peaks to a
distance of fifteen miles, some now being left by long erosion as
walls a hundred feet in height (Fig. 239). Intrusive sheets fed by
the dikes penetrate the surrounding strata, and their edges are
cut by canyons as much as twenty-five miles from the mountain. In
these strata are valuable beds of lignite, an imperfect coal,
which the heat of dikes and sheets has changed to coke.
LACCOLITHS. The laccolith (Greek laccos, cistern; lithos, stone)
is a variety of intrusive masses in which molten rock has spread
between the strata, and, lifting the strata above it to a dome-
shaped form, has collected beneath them in a lens-shaped body with
a flat base.
The HENRY MOUNTAINS, a small group of detached peaks in southern
Utah, rise from a plateau of horizontal rocks. Some of the peaks
are carved wholly in separate domelike uplifts of the strata of
the plateau. In others, as Mount Hillers, the largest of the
group, there is exposed on the summit a core of igneous rock from
which the sedimentary rocks of the flanks dip steeply outward in
all directions. In still others erosion has stripped off the
covering strata and has laid bare the core to its base; and its
shape is here seen to be that of a plano-convex lens or a baker's
bun, its flat base resting on the undisturbed bedded rocks
beneath. The structure of Mount Hillers is shown in Figure 248.
The nucleus of igneous rock is four miles in diameter and more
than a mile in depth.
REGIONAL INTRUSIONS. These vast bodies of igneous rock, which may
reach hundreds of miles in diameter, differ little from bosses
except in their immense bulk. Like bosses, regional intrusions
give off dikes and sheets and greatly change the rocks about them
by their heat. They are now exposed to view only because of the
profound denudation which has removed the upheaved dome of rocks
beneath which they slowly cooled. Such intrusions are accompanied
--whether as cause or as effect is still hardly known--by
deformations, and their masses of igneous rock are thus found as
the core of many great mountain ranges. The granitic masses of
which the Bitter Root Mountains and the Sierra Nevadas have been
largely carved are each more than three hundred miles in length.
Immense regional intrusions, the cores of once lofty mountain
ranges, are found upon the Laurentian peneplain.
PHYSIOGRAPHIC EFFECTS OF INTRUSIVE MASSES. We have already seen
examples of the topographic effects of intrusive masses in Mount
Hillers, the Spanish Peaks, and in the great mountain ranges
mentioned in the paragraph on regional intrusions, although in the
latter instances these effects are entangled with the effects of
other processes. Masses of igneous rock cannot be intruded within
the crust without an accompanying deformation on a scale
corresponding to the bulk of the intruded mass. The overlying
strata are arched into hills or mountains, or, if the molten
material is of great extent, the strata may conceivably be floated
upward to the height of a plateau. We may suppose that the
transference of molten matter from one region to another may be
among the causes of slow subsidences and elevations. Intrusions
give rise to fissures, dikes, and intrusive sheets, and these
dislocations cannot fail to produce earthquakes. Where intrusive
masses open communication with the surface, volcanoes are
established or fissure eruptions occur such as those of Iceland.
THE INTRUSIVE ROCKS
The igneous rocks are divided into two general classes,--the
VOLCANIC or ERUPTIVE rocks, which have been outpoured in open air
or on the floor of the sea, and the INTRUSIVE rocks, which have
been intruded within the rocks of the crust and have solidified
below the surface. The two classes are alike in chemical
composition and may be divided into acidic and basic groups. In
texture the intrusive rocks differ from the volcanic rocks because
of the different conditions under which they have solidified. They
cooled far more slowly beneath the cover of the rocks into which
they were pressed than is permitted to lava flows in open air.
Their constituent minerals had ample opportunity to sort
themselves and crystallize from the fluid mixture, and none of
that mixture was left to congeal as a glassy paste.
They consolidated also under pressure. They are never scoriaceous,
for the steam with which they were charged was not allowed to
expand and distend them with steam blebs. In the rocks of the
larger intrusive masses one may see with a powerful microscope
exceedingly minute cavities, to be counted by many millions to the
cubic inch, in which the gaseous water which the mass contained
was held imprisoned under the immense pressure of the overlying
rocks.
Naturally these characteristics are best developed in the
intrusives which cooled most slowly, i.e. in the deepest-seated
and largest masses; while in those which cooled more rapidly, as
in dikes and sheets, we find gradations approaching the texture of
surface flows.
VARIETIES OF THE INTRUSIVE ROCKS. We will now describe a few of
the varieties of rocks of deep-seated intrusions. All are even
grained, consisting of a mass of crystalline grains formed during
one continuous stage of solidification, and no porphyritic
crystals appear as in lavas.
GRANITE, as we have learned already, is composed of three
minerals,--quartz, feldspar, and mica. According to the color of
the feldspar the rock may be red, or pink, or gray. Hornblende--a
black or dark green mineral, an iron-magnesian silicate, about as
hard as feldspar--is sometimes found as a fourth constituent, and
the rock is then known as HORNBLENDIC GRANITE. Granite is an
acidic rock corresponding to rhyolite in chemical composition. We
may believe that the same molten mass which supplies this acidic
lava in surface flows solidifies as granite deep below ground in
the volcanic reservoir.
SYENITE, composed of feldspar and mica, has consolidated from a
less siliceous mixture than has granite.
DIORITE, still less siliceous, is composed of hornblende and
feldspar,--the latter mineral being of different variety from the
feldspar of granite and syenite.
GABBRO, a typical basic rock, corresponds to basalt in chemical
composition. It is a dark, heavy, coarsely crystalline aggregate
of feldspar and AUGITE (a dark mineral allied to hornblende). It
often contains MAGNETITE (the magnetic black oxide of iron) and
OLIVINE (a greenish magnesian silicate).
In the northern states all these types, and many others also of
the vast number of varieties of intrusive rocks, can be found
among the rocks of the drift brought from the areas of igneous
rock in Canada and the states of our northern border.
SUMMARY. The records of geology prove that since the earliest of
their annals tremendous forces have been active in the earth. In
all the past, under pressures inconceivably great, molten rock has
been driven upward into the rocks of the crust. It has squeezed
into fissures forming dikes; it has burrowed among the strata as
intrusive sheets; it has melted the rocks away or lifted the
overlying strata, filling the chambers which it has made with
intrusive masses. During all geological ages molten rock has found
way to the surface, and volcanoes have darkened the sky with
clouds of ashes and poured streams of glowing lava down their
sides. The older strata,--the strata which have been most deeply
buried,--and especially those which have suffered most from
folding and from fracture, show the largest amount of igneous
intrusions. The molten rock which has been driven from the earth's
interior to within the crust or to the surface during geologic
time must be reckoned in millions of cubic miles.
THE INTERIOR CONDITION OF THE EARTH AND CAUSES OF VULCANISM AND
DEFORMATION
The problems of volcanoes and of deformation are so closely
connected with that of the earth's interior that we may consider
them together. Few of these problems are solved, and we may only
state some known facts and the probable conclusions which may be
drawn as inferences from them.
THE INTERIOR OF THE EARTH IS HOT. Volcanoes prove that in many
parts of the earth there exist within reach of the surface regions
of such intense heat that the rock is in a molten condition. Deep
wells and mines show everywhere an increase in temperature below
the surface shell affected by the heat of summer and the cold of
winter,--a shell in temperate latitudes sixty or seventy feet
thick. Thus in a boring more than a mile deep at Schladebach,
Germany, the earth grows warmer at the rate of 1 degrees F. for
every sixty-seven feet as we descend. Taking the average rate of
increase at one degree for every sixty feet of descent, and
assuming that this rate, observed at the moderate distances open
to observation, continues to at least thirty-five miles, the
temperature at that depth must be more than three thousand
degrees,--a temperature at which all ordinary rocks would melt at
the earth's surface. The rate of increase in temperature probably
lessens as we go downward, and it may not be appreciable below a
few hundred miles. But there is no reason to doubt that THE
INTERIOR OF THE EARTH IS INTENSELY HOT. Below a depth of one or
two score miles we may imagine the rocks everywhere glowing with
heat.
Although the heat of the interior is great enough to melt all
rocks at atmospheric pressure, it does not follow that the
interior is fluid. Pressure raises the fusing point of rocks, and
the weight of the crust may keep the interior in what may be
called a solid state, although so hot as to be a liquid or a gas
were the pressure to be removed.
THE INTERIOR OF THE EARTH IS RIGID AND HEAVY. The earth behaves as
a globe more rigid than glass under the attractions of the sun and
moon. It is not deformed by these stresses as is the ocean in the
tides, proving that it is not a fluid ball covered with a yielding
crust a few miles thick. Earthquakes pass through the earth faster
than they would were it of solid steel. Hence the rocks of the
interior are highly elastic, being brought by pressure to a
compact, continuous condition unbroken by the cracks and vesicles
of surface rocks. THE INTERIOR OF THE EARTH IS RIGID
The common rocks of the crust are about two and a half times
heavier than water, while the earth as a whole weighs five and
six-tenths times as much as a globe of water of the same size. THE
INTERIOR IS THEREFORE MUCH MORE HEAVY THAN THE CRUST. This may be
caused in part by compression of the interior under the enormous
weight of the crust, and in part also by an assortment of
material, the heavier substances, such as the heavy metals, having
gravitated towards the center.
Between the crust, which is solid because it is cool, and the
interior, which is hot enough to melt were it not for the pressure
which keeps it dense and rigid, there may be an intermediate zone
in which heat and pressure are so evenly balanced that here rock
liquefies whenever and wherever the pressure upon it may be
relieved by movements of the crust. It is perhaps from such a
subcrustal layer that the lava of volcanoes is supplied.
THE CAUSES OF VOLCANIC ACTION. It is now generally believed that
the HEAT of volcanoes is that of the earth's interior. Other
causes, such as friction and crushing in the making of mountains
and the chemical reactions between oxidizing agents of the crust
and the unoxidized interior, have been suggested, but to most
geologists they seem inadequate.
There is much difference of opinion as to the FORCE which causes
molten rock to rise to the surface in the ducts of volcanoes.
Steam is so evidently concerned in explosive eruptions that many
believe that lava is driven upward by the expansive force of the
steam with which it is charged, much as a viscid liquid rises and
boils over in a test tube or kettle.
But in quiet eruptions, and still more in the irruption of
intrusive sheets and masses, there is little if any evidence that
steam is the driving force. It is therefore believed by many
geologists that it is PRESSURE DUE TO CRUSTAL MOVEMENTS AND
INTERNAL STRESSES which squeezes molten rock from below into
fissures and ducts in the crust. It is held by some that where
considerable water is supplied to the rising column of lava, as
from the ground water of the surrounding region, and where the
lava is viscid so that steam does not readily escape, the eruption
is of the explosive type; when these conditions do not obtain, the
lava outwells quietly, as in the Hawaiian volcanoes. It is held by
others not only that volcanoes are due to the outflow of the
earth's deep-seated heat, but also that the steam and other
emitted gases are for the most part native to the earth's interior
and never have had place in the circulation of atmospheric and
ground waters.
VOLCANIC ACTION AND DEFORMATION. Volcanoes do not occur on wide
plains or among ancient mountains. On the other hand, where
movements of the earth's crust are in progress in the uplift of
high plateaus, and still more in mountain making, molten rock may
reach the surface, or may be driven upward toward it forming great
intrusive masses. Thus extensive lava flows accompanied the
upheaval of the block mountains of western North America and the
uplift of the Colorado plateau. A line of recent volcanoes may be
traced along the system of rift valleys which extends from the
Jordan and Dead Sea through eastern Africa to Lake Nyassa. The
volcanoes of the Andes show how conspicuous volcanic action may be
in young rising ranges. Folded mountains often show a core of
igneous rock, which by long erosion has come to form the axis and
the highest peaks of the range, as if the molten rock had been
squeezed up under the rising upfolds. As we decipher the records
of the rocks in historical geology we shall see more fully how, in
all the past, volcanic action has characterized the periods of
great crustal movements, and how it has been absent when and where
the earth's crust has remained comparatively at rest.
THE CAUSES OF DEFORMATION. As the earth's interior, or nucleus, is
highly heated it must be constantly though slowly losing its heat
by conduction through the crust and into space; and since the
nucleus is cooling it must also be contracting. The nucleus has
contracted also because of the extrusion of molten matter, the
loss of constituent gases given off in volcanic eruptions, and
(still more important) the compression and consolidation of its
material under gravity. As the nucleus contracts, it tends to draw
away from the cooled and solid crust, and the latter settles,
adapting itself to the shrinking nucleus much as the skin of a
withering apple wrinkles down upon the shrunken fruit. The
unsupported weight of the spherical crust develops enormous
tangential pressures, similar to the stresses of an arch or dome,
and when these lateral thrusts accumulate beyond the power of
resistance the solid rock is warped and folded and broken.
Since the planet attained its present mass it has thus been
lessening in volume. Notwithstanding local and relative upheavals
the earth's surface on the whole has drawn nearer and nearer to
the center. The portions of the lithosphere which have been
carried down the farthest have received the waters of the oceans,
while those portions which have been carried down the least have
emerged as continents.
Although it serves our convenience to refer the movements of the
crust to the sea level as datum plane, it is understood that this
level is by no means fixed. Changes in the ocean basins increase
or reduce their capacity and thus lower or raise the level of the
sea. But since these basins are connected, the effect of any
change upon the water level is so distributed that it is far less
noticeable than a corresponding change would be upon the land.
CHAPTER XIII
METAMORPHISM AND MINERAL VEINS
Under the action of internal agencies rocks of all kinds may be
rendered harder, more firmly cemented, and more crystalline. These
processes are known as METAMORPHISM, and the rocks affected,
whether originally sedimentary or igneous, are called METAMORPHIC
ROCKS. We may contrast with metamorphism the action of external
agencies in weathering, which render rocks less coherent by
dissolving their soluble parts and breaking down their crystalline
grains.
CONTACT METAMORPHISM. Rocks beneath a lava flow or in contact with
igneous intrusions are found to be metamorphosed to various
degrees by the heat of the cooling mass. The adjacent strata may
be changed only in color, hardness, and texture. Thus, next to a
dike, bituminous coal may be baked to coke or anthracite, and
chalk and limestone to crystalline marble. Sandstone may be
converted into quartzite, and shale into ARGILLITE, a compact,
massive clay rock. New minerals may also be developed. In
sedimentary rocks there may be produced crystals of mica and of
GARNET (a mineral as hard as quartz, commonly occurring in red,
twelve-sided crystals). Where the changes are most profound, rocks
may be wholly made over in structure and mineral composition.
In contact metamorphism, thin sheets of molten rock produce less
effect than thicker ones. The strongest heat effects are naturally
caused by bosses and regional intrusions, and the zone of change
about them may be several miles in width. In these changes heated
waters and vapors from the masses of igneous rocks undoubtedly
play a very important part.
Which will be more strongly altered, the rocks about a closed dike
in which lava began to cool as soon as it filled the fissure, or
the rocks about a dike which opened on the surface and through
which the molten rock flowed for some time?
Taking into consideration the part played by heated waters, which
will produce the most far-reaching metamorphism, dikes which cut
across the bedding planes or intrusive sheets which are thrust
between the strata?
REGIONAL METAMORPHISM. Metamorphic rocks occur wide-spread in many
regions, often hundreds of square miles in area, where such
extensive changes cannot be accounted for by igneous intrusions.
Such are the dissected cores of lofty mountains, as the Alps, and
the worn-down bases of ancient ranges, as in New England, large
areas in the Piedmont Belt, and the Laurentian peneplain.
In these regions the rocks have yielded to immense pressure. They
have been folded, crumpled, and mashed, and even their minute
grains, as one may see with a microscope, have often been
puckered, broken, and crushed to powder. It is to these mechanical
movements and strains which the rocks have suffered in every part
that we may attribute their metamorphism, and the degree to which
they have been changed is in direct proportion to the degree to
which they have been deformed and mashed.
Other factors, however, have played important parts. Rock crushing
develops heat, and allows a freer circulation of heated waters and
vapors. Thus chemical reactions are greatly quickened; minerals
are dissolved and redeposited in new positions, or their chemical
constituents may recombine in new minerals, entirely changing the
nature of the rock, as when, for example, feldspar recrystallizes
as quartz and mica.
Early stages of metamorphism are seen in SLATE. Pressure has
hardened the marine muds, the arkose, or the volcanic ash from
which slates are derived, and has caused them to cleave by the
rearrangement of their particles.
Under somewhat greater pressure, slate becomes PHYLLITE, a clay
slate whose cleavage surfaces are lustrous with flat-lying mica
flakes. The same pressure which has caused the rock to cleave has
set free some of its mineral constituents along the cleavage
planes to crystallize there as mica.
FOLIATION. Under still stronger pressure the whole structure of
the rock is altered. The minerals of which it is composed, and the
new minerals which develop by heat and pressure, arrange
themselves along planes of cleavage or of shear in rudely parallel
leaves, or FOLIA. Of this structure, called FOLIATION, we may
distinguish two types,--a coarser feldspathic type, and a fine
type in which other minerals than feldspar predominate.
GNEISS is the general name under which are comprised coarsely
foliated rocks banded with irregular layers of feldspar and other
minerals. The gneisses appear to be due in many cases to the
crushing and shearing of deep-seated igneous rocks, such as
granite and gabbro.
THE CRYSTALLINE SCHISTS, representing the finer types of
foliation, consist of thin, parallel, crystalline leaves, which
are often remarkably crumpled. These folia can be distinguished
from the laminae of sedimentary rocks by their lenticular form and
lack of continuity, and especially by the fact that they consist
of platy, crystalline grains, and not of particles rounded by
wear.
MICA SCHIST, the most common of schists, and in fact of all
metamorphic rocks, is composed of mica and quartz in alternating
wavy folia. All gradations between it and phyllite may be traced,
and in many cases we may prove it due to the metamorphism of
slates and shales. It is widespread in New England and along the
eastern side of the Appalachians. TALC SCHIST consists of quartz
and TALC, a light-colored magnesian mineral of greasy feel, and so
soft that it can be scratched with the thumb nail.
HORNBLENDE SCHIST, resulting in many cases from the foliation of
basic igneous rocks, is made of folia of hornblende alternating
with bands of quartz and feldspar. Hornblende schist is common
over large areas in the Lake Superior region.
QUARTZ SCHIST is produced from quartzite by the development of
fine folia of mica along planes of shear. All gradations may be
found between it and unfoliated quartzite on the one hand and mica
schist on the other.
Under the resistless pressure of crustal movements almost any
rocks, sandstones, shales, lavas of all kinds, granites, diorites,
and gabbros may be metamorphosed into schists by crushing and
shearing. Limestones, however, are metamorphosed by pressure into
marble, the grains of carbonate of lime recrystallizing freely to
interlocking crystals of calcite.
These few examples must suffice of the great class of metamorphic
rocks. As we have seen, they owe their origin to the alteration of
both of the other classes of rocks--the sedimentary and the
igneous--by heat and pressure, assisted usually by the presence of
water. The fact of change is seen in their hardness arid
cementation, their more or less complete recrystallization, and
their foliation; but the change is often so complete that no trace
of their original structure and mineral composition remains to
tell whether the rocks from which they were derived were
sedimentary or igneous, or to what variety of either of these
classes they belonged.
In many cases, however, the early history of a metamorphic rock
can be deciphered. Fossils not wholly obliterated may prove it
originally water-laid. Schists may contain rolled-out pebbles,
showing their derivation from a conglomerate. Dikes of igneous
rocks may be followed into a region where they have been foliated
by pressure. The most thoroughly metamorphosed rocks may sometimes
be traced out into unaltered sedimentary or igneous rocks, or
among them may be found patches of little change where their
history maybe read.
Metamorphism is most common among rocks of the earlier geological
ages, and most rare among rocks of recent formation. No doubt it
is now in progress where deep-buried sediments are invaded
by heat either from intrusive igneous masses or from the earth's
interior, or are suffering slow deformation under the thrust of
mountain-making forces.
Suggest how rocks now in process of metamorphism may sometimes be
exposed to view. Why do metamorphic rocks appear on the surface
to-day?
MINERAL VEINS
In regions of folded and broken rocks fissures are frequently
found to be filled with sheets of crystalline minerals deposited
from solution by underground water, and fissures thus filled are
known as mineral veins. Much of the importance of mineral veins is
due to the fact that they are often metalliferous, carrying
valuable native metals and metallic ores disseminated in fine
particles, in strings, and sometimes in large masses in the midst
of the valueless nonmetallic minerals which make up what is known
as the VEIN STONE.
The most common vein stones are QUARTZ and CALCITE. FLUORITE
(calcium fluoride), a mineral harder than calcite and
crystallizing in cubes of various colors, and BARITE (barium
sulphate), a heavy white mineral, are abundant in many veins.
The gold-bearing quartz veins of California traverse the
metamorphic slates of the Sierra Nevada Mountains. Below the zone
of solution (p. 45) these veins consist of a vein stone of quartz
mingled with pyrite (p. 13), the latter containing threads and
grains of native gold. But to the depth of about fifty feet from
the surface the pyrite of the vein has been dissolved, leaving a
rusty, cellular quartz with grains of the insoluble gold scattered
through it.
The PLACER DEPOSITS of California and other regions are gold-
bearing deposits of gravel and sand in river beds. The heavy gold
is apt to be found mostly near or upon the solid rock, and its
grains, like those of the sand, are always rounded. How the gold
came in the placers we may leave the pupil to suggest.
Copper is found in a number of ores, and also in the native metal.
Below the zone of surface changes the ore of a copper vein is
often a double sulphide of iron and copper called CHALCOPYRITE, a
mineral softer than pyrite--it can easily be scratched with a
knife--and deeper yellow in color. For several score of feet below
the ground the vein may consist of rusty quartz from which the
metallic ores have been dissolved; but at the base of the zone of
solution we may find exceedingly rich deposits of copper ores,--
copper sulphides, red and black copper oxides, and green and blue
copper carbonates, which have clearly been brought down in
solution from the leached upper portion of the vein.
ORIGIN OF MINERAL VEINS. Both vein stones and ores have been
deposited slowly from solution in water, much as crystals of salt
are deposited on the sides of a jar of saturated brine. In our
study of underground water we learned that it is everywhere
circulating through the permeable rocks of the crust, descending
to profound depths under the action of gravity and again driven to
the surface by hydrostatic pressure. Now fissures, wherever they
occur, form the trunk channels of the underground circulation.
Water descends from the surface along these rifts; it moves
laterally from either side to the fissure plane, just as ground
water seeps through the surrounding rocks from every direction to
a well; and it ascends through these natural water ways as in an
artesian well, whenever they intersect an aquifer in which water
is under hydrostatic pressure.
The waters which deposit vein stones and ores are commonly hot,
and in many cases they have derived their heat from intrusions of
igneous rock still uncooled within the crust. The solvent power of
the water is thus greatly increased, and it takes up into solution
various substances from the igneous and sedimentary rocks which it
traverses. For various reasons these substances stances are
deposited in the vein as ores and vein stones. On rising through
the fissure the water cools and loses pressure, and its capacity
to hold minerals in solution is therefore lessened. Besides, as
different currents meet in the fissure, some ascending, some
descending, and some coming in from the sides, the chemical
reaction of these various weak solutions upon one another and upon
the walls of the vein precipitates the minerals of vein stuffs and
ores.
As an illustration of the method of vein deposits we may cite the
case of a wooden box pipe used in the Comstock mines, Nevada, to
carry the hot water of the mine from one level to another, which
in ten years was lined with calcium carbonate more than half an
inch thick.
The Steamboat Springs, Nevada, furnish examples of mineral veins
in process of formation. The steaming water rises through fissures
in volcanic rocks and is now depositing in the rifts a vein stone
of quartz, with metallic ores of iron, mercury, lead, and other
metals.
RECONCENTRATION. Near the base of the zone of solution veins are
often stored with exceptionally large and valuable ore deposits.
This local enrichment of the vein is due to the reconcentration of
its metalliferous ores. As the surface of the land is slowly
lowered by weathering and running water, the zone of solution is
lowered at an equal rate and encroaches constantly on the zone of
cementation. The minerals of veins are therefore constantly being
dissolved along their upper portions and carried down the fissures
by ground water to lower levels, where they are redeposited.
Many of the richest ore deposits are thus due to successive
concentrations: the ores were leached originally from the rocks to
a large extent by laterally seeping waters; they were concentrated
in the ore deposits of the vein chiefly by ascending currents;
they have been reconcentrated by descending waters in the way just
mentioned.
THE ORIGINAL SOURCE OF THE METALS. It is to the igneous rocks that
we may look for the original source of the metals of veins. Lavas
contain minute percentages of various metallic compounds, and no
doubt this was the case also with the igneous rocks which formed
the original earth crust. By the erosion of the igneous rocks the
metals have been distributed among sedimentary strata, and even
the sea has taken into solution an appreciable amount of gold and
other metals, but in this widely diffused condition they are
wholly useless to man. The concentration which has made them
available is due to the interaction of many agencies. Earth
movements fracturing deeply the rocks of the crust, the intrusion
of heated masses, the circulation of underground waters, have all
cooperated in the concentration of the metals of mineral veins.
While fissure veins are the most important of mineral veins, the
latter term is applied also to any water way which has been filled
by similar deposits from solution. Thus in soluble rocks, such as
limestones, joints enlarged by percolating water are sometimes
filled with metalliferous deposits, as, for example, the lead and
zinc deposits of the upper Mississippi valley. Even a porous
aquifer may be made the seat of mineral deposits, as in the case
of some copper-bearing and silver-bearing sandstones of New
Mexico.
PART III
HISTORICAL GEOLOGY
CHAPTER XIV
THE GEOLOGICAL RECORD
WHAT A FORMATION RECORDS. We have already learned that each
individual body of stratified rock, or formation, constitutes a
record of the time when it was laid. The structure and the
character of the sediments of each formation tell whether the area
was land or sea at the time when they were spread; and if the
former, whether the land was river plain, or lake bed, or was
covered with wind-blown sands, or by the deposits of an ice sheet.
If the sediments are marine, we may know also whether they were
laid in shoal water near the shore or in deeper water out at sea,
and whether during a period of emergence, or during a period of
subsidence when the sea transgressed the land. By the same means
each formation records the stage in the cycle of erosion of the
land mass from which its sediments were derived. An unconformity
between two marine formations records the fact that between the
periods when they were deposited in the sea the area emerged as
land and suffered erosion. The attitude and structure of the
strata tell also of the foldings and fractures, the deformation
and the metamorphism, which they have suffered; and the igneous
rocks associated with them as lava flows and igneous intrusions
add other details to the story. Each formation is thus a separate
local chapter in the geological history of the earth, and its
strata are its leaves. It contains an authentic record of the
physical conditions--the geography--of the time and place when and
where its sediments were laid.
PAST CYCLES OF EROSION. These chapters in the history of the
planet are very numerous, although much of the record has been
destroyed in various ways. A succession of different formations is
usually seen in any considerable section of the crust, such as a
deep canyon or where the edges of upturned strata are exposed to
view on the flanks of mountain ranges; and in any extensive area,
such as a state of the Union or a province of Canada, the number
of formations outcropping on the surface is large.
It is thus learned that our present continent is made up for. the
most part of old continental deltas. Some, recently emerged as the
strata of young coastal plains, are the records of recent cycles
of erosion; while others were deposited in the early history of
the earth, and in many instances have been crumpled into
mountains, which afterwards were leveled to their bases and
lowered beneath the sea to receive a cover of later sediments
before they were again uplifted to form land.
The cycle of erosion now in progress and recorded in the layers of
stratified rock being spread beneath the sea in continental deltas
has therefore been preceded by many similar cycles. Again and
again movements of the crust have brought to an end one cycle--
sometimes when only well under way, and sometimes when drawing
toward its close--and have begun another. Again and again they
have added to the land areas which before were sea, with all their
deposition records of earlier cycles, or have lowered areas of
land beneath the sea to receive new sediments.
THE AGE OF THE EARTH. The thickness of the stratified rocks now
exposed upon the eroded surface of the continents is very great.
In the Appalachian region the strata are seven or eight miles
thick, and still greater thicknesses have been measured in several
other mountain ranges. The aggregate thickness of all the
formations of the stratified rocks of the earth's crust, giving to
each formation its maximum thickness wherever found, amounts to
not less than forty miles. Knowing how slowly sediments accumulate
upon the sea floor, we must believe that the successive cycles
which the earth has seen stretch back into a past almost
inconceivably remote, and measure tens of millions and perhaps
even hundreds of millions of years.
HOW THE FORMATIONS ARE CORRELATED AND THE GEOLOGICAL RECORD MADE
UP. Arranged in the order of their succession, the formations of
the earth's crust would constitute a connected record in which the
geological history of the planet may be read, and therefore known
as the GEOLOGICAL RECORD. But to arrange the formations in their
natural order is not an easy task. A complete set of the volumes
of the record is to be found in no single region. Their leaves and
chapters are scattered over the land surface of the globe. In one
area certain chapters may be found, though perhaps with many
missing leaves, and with intervening chapters wanting, and these
absent parts perhaps can be supplied only after long search
through many other regions.
Adjacent strata in any region are arranged according to the LAW OF
SUPERPOSITION, i.e. any stratum is younger than that on which it
was deposited, just as in a pile of paper, any sheet was laid
later than that on which it rests. Where rocks have been
disturbed, their original attitude must be determined before the
law can be applied. Nor can the law of superposition be used in
identifying and comparing the strata of different regions where
the formations cannot be traced continuously from one region to
the other.
The formations of different regions are arranged in their true
order by the LAW OF INCLUDED ORGANISMS; i.e. formations, however
widely separated, which contain a similar assemblage of fossils
are equivalent and belong to the same division of geological time.
The correlation of formations by means of fossils may be explained
by the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental
shelf of North America limestones of different kinds are forming
off Florida, and sandstones and shales from Georgia northward.
Separated from them by the deep Atlantic oozes are other
sedimentary deposits now accumulating along the west coast of
Europe. If now all these offshore formations were raised to open
air, how could they be correlated? Surely not by lithological
likeness, for in this respect they would be quite diverse. All
would be similar, however, in the fossils which they contain. Some
fossil species would be identical in all these formations and
others would be closely allied. Making all due allowance for
differences in species due to local differences in climate and
other physical causes, it would still be plain that plants and
animals so similar lived at the same period of time, and that the
formations in which their remains were imbedded were
contemporaneous in a broad way. The presence of the bones of
whales and other marine mammals would prove that the strata were
laid after the appearance of mammals upon earth, and imbedded
relics of man would give a still closer approximation to their
age. In the same way we correlate the earlier geological
formations.
For example, in 1902 there were collected the first fossils ever
found on the antarctic continent. Among the dozen specimens
obtained were some fossil ammonites (a family of chambered shells)
of genera which are found on other continents in certain
formations classified as the Cretaceous system, and which occur
neither above these formations nor below them. On the basis of
these few fossils we may be confident that the strata in which
they were found in the antarctic region were laid in the same
period of geologic time as were the Cretaceous rocks of the United
States and Canada.
THE RECORD AS A TIME SCALE. By means of the law of included
organisms and the law of superposition the formations of different
countries and continents are correlated and arranged in their
natural order. When the geological record is thus obtained it may
be used as a universal time scale for geological history.
Geological time is separated into divisions corresponding to the
times during which the successive formations were laid. The
largest assemblages of formations are known as groups, while the
corresponding divisions of time are known as eras. Groups are
subdivided into systems, and systems into series. Series are
divided into stages and substages,--subdivisions which do not
concern us in this brief treatise. The corresponding divisions of
time are given in the following table.
STRATA TIME
Group Era
System Period
Series Epoch
The geologist is now prepared to read the physical history--the
geographical development--of any country or of any continent by
means of its formations, when he has given each formation its true
place in the geological record as a time scale.
The following chart exhibits the main divisions of the record, the
name given to each being given also to the corresponding time
division. Thus we speak of the CAMBRIAN SYSTEM, meaning a certain
succession of formations which are classified together because of
broad resemblances in their included organisms; and of the
CAMBRIAN PERIOD, meaning the time during which these rocks were
deposited.
Group and Era System and Period Series and Epoch
|Quaternary-----|Recent
Cenozoic------| |Pleistocene
|
|Tertiary-------|Pliocene
|Miocene
|Eocene
|Cretaceous
Mesozoic------|Jurassic
|Triassic
|Permian
|Carboniferous--|Pennsylvanian
| |Mississippian
Paleozoic-----|Devonian
|Silurian
|Ordovician
|Cambrian
Algonkian
Archean
FOSSILS AND WHAT THEY TEACH
The geological formations contain a record still more important
than that of the geographical development of the continents; the
fossils imbedded in the rocks of each formation tell of the kinds
of animals and plants which inhabited the earth at that time, and
from these fossils we are therefore able to construct the history
of life upon the earth.
FOSSILS. These remains of organisms are found in the strata in all
degrees of perfection, from trails and tracks and fragmentary
impressions, to perfectly preserved shells, wood, bones, and
complete skeletons. As a rule, it is only the hard parts of
animals and plants which have left any traces in the rocks.
Sometimes the original hard substance is preserved, but more often
it has been replaced by some less soluble material. Petrifaction,
as this process of slow replacement is called, is often carried on
in the most exquisite detail. When wood, for example, is
undergoing petrifaction, the woody tissue may be replaced,
particle by particle, by silica in solution through the action of
underground waters, even the microscopic structures of the wood
being perfectly reproduced. In shells originally made of
ARAGONITE, a crystalline form of carbonate of lime, that mineral
is usually replaced by CALCITE, a more stable form of the same
substance. The most common petrifying materials are calcite,
silica, and pyrite.
Often the organic substance has neither been preserved nor
replaced, but the FORM has been retained by means of molds and
casts. Permanent impressions, or molds, may be made in sediments
not only by the hard parts of organisms, but also by such soft and
perishable parts as the leaves of plants, and, in the rarest
instances, by the skin of animals and the feathers of birds. In
fine-grained limestones even the imprints of jellyfish have been
retained.
The different kinds of molds and casts may be illustrated by means
of a clam shell and some moist clay, the latter representing the
sediments in which the remains of animals and plants are entombed.
Imbedding the shell in the clay and allowing the clay to harden,
we have a MOLD OF THE EXTERIOR of the shell, as is seen on cutting
the clay matrix in two and removing the shell from it. Filling
this mold with clay of different color, we obtain a CAST OF THE
EXTERIOR, which represents accurately the original form and
surface markings of the shell. In nature, shells and other relics
of animals or plants are often removed by being dissolved by
percolating waters, and the molds are either filled with sediments
or with minerals deposited from solution.
Where the fossil is hollow, a CAST OF THE INTERIOR is made in the
same way. Interior casts of shells reproduce any markings on the
inside of the valves, and casts of the interior of the skulls of
ancient vertebrates show the form and size of their brains.
IMPERFECTION OF THE LIFE RECORD. At the present time only the
smallest fraction of the life on earth ever gets entombed in rocks
now forming. In the forest great fallen tree trunks, as well as
dead leaves, decay, and only add a little to the layer of dark
vegetable mold from which they grew. The bones of land animals
are, for the most part, left unburied on the surface and are soon
destroyed by chemical agencies. Even where, as in the swamps of
river, flood plains and in other bogs, there are preserved the
remains of plants, and sometimes insects, together with the bones
of some animal drowned or mired, in most cases these swamp and bog
deposits are sooner or later destroyed by the shifting channels of
the stream or by the general erosion of the land.
In the sea the conditions for preservation are more favorable than
on land; yet even here the proportion of animals and plants whose
hard parts are fossilized is very small compared with those which
either totally decay before they are buried in slowly accumulating
sediments or are ground to powder by waves and currents.
We may infer that during each period of the past, as at the
present, only a very insignificant fraction of the innumerable
organisms of sea and land escaped destruction and left in
continental and oceanic deposits permanent records of their
existence. Scanty as these original life records must have been,
they have been largely destroyed by metamorphism of the rocks in
which they were imbedded, by solution in underground waters, and
by the vast denudation under which the sediments of earlier
periods have been eroded to furnish materials for the sedimentary
records of later times. Moreover, very much of what has escaped
destruction still remains undiscovered. The immense bulk of the
stratified rocks is buried and inaccessible, and the records of
the past which it contains can never be known. Comparatively few
outcrops have been thoroughly searched for fossils. Although new
species are constantly being discovered, each discovery may be
considered as the outcome of a series of happy accidents,--that
the remains of individuals of this particular species happened to
be imbedded and fossilized, that they happened to escape
destruction during long ages, and that they happened to be exposed
and found.
SOME INFERENCES FROM THE RECORDS OF THE HISTORY OF LIFE UPON THE
PLANET. Meager as are these records, they set forth plainly some
important truths which we will now briefly mention.
1. Each series of the stratified rocks, except the very deepest,
contains vestiges of life. Hence THE EARTH WAS TENANTED BY LIVING
CREATURES FOR AN UNCALCULATED LENGTH OF TIME BEFORE HUMAN HISTORY
BEGAN.
2. LIFE ON THE EARTH HAS BEEN EVERCHANGING. The youngest strata
hold the remains of existing species of animals and plants and
those of species and varieties closely allied to them. Strata
somewhat older contain fewer existing species, and in strata of a
still earlier, but by no means an ancient epoch, no existing
species are to be found; the species of that epoch and of previous
epochs have vanished from the living world. During all geological
time since life began on earth old species have constantly become
extinct and with them the genera and families to which they
belong, and other species, genera, and families have replaced
them. The fossils of each formation differ on the whole from those
of every other. The assemblage of animals and plants (the FAUNA-
FLORA) of each epoch differs from that of every other epoch.
In many cases the extinction of a type has been gradual; in other
instances apparently abrupt. There is no evidence that any
organism once become extinct has ever reappeared. The duration of
a species in time, or its "vertical range" through the strata,
varies greatly. Some species are limited to a stratum a few feet
in thickness; some may range through an entire formation and be
found but little modified in still higher beds. A formation may
thus often be divided into zones, each characterized by its own
peculiar species. As a rule, the simpler organisms have a longer
duration as species, though not as individuals, than the more
complex.
3. THE LARGER ZOOLOGICAL AND BOTANICAL GROUPINGS SURVIVE LONGER
THAN THE SMALLER. Species are so short-lived that a single
geological epoch may be marked by several more or less complete
extinctions of the species of its fauna-flora and their
replacement by other species. A genus continues with new species
after all the species with which it began have become extinct.
Families survive genera, and orders families. Classes are so long-
lived that most of those which are known from the earliest
formations are represented by living forms, and no sub-kingdom has
ever become extinct.
Thus, to take an example from the stony corals,--the ZOANTHARIA,--
the particular characters--which constituted a certain SPECIES--
Facosites niagarensis--of the order are confined to the Niagara
series. Its GENERIC characters appeared in other species earlier
in the Silurian and continued through the Devonian. Its FAMILY
characters, represented in different genera and species, range
from the Ordovician to the close of the Paleozoic; while the
characters which it shares with all its order, the Zoantharia,
began in the Cambrian and are found in living species.
4. THE CHANGE IN ORGANISMS HAS BEEN GRADUAL. The fossils of each
life zone and of each formation of a conformable series closely
resemble, with some explainable exceptions, those of the beds
immediately above and below. The animals and plants which tenanted
the earth during any geological epoch are so closely related to
those of the preceding and the succeeding epochs that we may
consider them to be the descendants of the one and the ancestors
of the other, thus accounting for the resemblance by heredity. It
is therefore believed that the species of animals and plants now
living on the earth are the descendants of the species whose
remains we find entombed in the rocks, and that the chain of life
has been unbroken since its beginning.
5. THE CHANGE IN SPECIES HAS BEEN A GRADUAL DIFFERENTIATION.
Tracing the lines of descent of various animals and plants of the
present backward through the divisions of geologic time, we find
that these lines of descent converge and unite in simpler and
still simpler types. The development of life may be represented by
a tree whose trunk is found in the earliest ages and whose
branches spread and subdivide to the growing twigs of present
species.
6. THE CHANGE IN ORGANISMS THROUGHOUT GEOLOGIC TIME HAS BEEN A
PROGRESSIVE CHANGE. In the earliest ages the only animals and
plants on the earth were lowly forms, simple and generalized in
structure; while succeeding ages have been characterized by the
introduction of types more and more specialized and complex, and
therefore of higher rank in the scale of being. Thus the Algonkian
contains the remains of only the humblest forms of the
invertebrates. In the Cambrian, Ordovician, and Silurian the
invertebrates were represented in all their subkingdoms by a
varied fauna. In the Devonian, fishes--the lowest of the
vertebrates--became abundant. Amphibians made their entry on the
stage in the Carboniferous, and reptiles came to rule the world in
the Mesozoic. Mammals culminated in the Tertiary in strange forms
which became more and more like those of the present as the long
ages of that era rolled on; and latest of all appeared the noblest
product of the creative process, man.
Just as growth is characteristic of the individual life, so
gradual, progressive change, or evolution, has characterized the
history of life upon the planet. The evolution of the organic
kingdom from its primitive germinal forms to the complex and
highly organized fauna-flora of to-day may be compared to the
growth of some noble oak as it rises from the acorn, spreading
loftier and more widely extended branches as it grows.
7. While higher and still higher types have continually been
evolved, until man, the highest of all, appeared, THE LOWER AND
EARLIER TYPES HAVE GENERALLY PERSISTED. Some which reached their
culmination early in the history of the earth have since changed
only in slight adjustments to a changing environment. Thus the
brachiopods, a type of shellfish, have made no progress since the
Paleozoic, and some of their earliest known genera are represented
by living forms hardly to be distinguished from their ancient
ancestors. The lowest and earliest branches of the tree of life
have risen to no higher levels since they reached their climax of
development long ago.
8. A strange parallel has been found to exist between the
evolution of organisms and the development of the individual. In
the embryonic stages of its growth the individual passes swiftly
through the successive stages through which its ancestors evolved
during the millions of years of geologic time. THE DEVELOPMENT OF
THE INDIVIDUAL RECAPITULATES THE EVOLUTION OF THE RACE.
The frog is a typical amphibian. As a tadpole it passes through a
stage identical in several well-known features with the maturity
of fishes; as, for example, its aquatic life, the tail by which it
swims, and the gills through which it breathes. It is a fair
inference that the tadpole stage in the life history of the frog
represents a stage in the evolution of its kind,--that the
Amphibia are derived from fishlike ancestral forms. This inference
is amply confirmed in the geological record; fishes appeared
before Amphibia and were connected with them by transitional
forms.
THE GREAT LENGTH OF GEOLOGIC TIME INFERRED FROM THE SLOW CHANGE OF
SPECIES. Life forms, like land forms, are thus subject to change
under the influence of their changing environment and of forces
acting from within. How slowly they change may be seen in the
apparent stability of existing species. In the lifetime of the
observer and even in the recorded history of man, species seem as
stable as the mountain and the river. But life forms and land
forms are alike variable, both in nature and still more under the
shaping hand of man. As man has modified the face of the earth
with his great engineering works, so he has produced widely
different varieties of many kinds of domesticated plants and
animals, such as the varieties of the dog and the horse, the apple
and the rose, which may be regarded in some respects as new
species in the making. We have assumed that land forms have
changed in the past under the influence of forces now in
operation. Assuming also that life forms have always changed as
they are changing at present, we come to realize something of the
immensity of geologic time required for the evolution of life from
its earliest lowly forms up to man.
It is because the onward march of life has taken the same general
course the world over that we are able to use it as a UNIVERSAL
TIME SCALE and divide geologic time into ages and minor
subdivisions according to the ruling or characteristic organisms
then living on the earth. Thus, since vertebrates appeared, we
have in succession the Age of Fishes, the Age of Amphibians, the
Age of Reptiles, and the Age of Mammals.
The chart given on page 295 is thus based on the law of
superposition and the law of the evolution of organisms. The first
law gives the succession of the formations in local areas. The
fossils which they contain demonstrate the law of the progressive
appearance of organisms, and by means of this law the formations
of different countries are correlated and set each in its place in
a universal time scale and grouped together according to the
affinities of their imbedded organic remains.
GEOLOGIC TIME DIVISIONS COMPARED WITH THOSE OF HUMAN HISTORY. We
may compare the division of geologic time into eras, periods, and
other divisions according to the dominant life of the time, to the
ill-defined ages into which human history is divided according to
the dominance of some nation, ruler, or other characteristic
feature. Thus we speak of the DARK AGES, the AGE OF ELIZABETH, and
the AGE OF ELECTRICITY. These crude divisions would be of much
value if, as in the case of geologic time, we had no exact
reckoning of human history by years.
And as the course of human history has flowed in an unbroken
stream along quiet reaches of slow change and through periods of
rapid change and revolution, so with the course of geologic
history. Periods of quiescence, in which revolutionary forces are
perhaps gathering head, alternate with periods of comparatively
rapid change in physical geography and in organisms, when new and
higher forms appear which serve to draw the boundary line of new
epochs. Nevertheless, geological history is a continuous progress;
its periods and epochs shade into one another by imperceptible
gradations, and all our subdivisions must needs be vague and more
or less arbitrary.
HOW FOSSILS TELL OF THE GEOGRAPHY OF THE PAST. Fossils are used
not only as a record of the development of life upon the earth,
but also in testimony to the physical geography of past epochs.
They indicate whether in any region the climate was tropical,
temperate, or arctic. Since species spread slowly from some center
of dispersion where they originate until some barrier limits their
migration farther, the occurrence of the same species in rocks of
the same system in different countries implies the absence of such
barriers at the period. Thus in the collection of antarctic
fossils referred to on page 294 there were shallow-water marine
shells identical in species with Mesozoic shells found in India
and in the southern extremity of South America. Since such
organisms are not distributed by the currents of the deep sea and
cannot migrate along its bottom, we infer a shallow-water
connection in Mesozoic times between India, South America, and the
antarctic region. Such a shallow-water connection would be offered
along the marginal shelf of a continent uniting these now widely
separated countries.
CHAPTER XV
THE PRE-CAMBRIAN SYSTEMS
THE EARTH'S BEGINNINGS. The geological record does not tell us of
the beginnings of the earth. The history of the planet, as we have
every reason to believe, stretches far back beyond the period of
the oldest stratified rocks, and is involved in the history of the
solar system and of the nebula,--the cloud of glowing gases or of
cosmic dust,--from which the sun and planets are believed to have
been derived.
THE NEBULAR HYPOTHESIS. It is possible that the earth began as a
vaporous, shining sphere, formed by the gathering together of the
material of a gaseous ring which had been detached from a cooling
and shrinking nebula. Such a vaporous sphere would condense to a
liquid, fiery globe, whose surface would become cold and solid,
while the interior would long remain intensely hot because of the
slow conductivity of the crust. Under these conditions the
primeval atmosphere of the earth must have contained in vapor the
water now belonging to the earth's crust and surface. It held also
all the oxygen since locked up in rocks by their oxidation, and
all the carbon dioxide which has since been laid away in
limestones, besides that corresponding to the carbon of
carbonaceous deposits, such as peat, coal, and petroleum. On this
hypothesis the original atmosphere was dense, dark, and noxious,
and enormously heavier than the atmosphere at present.
THE ACCRETION HYPOTHESIS. On the other hand, it has been recently
suggested that the earth may have grown to its present size by the
gradual accretion of meteoritic masses. Such cold, stony bodies
might have come together at so slow a rate that the heat caused by
their impact would not raise sensibly the temperature of the
growing planet. Thus the surface of the earth may never have been
hot and luminous; but as the loose aggregation of stony masses
grew larger and was more and more compressed by its own
gravitation, the heat thus generated raised the interior to high
temperatures, while from time to time molten rock was intruded
among the loose, cold meteoritic masses of the crust and outpoured
upon the surface.
It is supposed that the meteorites of which the earth was built
brought to it, as meteorites do now, various gases shut up within
their pores. As the heat of the interior increased, these gases
transpired to the surface and formed the primitive atmosphere and
hydrosphere. The atmosphere has therefore grown slowly from the
smallest beginnings. Gases emitted from the interior in volcanic
eruptions and in other ways have ever added to it, and are adding
to it now. On the other hand, the atmosphere has constantly
suffered loss, as it has been robbed of oxygen by the oxidation of
rocks in weathering, and of carbon dioxide in the making of
limestones and carbonaceous deposits.
While all hypotheses of the earth's beginnings are as yet unproved
speculations, they serve to bring to mind one of the chief lessons
which geology has to teach,--that the duration of the earth in
time, like the extension of the universe in space, is vastly
beyond the power of the human mind to realize. Behind the history
recorded in the rocks, which stretches back for many million
years, lies the long unrecorded history of the beginnings of the
planet; and still farther in the abysses of the past are dimly
seen the cycles of the evolution of the solar system and of the
nebula which gave it birth.
We pass now from the dim realm of speculation to the earliest era
of the recorded history of the earth, where some certain facts may
be observed and some sure inferences from them may be drawn.
THE ARCHEAN.
The oldest known sedimentary strata, wherever they are exposed by
uplift and erosion, are found to be involved with a mass of
crystalline rocks which possesses the same characteristics in all
parts of the world. It consists of foliated rocks, gneisses, and
schists of various kinds, which have been cut with dikes and other
intrusions of molten rock, and have been broken, crumpled, and
crushed, and left in interlocking masses so confused that their
true arrangement can usually be made out only with the greatest
difficulty if at all. The condition of this body of crystalline
rocks is due to the fact that they have suffered not only from the
faultings, foldings, and igneous intrusions of their time, but
necessarily, also, from those of all later geological ages.
At present three leading theories are held as to the origin of
these basal crystalline rocks.
1. They are considered by perhaps the majority of the geologists
who have studied them most carefully to be igneous rocks intruded
in a molten state among the sedimentary rocks involved with them.
In many localities this relation is proved by the phenomena of
contact; but for the most part the deformations which the rocks
have since suffered again and again have been sufficient to
destroy such evidence if it ever existed.
2. An older view regards them as profoundly altered sedimentary
strata, the most ancient of the earth.
3. According to a third theory they represent portions of the
earth's original crust; not, indeed, its original surface, but
deeper portions uncovered by erosion and afterwards mantled with
sedimentary deposits. All these theories agree that the present
foliated condition of these rocks is due to the intense
metamorphism which they have suffered.
It is to this body of crystalline rocks and the stratified rocks
involved with it, which form a very small proportion of its mass,
that the term ARCHEAN (Greek, ARCHE, beginning) is applied by many
geologists.
THE ALGONKIAN
In some regions there rests unconformably on the Archean an
immense body of stratified rocks, thousands and in places even
scores of thousands of feet thick, known as the ALGONKIAN. Great
unconformities divide it into well-defined systems, but as only
the scantiest traces of fossils appear here and there among its
strata, it is as yet impossible to correlate the formations of
different regions and to give them names of more than local
application. We will describe the Algonkian rocks of two typical
areas.
THE GRAND CANYON OF THE COLORADO. We have already studied a very
ancient peneplain whose edge is exposed to view deep on the walls
of the Colorado Canyon. The formation of flat-lying sandstone
which covers this buried land surface is proved by its fossils to
belong to the Cambrian,--the earliest period of the Paleozoic era.
The tilted rocks on whose upturned edges the Cambrian sandstone
rests are far older, for the physical break which separates them
from it records a time interval during which they were upheaved to
mountainous ridges and worn down to a low plain. They are
therefore classified as Algonkian. They comprise two immense
series. The upper is more than five thousand feet thick and
consists of shales and sandstones with some limestones. Separated
from it by an unconformity which does not appear in Figure 207,
the lower division, seven thousand feet thick, consists chiefly of
massive reddish sandstones with seven or more sheets of lava
interbedded. The lowest member is a basal conglomerate composed of
pebbles derived from the erosion of the dark crumpled schists
beneath,--schists which are supposed to be Archean. As shown in
Figure 207, a strong unconformity parts the schists and the
Algonkian. The floor on which the Algonkian rests is remarkably
even, and here again is proved an interval of incalculable length,
during which an ancient land mass of Archean rocks was baseleveled
before it received the cover of the sediments of the later age.
THE LAKE SUPERIOR REGION. In eastern Canada an area of pre-
Cambrian rocks, Archean and Algonkian, estimated at two million
square miles, stretches from the Great Lakes and the St. Lawrence
River northward to the confines of the continent, inclosing Hudson
Bay in the arms of a gigantic U. This immense area, which we have
already studied as the Laurentian peneplain, extends southward
across the Canadian border into northern Minnesota, Wisconsin, and
Michigan. The rocks of this area are known to be pre-Cambrian; for
the Cambrian strata, wherever found, lie unconformably upon them.
The general relations of the formations of that portion of the
area which lies about Lake Superior are shown in Figure 262. Great
unconformities, UU' separate the Algonkian both from the Archean
and from the Cambrian, and divide it into three distinct systems,
--the LOWER HURONIAN, the UPPER HURONIAN, and the KEWEENAWAN. The
Lower and the Upper Huronian consist in the main of old sea muds
and sands and limy oozes now changed to gneisses, schists,
marbles, quartzites, slates, and other metamorphic rocks. The
Keweenawan is composed of immense piles of lava, such as those of
Iceland, overlain by bedded sandstones. What remains of these rock
systems after the denudation of all later geologic ages is
enormous. The Lower Huronian is more than a mile thick, the Upper
Huronian more than two miles thick, while the Keweenawan exceeds
nine miles in thickness. The vast length of Algonkian time is
shown by the thickness of its marine deposits and by the cycles of
erosion which it includes. In Figure 262 the student may read an
outline of the history of the Lake Superior region, the
deformations which it suffered, their relative severity, the times
when they occurred, and the erosion cycles marked by the
successive unconformities.
OTHER PRE-CAMBRIAN AREAS IN NORTH AMERICA. Pre-Cambrian rocks are
exposed in various parts of the continent, usually by the erosion
of mountain ranges in which their strata were infolded. Large
areas occur in the maritime provinces of Canada. The core of the
Green Mountains of Vermont is pre-Cambrian, and rocks of these
systems occur in scattered patches in western Massachusetts. Here
belong also the oldest rocks of the Highlands of the Hudson and of
New Jersey. The Adirondack region, an outlier of the Laurentian
region, exposes pre-Cambrian rocks, which have been metamorphosed
and tilted by the intrusion of a great boss of igneous rock out of
which the central peaks are carved. The core of the Blue Ridge and
probably much of the Piedmont Belt are of this age. In the Black
Hills the irruption of an immense mass of granite has caused or
accompanied the upheaval of pre-Cambrian strata and metamorphosed
them by heat and pressure into gneisses, schists, quartzites, and
slates. In most of these mountainous regions the lowest strata are
profoundly changed by metamorphism, and they can be assigned to
the pre-Cambrian only where they are clearly overlain
unconformably by formations proved to be Cambrian by their
fossils. In the Belt Mountains of Montana, however, the Cambrian
is underlain by Algonkian sediments twelve thousand feet thick,
and but little altered.
MINERAL WEALTH OF THE PRE-CAMBRIAN ROCKS. The pre-Cambrian rocks
are of very great economic importance, because of their extensive
metamorphism and the enormous masses of igneous rock which they
involve. In many parts of the country they are the source of
supply of granite, gneiss, marble, slate, and other such building
materials. Still more valuable are the stores of iron and copper
and other metals which they contain.
At the present time the pre-Cambrian region about Lake Superior
leads the world in the production of iron ore, its output for 1903
being more than five sevenths of the entire output of the whole
United States, and exceeding that of any foreign country. The ore
bodies consist chiefly of the red oxide of iron (hematite) and
occur in troughs of the strata, underlain by some impervious rock.
A theory held by many refers the ultimate source of the iron to
the igneous rocks of the Archean. When these rocks were upheaved
and subjected to weathering, their iron compounds were decomposed.
Their iron was leached out and carried away to be laid in the
Algonkian water bodies in beds of iron carbonate and other iron
compounds. During the later ages, after the Algonkian strata had
been uplifted to form part of the continent, a second
concentration has taken place. Descending underground waters
charged with oxygen have decomposed the iron carbonate and
deposited the iron, in the form of iron oxide, in troughs of the
strata where their downward progress was arrested by impervious
floors.
The pre-Cambrian rocks of the eastern United States also are rich
in iron. In certain districts, as in the Highlands of New Jersey,
the black oxide of iron (magnetite) is so abundant in beds and
disseminated grains that the ordinary surveyor's compass is
useless.
The pre-Cambrian copper mines of the Lake Superior region are
among the richest on the globe. In the igneous rocks copper, next
to iron, is the most common of all the useful metals, and it was
especially abundant in the Keweenawan lavas. After the Keweenawan
was uplifted to form land, percolating waters leached out much of
the copper diffused in the lava sheets and deposited it within
steam blebs as amygdules of native copper, in cracks and fissures,
and especially as a cement, or matrix, in the interbedded gravels
which formed the chief aquifers of the region. The famous Calumet
and Hecla mine follows down the dip of the strata to the depth of
nearly a mile and works such an ancient conglomerate whose matrix
is pure copper.
THE APPEARANCE OF LIFE. Sometime during the dim ages preceding the
Cambrian, whether in the Archean or in the Algonkian we know not,
occurred one of the most important events in the history of the
earth. Life appeared for the first time upon the planet. Geology
has no evidence whatever to offer as to whence or how life came.
All analogies lead us to believe that its appearance must have
been sudden. Its earliest forms are unknown, but analogy suggests
that as every living creature has developed from a single cell, so
the earliest organisms upon the globe--the germs from which all
later life is supposed to have been evolved--were tiny,
unicellular masses of protoplasm, resembling the amoeba of to-day
in the simplicity of their structure.
Such lowly forms were destitute of any hard parts and could leave
no evidence of their existence in the record of the rocks. And of
their supposed descendants we find so few traces in the pre-
Cambrian strata that the first steps in organic evolution must be
supplied from such analogies in embryology as the following. The
fertilized ovum, the cell with which each animal begins its life,
grows and multiplies by cell division, and develops into a hollow
globe of cells called the BLASTOSPHERE. This stage is succeeded by
the stage of the GASTRULA,--an ovoid or cup-shaped body with a
double wall of cells inclosing a body cavity, and with an opening,
the primitive mouth. Each of these early embryological stages is
represented by living animals,--the undivided cell by the
PROTOZOA, the blastosphere by some rare forms, and the gastrula in
the essential structure of the COELENTERATES,--the subkingdom to
which the fresh-water hydra and the corals belong. All forms of
animal life, from the coelenterates to the mammals, follow the
same path in their embryological development as far as the
gastrula stage, but here their paths widely diverge, those of each
subkingdom going their own separate ways.
We may infer, therefore, that during the pre-Cambrian periods
organic evolution followed the lines thus dimly traced. The
earliest one-celled protozoa were probably succeeded by many-
celled animals of the type of the blastosphere, and these by
gastrula-like organisms. From the gastrula type the higher sub-
divisions of animal life probably diverged, as separate branches
from a common trunk. Much or all of this vast differentiation was
accomplished before the opening of the next era; for all the
subkingdoms are represented in the Cambrian except the
vertebrates.
EVIDENCES OF PRE-CAMBRIAN LIFE. An indirect evidence of life
during the pre-Cambrian periods is found in the abundant and
varied fauna of the next period; for, if the theory of evolution
is correct, the differentiation of the Cambrian fauna was a long
process which might well have required for its accomplishment a
large part of pre-Cambrian time.
Other indirect evidences are the pre-Cambrian limestones, iron
ores, and graphite deposits, since such minerals and rocks have
been formed in later times by the help of organisms. If the
carbonate of lime of the Algonkian limestones and marbles was
extracted from sea water by organisms, as is done at present by
corals, mollusks, and other humble animals and plants, the life of
those ancient seas must have been abundant. Graphite, a soft black
mineral composed of carbon and used in the manufacture of lead
pencils and as a lubricant, occurs widely in the metamorphic pre-
Cambrian rocks. It is known to be produced in some cases by the
metamorphism of coal, which itself is formed of decomposed vegetal
tissues. Seams of graphite may therefore represent accumulations
of vegetal matter such as seaweed. But limestone, iron ores, and
graphite can be produced by chemical processes, and their presence
in the pre-Cambrian makes it only probable, and not certain, that
life existed at that time.
PRE-CAMBRIAN FOSSILS. Very rarely has any clear trace of an
organism been found in the most ancient chapters of the geological
record, so many of their leaves have been destroyed and so far
have their pages been defaced. Omitting structures whose organic
nature has been questioned, there are left to mention a tiny
seashell of one of the most lowly types,--a DISCINA from the pre-
Cambrian rocks of the Colorado Canyon,--and from the pre-Cambrian
rocks of Montana trails of annelid worms and casts of their
burrows in ancient beaches, and fragments of the tests of
crustaceans. These diverse forms indicate that before the
Algonkian had closed, life was abundant and had widely
differentiated. We may expect that other forms will be discovered
as the rocks are closely searched.
PRE-CAMBRIAN GEOGRAPHY. Our knowledge is far too meager to warrant
an attempt to draw the varying outlines of sea and land during the
Archean and Algonkian eras. Pre-Cambrian time probably was longer
than all later geological time down to the present, as we may
infer from the vast thicknesses of its rocks and the
unconformities which part them. We know that during its long
periods land masses again and again rose from the sea, were worn
low, and were submerged and covered with the waste of other lands.
But the formations of separated regions cannot be correlated
because of the absence of fossils, and nothing more can be made
out than the detached chapters of local histories, such as the
outline given of the district about Lake Superior.
The pre-Cambrian rocks show no evidence of any forces then at work
upon the earth except the forces which are at work upon it now.
The most ancient sediments known are so like the sediments now
being laid that we may infer that they were formed under
conditions essentially similar to those of the present time. There
is no proof that the sands of the pre-Cambrian sandstones were
swept by any more powerful waves and currents than are offshore
sands to-day, or that the muds of the pre-Cambrian shales settled
to the sea floor in less quiet water than such muds settle in at
present. The pre-Cambrian lands were, no doubt, worn by wind and
weather, beaten by rain, and furrowed by streams as now, and, as
now, they fronted the ocean with beaches on which waves dashed and
along which tidal currents ran.
Perhaps the chief difference between the pre-Cambrian and the
present was the absence of life upon the land. So far as we have
any knowledge, no forests covered the mountain sides, no verdure
carpeted the plains, and no animals lived on the ground or in the
air. It is permitted to think of the most ancient lands as deserts
of barren rock and rock waste swept by rains and trenched by
powerful streams. We may therefore suppose that the processes of
their destruction went on more rapidly than at present.
CHAPTER XVI
THE CAMBRIAN
THE PALEOZOIC ERA. The second volume of the geological record,
called the Paleozoic (Greek, PALAIOS, ancient; ZOE, life), has
come down to us far less mutilated and defaced than has the first
volume, which contains the traces of the most ancient life of the
globe. Fossils are far more abundant in the Paleozoic than in the
earlier strata, while the sediments in which they were entombed
have suffered far less from metamorphism and other causes, and
have been less widely buried from view, than the strata of the
pre-Cambrian groups. By means of their fossils we can correlate
the formations of widely separated regions from the beginning of
the Paleozoic on, and can therefore trace some outline of the
history of the continents.
Paleozoic time, although shorter than the pre-Cambrian as measured
by the thickness of the strata, must still be reckoned in millions
of years. During this vast reach of time the changes in organisms
were very great. It is according to the successive stages in the
advance of life that the Paleozoic formations are arranged in five
systems,--the CAMBRIAN, the ORDOVICIAN, the SILURIAN, the
DEVONIAN, and the CARBONIFEROUS. On the same basis the first three
systems are grouped together as the older Paleozoic, because they
alike are characterized by the dominance of the invertebrates;
while the last two systems are united in the later Paleozoic, and
are characterized, the one by the dominance of fishes, and the
other by the appearance of amphibians and reptiles.
Each of these systems is world-wide in its distribution, and may
be recognized on any continent by its own peculiar fauna. The
names first given them in Great Britain have therefore come into
general use, while their subdivisions, which often cannot be
correlated in different countries and different regions, are
usually given local names.
The first three systems were named from the fact that their strata
are well displayed in Wales. The Cambrian carries the Roman name
of Wales, and the Ordovician and Silurian the names of tribes of
ancient Britons which inhabited the same country. The Devonian is
named from the English county Devon, where its rocks were early
studied. The Carboniferous was so called from the large amount of
coal which it was found to contain in Great Britain and
continental Europe.
THE CAMBRIAN
DISTRIBUTION OF STRATA. The Cambrian rocks outcrop in narrow belts
about the pre-Cambrian areas of eastern Canada and the Lake
Superior region, the Adirondacks and the Green Mountains. Strips
of Cambrian formations occupy troughs in the pre-Cambrian rocks of
New England and the maritime provinces of Canada; a long belt
borders on the west the crystalline rocks of the Blue Ridge; and
on the opposite side of the continent the Cambrian reappears in
the mountains of the Great Basin and the Canadian Rockies. In the
Mississippi valley it is exposed in small districts where uplift
has permitted the stripping off of younger rocks. Although the
areas of outcrop are small, we may infer that Cambrian rocks were
widely deposited over the continent of North America.
PHYSICAL GEOGRAPHY. The Cambrian system of North America comprises
three distinct series, the LOWER CAMBRIAN, the MIDDLE CAMBRIAN,
and the UPPER CAMBRIAN, each of which is characterized by its own
peculiar fauna. In sketching the outlines of the continent as it
was at the beginning of the Paleozoic, it must be remembered that
wherever the Lower Cambrian formations now are found was certainly
then sea bottom, and wherever |
