HOW MOUNTAINS COME AND GO
Mountains constitute the grandest relief features of the earth, and
some of the most profound lessons of earth changes may be learned by
studying them. To the layman who views great mountains in all their
grandeur and massiveness, the expression "everlasting hills" seems
appropriate. But the geologist knows that even the loftiest mountains
are only temporary features on the face of the earth. Like organisms,
they come and go. For example, where the great Rocky Mountains now
stand was only a few million years ago (in late Mesozoic time) the
bottom of an interior sea. Where the Appalachians now stand there were
no mountains late in the Paleozoic era (not less than ten or twelve
million years ago), but instead sea water covered the district. Then
the Appalachians were formed, lifting their heads much higher than at
present, after which they were cut down almost to sea level, and then
once more upraised. The Coast Range Mountains of our Pacific Coast have
come into existence since the middle of the present (Cenozoic) geologic
era. Every mountain, like every organism, has a life history, in some
cases simple, and other cases complex. All pass through stages of
birth, youth, maturity, old age, and death. Some rear their heads and
disappear after a short (geological) existence. Others continue their
growth and persist much longer, while still others undergo periods of
profound rejuvenation.
Among the various processes by which mountain ranges have been
formed, the folding and accompanying general uplift of strata are
the most important. In fact, in most of the great mountain ranges of
the world the folded structure is conspicuously developed, so much
so that they may well be called "folded mountains." Very commonly,
however, mountains of this type have also been subjected to more or
less fracturing of the rocks (faulting), and not uncommonly they have
also been subjected to igneous activity, including both intrusion and
extrusion of molten material. It is among the folded mountains of
greater or less degree of complexity that the "greatest exhibitions of
geologic phenomena are seen and the lessons which geology as a sciences
teaches may be learned. If one desires to know the history of a region,
one turns naturally to its mountain ranges, for here may be found the
upturned and dissected strata, a study of whose kinds, thickness,
and fossils throws light upon past events, while their foldings and
dislocations show the nature and results of those great dynamic
agencies which, from time to time, have operated upon the outer portion
of the earth, and given to it the broad distinctive features which
characterize it to-day." (L. V. Pirsson.) Among the great mountains we
may also see wonderful exhibitions of the results of weathering and
erosion, especially the work of rivers and glaciers.
We can, perhaps, best convey to the reader some of the main facts
and principles regarding folded mountains by considering certain
observations which may be readily made in a short trip across a folded
range of not too complex kind--for example, across the Appalachian
range along the line of the Baltimore and Ohio Railroad, west of
Washington, or the Pennsylvania Railroad, west of Philadelphia. It
would be most evident that the mountains consist of strata, that is
sedimentary rocks, such as sandstone, shale and limestone, which were
deposited under water. A few measurements would reveal the fact that
thousands of feet in thickness of strata are represented. Careful
measurements by geologists have, in fact, shown that the strata were
originally piled up layer upon layer to a thickness of 25,000 to 30,000
feet. The fact that they are strata of such great thickness proves that
sediments must there have accumulated under water for some millions
of years at least. Closer examination of a few good exposures (i.e.,
outcrops) would further reveal the presence of fossil shells and
impressions of marine organisms, thus definitely leading us to conclude
that the strata were accumulated under sea water, which, of course,
means that the present site of the mountain range was once sea floor.
Examination of the rock materials also establishes the fact that the
strata are such as were deposited in relatively shallow sea water--that
is to say, none are at all of the sort which are now forming under
really deep ocean water. Most of the strata represent original
sands (and even gravels) and muds which could have accumulated only
relatively near shore, that is within about 100 miles, which harmonizes
with a statement made in a preceding chapter to the effect that very
little land-derived sediment is at present depositing more than 100
miles out from shore. The coarse materials (sands and gravels) could
not, of course, be carried many miles out, while many of the strata are
covered with ripple marks, thus positively proving their shallow-water
origin. We conclude, therefore, that the Appalachian strata are of
marine, shallow-water origin. But we have already stated that these
strata are at least 25,000 feet thick. How, then, do we reconcile
these two seemingly paradoxical statements? All that is necessary is
to realize that the floor of the shallow sea, in which the sediments
eroded from adjacent land were being deposited, slowly, though more
or less irregularly, subsided or sank during the long ages (millions
of years) of their accumulation. It would carry us too far afield to
really attempt an explanation of this remarkable type of geologic
phenomenon, and it must suffice to suggest that, starting with the
earth's crust in equilibrium, the very weight of accumulating strata
would tend to destroy that equilibrium and so cause subsidence.
In our trip across the mountains it would be strikingly evident that
the strata are no longer in their original horizontal position, as
they were piled up layer upon layer, but that they have been notably
disturbed and thrown into folds (Plate 7), large and small, some masses
of the strata having been bent upward (anticlines) and others downward
(synclines). Such folded structures could have been developed only by a
great force of lateral compression in the earth's crust within the zone
of flowage. Under compression the strata were mashed together, notably
bent into curves (folds), and more or less upraised. It would also be
readily observed that the main axes of the folds extend essentially
parallel to the main trend of the mountain range, thus proving that
the force of compression was exerted at right angles to the trend of
the range.
[Illustration: Fig. 24.--Diagrammatic sections illustrating the
development of a typical folded mountain range. Upper figure: A, the
old land eroded to furnish sediments deposited under the adjacent
sea at C. Middle figure: strata (C) folded as they would appear if
unaffected by erosion, and a down-warp (B) between A and C. Lower
figure: condition after profound erosion, and filling of B with
sediment. (Drawn by the author.)]
Using a biological analogy, a brief history of a typical folded
mountain range may be stated as follows: First, there is the prenatal
or embryonic stage when the materials of the range are gathering, that
is when the sediments are piling up layer upon layer relatively near
shore on a sinking sea bottom. Next comes the birth of the range when,
due to the great lateral compressive force, the strata are thrown into
folds and forced to appear above sea level, the range thus literally
being born out of the sea. During the next, or youthful stage,
the range grows (with increasing altitudes) because of continued
application of the compressive force. Even during the youthful growing
stage weathering and erosion attack the range and tend to reduce it.
Then comes the stage of maturity, when the upbuilding (compressive)
force and the tearing down (erosive) force about counterbalance each
other. At this time the range has reached its maximum height and
ruggedness of relief, with ridges and valleys higher and deeper than
at any other time. The old-age stage sets in when the upbuilding power
wanes or actually ceases, and erosion dominates or reigns supreme.
Slowly but surely, unless there be a renewal by an upbuilding power,
the range is cut down until little or nothing of it remains well
above sea level, or, in other words, until a peneplain is developed.
This last stage may truly be called the death of the range. Usually,
however, some local portions of the disappearing range, which are more
resistant or more favorably situated against erosion, are left standing
to at least moderate heights above the general level of the plain of
erosion.
The above normal order of events may be disturbed at any stage,
especially after maturity, by renewed uplift when the streams are
revived in activity and increased ruggedness results. Even after the
whole range as a relief feature has been planed away, the site of the
range may be uplifted and a new cycle of erosion started.
By the use of two well-known examples we shall not only illustrate the
above principles of mountain history, but also show that no less than a
few million years must be allowed for the growth and decay of a great
folded range. During the last (Permian) period of the Paleozoic era
the Appalachian strata began to buckle and the yielding to pressure
continued till well into the succeeding (Triassic) period. The climax
was reached about the close of the Permian. Then, throughout the
Mesozoic era, erosion reduced the central Appalachians to a great plain
(peneplain) near sea level, after which, about the beginning of the
present (Cenozoic era), the site of the former range was distinctly
upraised (without folding of the rocks), causing the revived streams to
begin their work of carving out the present ridges and valleys, this
work still being in progress.
In the case of the Sierra Nevadas, the strata were folded into a lofty
mountain range relatively late in the Mesozoic era and, by the middle
of the Cenozoic era, the old-age stage of erosion was well advanced
when the range was not more than a few thousand feet high. Then (in
the middle of the Cenozoic era) uplift, accompanied by faulting on a
large scale, but not by folding, took place, and the range was notably
rejuvenated to about its present height. All the remarkably deep
canyons of the Sierras have been carved out since the rejuvenation.
How is the geological birthday of a mountain range determined? In the
preceding paragraph we stated that the Appalachians were folded and
born out of the sea about the close of the Paleozoic era. This is
readily proved by calling attention to two facts. First, the youngest
strata involved in the folding are of Permian, or late Paleozoic Age
in the geologic column, as proved by their fossil content, etc.,
and obviously the folding must have taken place after they had been
deposited. Clearly, then, the folding could not have taken place before
very late Paleozoic time. Second, the oldest strata resting upon the
folded rocks are of early (not the very earliest) Mesozoic Age, and
these strata are somewhat tilted but not folded. Obviously, then, the
folding must have occurred before the nonfolded strata were deposited,
which means that the folding must have been essentially completed
in not later than early Mesozoic time. Or, in the case of the Rocky
Mountains, we know that strata were there folded late in the Mesozoic
era or very early in the Cenozoic era, because folded rocks as late
in age as late Mesozoic (Cretaceous) have resting upon them, in some
places, nonfolded strata of early Cenozoic (Tertiary) Age. The figure
clearly shows how the Ordovician strata must have been folded before
the next (Silurian and Devonian) strata were deposited upon them in
southeastern New York.
[Illustration: Fig. 25.--Diagram illustrating the topography and
folded structure of the Appalachian Mountains west of Harrisburg,
Pennsylvania. The valleys have been etched out of belts of weak rocks,
while outcropping resistant rocks stand out to form ridges. Note the
course of the Susquehanna River across the mountain ridges, this being
a "superimposed river" (see text, p. 233). (Drawn by A. K. Lobeck.)]
[Illustration: Fig. 26.--Only slightly tilted strata of Silurian and
Devonian ages resting upon folded strata of Cambrian and Ordovician
ages in an east-west section across the Catskill Mountains and Hudson
Valley of New York. The folding took place at the time of the Taconic
Mountain Revolution toward the end of the Ordovician period. (Drawn by
the author.)]
As already suggested, however, folding is not the only method by
which mountains are formed. Many ranges are either entirely due to
the tilting of earth blocks by faulting or fracturing of the earth,
or their present altitude, at least, is a direct result of faulting.
Such may be called block mountains. They are wonderfully represented
by the various north-south ranges rising some thousands of feet above
the general level of the Great Basin region of Utah and Nevada.
These ranges are, in short, somewhat eroded edges of approximately
parallel-tilted fault blocks lying between the Sierra Nevada Range and
the Wasatch Range. In southeastern Oregon a series of nearly parallel
block mountains, up to forty miles in length and over 1,000 feet in
height, show very steep eastern fronts only slightly modified by
erosion.
Another mode of origin of mountains is by the rise of molten material
to the surface, especially where a chain of volcanoes is located. Thus
the Cascade Mountains from northern California through Oregon and
Washington, including Mounts Lassen, Shasta, Pitt, Baker, St. Helens,
and Rainier, are very largely the result of volcanic action. The long
chain of Aleutian Islands of Alaska, referred to in our study of
volcanoes, is an excellent example of a great mountain range now being
built up out of the sea by volcanic action. More locally molten rocks
under pressure may not reach the surface but instead simply bulge or
dome the strata over them, as in the case of the group known as the
Henry Mountains of Utah, and also in other parts of the West.
In still other cases mountains of considerable area and altitude have
resulted from erosion of uplifted regions where the uplift has been
practically unaccompanied by either folding, faulting, or igneous
activity. Any low-lying area, regardless of the character of its rocks,
structure, or previous history, may be notably upraised and simply
subjected to erosion. An excellent illustration is afforded by the
Catskill Mountains of New York, where numerous deep valleys and narrow
ridges have been carved out of upraised nearly horizontal strata. The
so-called "Bad Lands" region of parts of South Dakota and Wyoming is
also essentially of this type, where deep, narrow valleys and sharp
ridges have been etched out of high, relatively soft, nearly horizontal
strata, resulting in an almost impassable maze of mountains. In the
high, recently upraised Colorado Plateau of parts of Arizona, New
Mexico, Colorado, and Utah, nearly horizontal strata are being etched
out, the result being numerous buttes, mesas (flat-topped hills and
mountains) and deep canyons, including the Grand Canyon with its maze
of peaks and pinnacles, many of them rising like mountains out of the
canyon depths.
Mountains of the pure types just described are not the prevailing
ones of the earth. Most mountains and their structures, as we see
them to-day, are products of two or more of the processes of folding,
faulting, igneous action, and erosion. A few well-known examples will
suffice to make this matter clearer. Thus, the Appalachian Mountains
originally developed by severe folding of thick strata. After
considerable erosion, numerous small and large thrust faults developed,
some of the dislocations amounting to miles. Then the whole range was
cut down nearly to sea level by erosion, after which the district was
upraised (without folding) mostly from 2,000 to 4,000 feet, and the
present long, narrow mountain ridges and valleys have been carved out
by stream erosion. Thus folding, faulting, and erosion all enter into
the height and structure of the Appalachians.
A lofty mountain range still more complex in its history is the Sierra
Nevada of California. First, thick strata were highly folded, upraised,
and intruded by great masses of molten granite. Erosion then proceeded
to cut the range down to hills, after which a great fracture (fault)
developed along the eastern side and the Sierra Nevada earth block was
notably tilted with steep eastern front and long western slope. Erosion
has considerably modified the eastern fault face, and the deep canyons
like Yosemite, King's River and American River, have been carved out
of the western slope of the great tilted fault block. Geologically
recently the central to northern portion of the range has been affected
by volcanic action, streams of lava in some cases having flowed down
the valleys.