Books: The Elements of Geology
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William Harmon Norton >> The Elements of Geology
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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.
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