MODERN EARTH HISTORY
(_Cenozoic Era_)
Since the Cenozoic era is the last one of geologic time, it will be of
particular interest to trace out the main events which have led up to
the present day conditions, especially in North America. Both because
of the recency of the time and the unusual accessibility of the rocks,
which are mostly at or near the surface, our knowledge of the Cenozoic
era is exceptionally detailed and accurate. It will, therefore, be more
necessary than ever to select only the very significant features of
this history for our brief discussion.
During the first half of the Tertiary period portions only of the
Atlantic coastal plain were submerged under shallow water, but soon
after the middle of the period (Miocene epoch) the sea spread over
practically the whole Atlantic coastal plain area from Martha's
Vineyard south to and including Florida. During the late Tertiary the
marine waters had become greatly restricted, and by the close of the
period the sea was entirely excluded from the Atlantic seaboard. The
total thickness of these Tertiary strata is less than 1,000 feet, and
they all tilt downward gently toward the sea. The strata consist mostly
of unconsolidated sands, gravels, clays, marls, etc.
The Gulf coastal plain area from Florida through Texas and south
through eastern Mexico was largely overspread by the sea during most
of Tertiary time, except the latest. During early Tertiary time an arm
of the Gulf reached north to the mouth of the Ohio River. Late in the
period but little of the Gulf Plain was submerged, and at its close sea
water was wholly excluded. On the Gulf Coast the Tertiary strata from
2,000 to 4,000 feet thick are also mainly sands, gravels, clays, and
marls. They are commonly rich in fossils, and they show a gentle tilt
downward toward the Gulf.
Throughout Tertiary time local portions of the Pacific border of
the continent were submerged, this having been especially true of
portions of California, Oregon, and Washington. In spite of the very
restricted marine waters, the Tertiary strata of the Pacific Coast,
especially in California, are remarkably thick, 10,000 to 20,000 feet
being common, while the maximum thickness is fully 30,000 feet. Such
great thicknesses are readily explained when we realize that erosion
was notably speeded up by pronounced uplifts resulting from crustal
disturbances toward the close of the preceding period, and again in the
midst of the Tertiary period itself.
To summarize the Tertiary relations of sea and land for North America
we may say that only local portions of the continental border ever
became submerged, and that, by late Tertiary time, practically the
whole continent was a land area. At the close of the period the
continent was, as we shall see, even larger than now because the
continental shelves of the ocean were then also largely above water.
[Illustration: Fig. 45.--Map showing the general relations of land
and water in North America during part of the middle Tertiary period.
(After Willis, courtesy of the Journal of Geology.)]
The whole of the Cenozoic era, including both the Tertiary and
Quaternary periods, has been a time of profound crustal disturbances
throughout much of the continent, certain of these movements having
continued right up to the present time, with positive evidence that
some of them are still continuing. These great movements have included
notable foldings of strata, uplifts without folding, faulting, and
igneous activity, the whole effect having been to greatly increase
the general altitude and ruggedness of the continent. In fact, North
America is not known ever to have been at once higher, broader, and
more rugged than it was very late in the Tertiary, or early in the
Quaternary, period. Since that time the only notable change (barring
the great Ice Age and its effects) has been a restriction of the area
of the continent to its present size by spreading of sea waters over
the borders of the continent, that is over the continental shelves.
We shall now rather systematically consider the more profound earth
changes which have affected the continent, producing the existing major
relief features, from west to east.
The "Coast Range Revolution" took place in the midst of the Tertiary
period. Over the site of the Coast Ranges, strata had accumulated,
especially during Cretaceous and earlier Tertiary times, to a
thickness of thousands of feet. In middle Tertiary time these strata
were subjected to a mountain-making force of compression and more or
less folded, faulted (fractured), and uplifted into the Coast Range
Mountains. Some portions of the range were intensely folded and faulted
and upraised many thousands of feet, while other portions were only
moderately folded and uplifted. It is an interesting fact that the
great San Francisco earthquake rift or fault originated at this time.
It was a renewed, sudden movement of a few feet along this fault which
caused the disastrous earthquake of 1906. Still other considerable
earth movements took place in the Coast Range region during late
Tertiary and Quaternary times, as, for example, uplift without folding,
as proved by distinct sea-cut terraces at altitudes of more than a
thousand feet, like those north of San Francisco and south of Los
Angeles. A moderate amount of still later subsidence has caused the
development of San Francisco Bay. The large islands off the coast
of southern California have in very recent geologic time (probably
Quaternary) been cut off from the mainland by sinking of the land.
[Illustration: Plate 11.--(_a_) Part of the Mammoth Hot Springs Terrace
in the Yellowstone Park. The view shows the deposit with boiling water
flowing over it. The water enters the earth back on the mountain,
travels underground in contact with hot lava, rises through limestone,
from which the boiling water takes into solution much carbonate of
lime which is deposited when the water reaches the surface. (_Photo by
Jackson, U. S. Geological Survey._)]
[Illustration: Plate 11.--(_b_) View Across Part of Crater Lake,
Oregon. This great hole, 3,000 to 4,000 feet deep and 6 miles in
diameter and now partly filled with a lake 2,000 feet deep, was formed
by a subsidence of the top of a once great cone-shaped volcano fully
14,000 feet high above the sea. The high rock in the distance rises
2,000 feet above the lake which is over 6,000 feet above sea level. The
island is a small volcano of recent origin. (_Photo by Russell, U. S.
Geological Survey._)]
[Illustration: Plate 12.--(_a_) Detailed View of Part of the Very
Oldest Known (Archeozoic) Rock Formation of the Earth. The rock is
distinctly stratified and represents sands and muds deposited layer
upon layer upon a sea floor at least 50,000,000 years ago. The sands
and muds first consolidated into sandstone and shale below the earth's
surface. Then, under conditions of heat, moisture, and pressure, they
were notably altered, mainly by crystallization of minerals, and raised
high above sea level. Finally the strata were laid bare by erosion.
(_Photo by the author._)]
[Illustration: Plate 12.--(_b_) A Twisted Mass of Stratified Archeozoic
Limestone Surrounded by Granite in Northern New York. The limestone was
enveloped in the granite while it was being forced in molten condition
into the earth's crust. (_Photo by the author._)]
The Sierra Nevada Range, which originated by intense folding of rocks
late in the Jurassic period, underwent profound erosion until about
the middle of the Tertiary period, by which time it had been cut down
to a range of hills or low mountains. Then the great fault (fracture)
previously described began to develop along the eastern side. As a
result of many sudden movements along this fault, which is hundreds of
miles long, the vast earth block has been tilted westward with a very
steep eastern face and a long, more gradual western slope, the crest of
the fault block forming the summit of the range. The amount of nearly
vertical displacement along this fault has been commonly from 10,000
to 20,000 feet, and, in spite of considerable erosion of the top of
the fault block and accumulation of sediment at its eastern base, the
modified fault face now usually stands out boldly from 2,000 to 10,000
feet high. As an evidence that this movement of faulting has not yet
ceased we may cite the Inyo earthquake of 1872, when there was a sudden
renewal of movement of ten to twenty-five feet along this fault for
many miles. Since the great Sierra block began to tilt, the many mighty
canyons, like Yosemite, Hetch-Hetchy, King's River, and Feather River,
have been carved out by the action of streams, in some cases aided by
former glaciers. King's River canyon has been sunk to a maximum depth
of 6,900 feet in solid granite solely by the erosive action of the
river!
The Cascade Mountains, too, were reduced to nearly a peneplain
condition by late Tertiary time when they began to be rejuvenated by
arching or bowing of the surface unaccompanied by great faulting or
fracturing, and many canyons, like that of the Columbia River, have
since been carved out.
Mention should now be made of the vigorous volcanic activity which took
place in the Cascade and Sierra Nevada Ranges. Most of this activity
occurred during Tertiary time (particularly in the latter part) and it
has continued with diminishing force practically to the present time.
In California streams of lava buried many gold-bearing river gravels
which have yielded rich mines. Many well-known mountain peaks, such
as Shasta, Lassen, Pitt, Hood, and Rainier, from northern California
to Washington, are great volcanic cones which date from Tertiary
time, and which are now mostly inactive. That this volcanic activity
has not yet altogether ceased is shown by renewed eruptions of Mount
Lassen (or Lassen Peak, altitude 10,437 feet) in northern California.
Since the beginning of this renewed activity in 1914, several hundred
outbursts have occurred. No molten rock has flowed out, but large
quantities of rock fragments, dust and steam have been erupted, in many
cases forming great clouds two or three miles high over the top of the
mountain (Plate 10). At this writing (October, 1920), Mount Lassen is
still showing vigorous activity. At Cinder Cone, only ten miles from
Mount Lassen, there were two eruptions of cinders and a considerable
outpouring of lava within the last 200 years. Still other very recent
cinder cones occur in southeastern California and Arizona.
[Illustration: Fig. 46.--Sketch map showing the distribution of
volcanic rocks of Cenozoic (mostly Tertiary) Age in western North
America. Only one volcano (Mount Lassen, California) is now active
in the United States proper, but a number are more or less active
in Mexico and Central America. (Data from Willis, U. S. Geological
Survey.)]
One of the greatest lava fields in the world forms the Columbian
Plateau between western Wyoming (including the Yellowstone National
Park) and the Cascade Mountains from northeastern California to
northern Washington. It covers fully 200,000 square miles and is really
considerably larger than shown on the map because the lava in parts of
the plateau region are covered by very recent sedimentary materials.
The great lava fields of the Deccan, India, and of the plateau region
of western Mexico are comparable in size to the Columbian field and
these lava fields are all of the same age. In the Columbian Plateau
most of the lava was poured out during later Tertiary time. Sheets of
molten rock, averaging fifty to one hundred feet in thickness, spread
out over various parts of the region and piled up by overlapping
layers one over another until the lava plateau more than a mile high
was built up. Many hills and low mountains were completely buried
under the molten floods, and in other places the liquid rock masses
flowed against the higher mountains. "For thousands of square miles
the surface is a lava plain which meets the boundary mountains as a
lake or sea meets a rugged and deeply indented coast.... The plateau
was long in building. Between the layers are found in places old soil
beds and forest grounds and the sediments of lakes.... So ancient are
the latest floods in the Columbia River Basin that they have weathered
to a residual yellow clay from thirty to sixty feet in depth, and
marvelously rich in the mineral substances on which plants feed. In
the Snake River Valley the latest lavas are much younger (Quaternary).
Their surfaces are so fresh and undecayed that here the effusive
eruptions may have continued to within the period of human history."
(W. H. Norton.) Many of the lava layers are plainly visible where the
Columbia River has cut its great gorge or canyon. The Snake River in
places has sunk its channel several thousand feet into the lava plateau
without reaching underlying rock.
Both north and south of the Columbian Plateau there was also much
volcanic activity in the Rocky Mountain region during Tertiary time. A
single formation in Colorado consists mostly of volcanic "ash" or dust
over 2,000 feet thick. There was also much volcanic activity over the
Colorado Plateau area of southern Utah, New Mexico, and Arizona. The
volcanoes there exhibit all stages from those which are very recent and
practically unaffected by erosion to others which have been completely
cut away with the exception of the cores or "volcanic necks."
During the second half of the Tertiary period the whole region known
as the Great Basin, between the Sierra Nevada Mountains of California
and the Wasatch Mountains of Utah, began to be affected by profound
faulting or fracturing and tilting of portions of the earth's crust.
The two largest faults, one on the western side of the Wasatch Range
and the other on the eastern side of the Sierra Range, are each
hundreds of miles long. Each of these ranges owes most of its present
altitude to the uptilting of great fault blocks, and most of the many
nearly north-south Basin Ranges of Nevada and Utah are in reality
recently tilted fault blocks.
Turning now to the Colorado Plateau, studies have shown that region
to have been more or less periodically raised fully 20,000 feet since
the beginning of Tertiary time, but because of profound erosion in
the meantime its present altitude is only 6,000 to 9,000 feet. During
late Tertiary time the land stood at a much lower level than to-day,
so that, practically during the last period (Quaternary) of geologic
time, the region has been elevated to its present position. As a
direct result of this profound rejuvenation the Colorado River has
had its erosive activity tremendously increased, and it has carved
out the mightiest of all existing canyons--the Grand Canyon. The work
of deepening and widening the canyon is still proceeding at a rapid
geologic rate.
As we have learned, the Rocky Mountains and many of its subsidiary
ranges were formed by folding and uplift of strata toward the close
of the Mesozoic era (Cretaceous period). During much of Tertiary time
the newly formed mountains had been considerably reduced by erosion.
Then, late in the Tertiary period, much of the Rocky Mountain region,
as well as much of the Great Plains area just east of the mountains,
became rejuvenated by differential uplift without any notable folding
of strata. We can tell that this general uplift amounted to at least
several thousand feet because definite formations of relatively late
Tertiary strata, originally horizontally deposited under inland bodies
of water, gradually rise so that at the base of the Front Range of
the Rockies they are fully 3,000 feet higher than they are 200 miles
or more farther east. Thus, the original folding and faulting of the
Rockies, Tertiary volcanic activity, late Tertiary rejuvenation, and
subsequent erosion account for the present altitude and relief features
of the great Rocky Mountain system.
Portions of the rejuvenated Great Plains region have been notably
dissected by erosion since the late Tertiary, this being particularly
true of the so-called "Bad Lands," especially in parts of Wyoming and
South Dakota, where mostly relatively soft Tertiary strata have been
cut to pieces.
Turning our attention now to the eastern half of the continent we find
that all, or nearly all, of it was more or less raised toward the
close of the Tertiary period. Practically the whole Mississippi Valley
east of the Great Plains, as well as much of the country to the north
in Canada, was elevated some hundreds of feet and the streams have
since the late Tertiary uplift (except where the land was ice-covered
during the Ice Age) been at work sinking their channels below the newly
upraised surface.
As already pointed out, the lowlands of the Atlantic and Gulf Coastal
Plains were mostly submerged under the sea during early middle Tertiary
time. By the close of the period they had emerged practically to their
present positions, and they have been only moderately affected by
erosion.
We have still to explain the existing topography or relief of a large
and important part of eastern North America, including the whole of
the Appalachian Mountains, Allegheny Plateau, Piedmont Plateau, New
York, New England, and the Canadian region to the north. As a starting
point in this discussion we should recall the fact that, after the
great Appalachian Mountain Revolution toward the close of the Paleozoic
era, the predominant geologic process which affected the region under
consideration was erosion throughout the succeeding Mesozoic era. By
about the close of the Mesozoic (Cretaceous period) the whole region,
with some local exceptions, has been worn down to a comparatively
smooth plain (peneplain) not far above sea level. Local exceptions
were mainly in the New York and New England region as, for example,
some of the higher parts of the Adirondack and White Mountains, Mount
Monadnock in southern New Hampshire, and Mount Greylock in western
Massachusetts. These and other masses rose rather conspicuously above
the general level of the great plain of erosion commonly called the
"Cretaceous peneplain" because it is believed to have been well
developed by the close of that period.
The uplift of the vast Cretaceous peneplain about the beginning of
the Cenozoic era (Tertiary period) was an event of prime importance
in the recent geological history of eastern North America because
it was literally the initial step in bringing about nearly all of
the existing major relief features of the Appalachian-New York-New
England-St. Lawrence region. The amount of uplift (unaccompanied by
folding) of the peneplain was commonly from a few hundred to a few
thousand feet with the greatest amount in general along the main trend
of the Appalachians. The fact should be emphasized that nearly all the
principal topographic features of the great upraised region have been
produced by dissection (erosion) of the uplifted peneplain surface.
Thus nearly all the valleys, small and large, including those of the
St. Lawrence, Hudson, Mohawk, Connecticut, and Susquehanna, have been
carved out by streams since the uplift of the great peneplain.
The streams which flowed upon the old low-lying peneplain surface
meandered sluggishly over deep alluvial or flood-plain deposits, and
their courses were little if any determined by the character and
structure of the underlying rocks, because, with few exceptions, all
rocks were worn down to the general plain level. The uplift of the
peneplain, however, caused great revival of activity of erosive power
by the streams, the larger ones of which soon cut through the loose
superficial alluvial deposits and then into the underlying bedrock.
Thus the large, original streams had their courses well determined in
the overlying deposits, and when the underlying rocks were reached
the same courses had to be pursued entirely without reference to the
underlying rock character and structure. Such streams are said to be
"superimposed" because they have, so to speak, been let down upon and
into the underlying rock masses. As Professor Berkey has well said:
"The larger rivers, the great master streams, of the superimposed
drainage system, in some cases were so efficient in the corrosion of
their channels that the discovery of discordant structures (in the
underlying rocks) has not been of sufficient influence to displace
them, or reverse them, or even to shift them very far from their
original direct course to the sea. They cut directly across mountain
ridges because they flowed over the plain out of which these ridges
have been carved, and because their own erosive and transporting power
have exceeded those of any of their tributaries or neighbors."
Fine examples of such superimposed streams which are now entirely
out of harmony with the structure of regions through which they flow
are the Susquehanna, Delaware, and Hudson. Thus the Susquehanna cuts
across a whole succession of Appalachian ridges while, in accordance
with the same explanation, the Delaware cuts through the Kittatiny
range or ridge at the famous Delaware Water Gap. The ridges are
explained as follows: while the great master streams were cutting deep
trenches or channels in hard and soft rock alike, numerous side streams
(tributaries) came into existence and naturally mostly developed
along belts of weak, easily eroded rock parallel to geologic (folded)
structure. Thus the Appalachian valleys have been, and are being,
formed, while the ridges represent the more resistant rock formations
which have more effectually stood out against erosion. The lower Hudson
River flows at a considerable angle across folded formations above the
Highlands, after which it passes though a deep gorge which it has cut
into the hard granite and other rocks of the Highlands. The simple
explanation is that the Hudson had its course determined upon the
surface of the upraised Cretaceous peneplain, and that it has been able
to keep that course in spite of discordant structure and character of
the underlying rocks. In a similar manner we may readily account for
the passage of the Connecticut River through a great gap in the Holyoke
ridge or range of hard lava in western Massachusetts.
Before leaving this part of our discussion we shall briefly present
some evidence showing that the New York-New England-St. Lawrence region
at least must have been considerably higher shortly before the Ice Age
(Quaternary period). An old channel of the Hudson River has been traced
about 100 miles eastward beyond the present mouth of the river and it
forms a distinct trench under the shallow sea in the continental shelf.
Even in the Hudson Valley, many miles above New York City, the bedrock
bottom of the river lies hundreds of feet (near West Point, 800 feet)
below sea level. Obviously this submerged channel must have been cut
when the land in the general vicinity of New York City was fully 1,000
feet higher than at present. That the land thus stood higher late in
the Tertiary and possibly early in the Quaternary periods is proved as
follows: (1) because most of Tertiary time must have been needed for
the river to erode such a deep valley after the initial uplift of the
peneplain about the beginning of the period; and (2) because glacial
deposits of Quaternary age filled the former channel to a considerable
depth. The valleys of the coast of Maine, and the submerged lower St.
Lawrence Valley (Gulf of St. Lawrence), in a similar way lead us to
conclude that the region farther north was also notably higher just
before the Ice Age.
In the eastern hemisphere early in the Tertiary period a great
submergence set in and marine waters spread over much of western and
southern Europe, northern Africa, and southern Asia. The sites of the
Himalayas, Alps, Pyrenees, Apennines and other mountains were then
mostly submerged. A very remarkable marine deposit, made up almost
wholly of carbonate of lime shells of a single-celled animal called
Nummulites, formed on the floor of this vastly expanded early Tertiary
mediterranean. This rock attains a thickness of several thousand feet.
It is doubtful if any other single formation made up almost entirely of
the shells of but one species is at once so widespread and thick. In
the Alps this remarkable marine deposit may be seen 10,000 feet above
sea level, and in Tibet fully 20,000 feet. Much of the rock in the
Egyptian pyramids was quarried from this formation.
Later in the Tertiary in Eurasia and Africa the marine waters
gradually became very restricted, so that by the close of the period
the relations of land and sea were not strikingly different from
the present, although northwestern Europe, like northeastern North
America, was notably higher just before the Ice Age than it is to-day.
Eurasia witnessed tremendous crustal disturbances during the middle and
later Tertiary time when, due to intense folding and uplift of great
zones, the Himalayas, Caucasus, Alps, Pyrenees, Apennines, and other
great ranges were formed. The crustal disturbance was most remarkable
in the region of the Alps, where the movement resulted in "elevating
and folding the Tertiary and older strata into overturned, recumbent,
and nearly horizontal folds, and pushing the southern or Lepontine Alps
about sixty miles (over a low angle fault fracture) to the northward
into the Helvetic region. Erosion has since carved up these overthrust
sheets, leaving remnants lying on foundations which belong to a more
northern portion of the ancient (early Tertiary) sea. Most noted of
these residuals of overthrust masses is the Matterhorn, a mighty
mountain without roots, a stranger in a foreign geologic environment."
(C. Schuchert.)
The last period of geological time--the Quaternary--was ushered in
by the spreading of vast sheets of ice over much of northern North
America and northern Europe, and this ranks among the most interesting
and remarkable events of known geological time. On first thought the
former existence of such vast ice sheets seems unbelievable, but the
Ice Age occurred so short a time ago that the records of the event
are perfectly clear and conclusive. The fact of this great Ice Age
was discovered by Louis Agassiz in 1837, and fully announced before
the British Scientific Association in 1840. For some years the idea
was opposed, especially by advocates of the so-called iceberg theory.
Now, however, no important event of earth history is more firmly
established, and no student of the subject ever questions the fact of
the Quaternary Ice Age.
Some of the proofs of the former presence of the great ice sheet
are as follows: (1) polished and striated rock surfaces which are
precisely like those produced by existing glaciers, and which could not
possibly have been produced by any other agency; (2) glacial bowlders
or "erratics" which are often somewhat rounded and scratched, and
which have often been transported many miles from their parent rock
ledges (Plate 20); (3) true glacial moraines, especially terminal
moraines, like that which extends the full length of Long Island and
marks the southernmost limit of the great ice sheet; and (4) the
generally widespread distribution over most of the glaciated area of
heterogeneous glacial débris, both unstratified and stratified, which
is clearly transported material and typically rests upon the bedrock by
sharp contact.
The best known existing great ice sheets are those of Greenland and
Antarctica, especially the former, which covers about 500,000 square
miles. This glacier is so large and deep that only an occasional high
rocky mountain projects above its surface, and the ice is known to
be slowly moving outward in all directions from the interior to the
margins of Greenland. Along the margins, where melting is more rapid,
some land is exposed, and often the ice flows out into the ocean where
it breaks off to form large icebergs.
[Illustration: Fig. 47.--Map of North America showing the area buried
under ice during the Great Ice Age of the Quaternary period; the three
great glacial centers; and the extent of mountain glaciers in the west.
(After U. S. Geological Survey.)]
The accompanying map shows the area of nearly 4,000,000 square miles
of North America covered by ice at the time of maximum glaciation, and
also the three great centers of accumulation and dispersal of the ice.
The directions of flow from these centers have been determined by the
study of the directions of many thousands of glacial scratches on rock
ledges. The Labradorean (or Laurentide) glacier spread out 1,600 miles
to the south to Long Island and near the mouth of the Ohio River. The
vast Keewatin glacier sent a great lobe of ice nearly as far south,
that is into northern Missouri. "One of the most marvelous features of
the ice dispersion was the great extension of the Keewatin sheet from
a low flat center westward and southward over what is now a semiarid
plain, rising in the direction in which the ice moved, while the
mountain glaciers on the west (Cordilleran region), where now known,
pushed eastward but little beyond the foot-hills." (Chamberlin and
Salisbury.)
The Labradorean and Keewatin ice sheets everywhere coalesced except in
two places. One of these is an area of about 10,000 square miles mostly
in southwestern Wisconsin. In spite of several ice invasions during
the Ice Age, this area, hundreds of miles north of the southern limit
of the ice sheets, was never ice-covered. There is a total absence
of records of glaciation within this area, and so we here have an
excellent sample of the kind of topography which prevailed over the
northern Mississippi Valley just before the advent of the ice. A much
smaller, nonglaciated area occurs in northeastern Missouri near the
southern limit of ice extension.
The Cordilleran ice sheet was the smallest of the three, and it was
probably not such a continuous mass of ice, the higher mountains
projecting above its surface. A surprising fact is that neither
this ice sheet nor any other overspread northern Alaska, which is
well within the Arctic Circle, during the Ice Age. More than likely
the temperature was low enough, but precipitation of snow was not
sufficient to permit the building up of a great glacier.
At the same time that nearly 4,000,000 square miles of North America
were ice-covered, about 600,000 square miles of northern Europe
were buried under ice which spread from the one great center over
Scandinavia southwest, south, and southeast over most of the British
Isles, well into Germany, and well into Russia.
In both North America and Europe the high mountains, well south of the
great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees,
and Caucasus, supported many large local glaciers in valleys which now
contain none at all or only relatively small ones.
Records of glaciation, such as glacial scratches, bowlders, lakes,
etc., occur high up in the White and Green Mountains, Adirondacks,
Catskills, and the Berkshire Hills, thus proving that the ice must
have been at least some thousands of feet thick over New England
and New York. We have good reason to believe that even the highest
summits, except possibly in the Catskills, from 4,000 to over 6,000
feet above sea level, were completely submerged under the ice. On top
of a mountain of Archeozoic granite nearly 4,000 feet in altitude,
facing the St. Lawrence Valley in northern New York, the writer has
found many fragments of sandstone which were picked off by the ice in
the low valley, moved southward a good many miles, and uphill several
thousand feet to the top of the mountain. The reader may wonder how a
great glacier at least a mile thick in northern New York could have
thinned out to disappearance within the short distance to the southern
border of the State, but observations on existing large glaciers show
that it is quite the habit for them to thin out very rapidly near their
margins, thus producing steep ice fronts.
The fact that glacial ice flows as though it were a viscous substance
is well known from studies of valley glaciers in the Alps and Alaska,
and the great ice sheet of Greenland. A common assumption, either that
the land at one of the great centers of ice accumulation during the Ice
Age must have been many thousands of feet higher, or that the ice must
there have been immensely thick, in order to permit ice flowage so far
out from the center, is not necessary. Viscous tar slowly poured upon
a level surface will gradually flow out in all directions, and at no
time need the tar at the center of accumulation be very much thicker
than elsewhere. The movement of glacial ice from the great centers of
dispersal during the Ice Age was much the same in principle, only in
the case of the glaciers the accumulations of snow and ice were by no
means confined to the immediate centers.
The fronts of the vast ice sheets, like those of ordinary valley
glaciers, must have undergone many advances and retreats of greater
or less consequence. In the northern Mississippi Valley, and also in
Europe, there is positive proof for five or six important advances and
retreats of the ice which gave rise to the true interglacial stages.
The strongest evidence is the presence of successive layers of glacial
(morainic) débris piled one upon another, a given layer often having
been oxidized, eroded, and even covered with plant life before the
next or overlying layer was deposited. Such is the condition of things
throughout much of Iowa, where wells sunk into the glacial deposits
commonly pass through layers of partly decomposed vegetable matter
at depths of from 100 to 300 feet. Near Toronto, Canada, the finding
of warm climate plants between two glacial deposits proves that the
climate there during an interglacial stage was much like that of the
southern States to-day. During the great interglacial stages the vast
glaciers were notably restricted in size, and in some or possibly all,
cases they may have wholly disappeared from the continent.
In former years there was a tendency to ascribe mighty erosive power
to the vast slow-moving ice sheets, but to-day scarcely any geologist
would hold that the ice really produced large valleys solely by ice
erosion, or that mountains were notably cut down. Throughout the
glaciated region, especially toward the north, the deep preglacial
residual soils and rotten rocks were nearly all scoured off by the
passage of the ice. That the ice, where properly shod with rock
fragments, actually eroded to at least little depths into hard and
fresh rocks is well known, but the evidence is clear and conclusive
that the preglacial hills and mountains, and most of the valleys
(including all the large ones), were rarely more than a little modified
in shape and size.
One of the principal effects of the Ice Age is the widespread
distribution of glacial deposits, and other deposits which were formed
under water in direct association with the ice. Such materials have
been described in the chapter on "Glaciers and Their Work."
As a direct result of the Ice Age, many thousands of lakes came into
existence throughout the glaciated region where few, if any, previously
existed. Many of these lasted only while the ice was present because
their waters were held up by walls of ice acting as dams. Thousands
of others still persist, most of these having their water levels
maintained by dams of glacial débris left by the ice across valleys.
Good examples of lakes of both types, including a summary of the
remarkable history of the Great Lakes, are considered in the chapter on
"A Study of Lakes."
Many drainage changes, gorges, and waterfalls have also directly
resulted from the great Ice Age. In fact it is not too much to say
that practically all true gorges and waterfalls of the glaciated
region have originated as a direct result of the Ice Age. The most
remarkable combination of waterfall and gorge thus produced is that of
the world-famous Niagara, described in the chapter on "Stream Work."
Not only are Niagara Falls and gorge of postglacial origin but there
was no Niagara River as such before the Ice Age. In New York the
well-known Ausable Chasm, Trenton Falls Gorge, and Watkins Glen are all
excellent examples of gorges cut since the Ice Age by streams which,
because their old valleys were filled with glacial débris, have been
forced to take new courses. A gorge of very special interest is that at
Little Falls in central New York. This gorge, two miles long, with its
precipitous walls hundreds of feet high, is the most important gateway
for traffic between the Atlantic border and the Great Lakes region.
The bottom of this defile contains six tracks of the New York Central
and West Shore Railroads, the Barge Canal, an important highway, and
the Mohawk River. Before the Ice Age there was a stream divide instead
of a gorge, several hundred feet above the present river level. During
a late stage of the Ice Age, when the Great Lakes drained through the
Mohawk Valley, a tremendous volume of water passed over the divide and
cut it down to form nearly all of the gorge except the inner or bottom
trench which has since been eroded by the Mohawk River.
[Illustration: Fig. 48.--Sketch map of the region between Lake George
and Schenectady, New York, showing how certain of the main drainage
courses have been revolutionized by the great retreating ice sheet and
the deposits it left. Preglacial courses shown by dotted lines only
where essentially different from the present streams. (By the author,
as published by New York State Museum.)]
Only a few of the numerous stream changes directly due to the Ice Age
will be briefly referred to. Certain of the principles involved are
exceptionally well illustrated in the general vicinity of Saratoga
Springs and Lake George, New York. During the retreat of the great
glacier a lobe of ice occupied the Lake George Valley and forced the
Hudson River west over a divide at Stony Creek. Then, because of heavy
glacial deposits near Corinth, the Hudson could not continue south
through what had been the preglacial valley of Luzerne River, but it
was forced eastward over a divide in a low mountain ridge to Glens
Falls. The remarkable shift of the Sacandaga River from its preglacial
channel was caused by the building up of a great morainic ridge across
the valley in the vicinity of Broadalbin.
The drainage of the basin of the upper Ohio River has also been
revolutionized as a result of the glaciation. All the drainage of
western Pennsylvania passed northward into Lake Erie just before the
Ice Age instead of southwestward through the Ohio River as at present.
Rivers as large as the Mississippi and the Missouri were also more
or less locally deflected from their preglacial courses. Thus the
Missouri, which in preglacial time followed the James River Valley of
eastern South Dakota, was forced, by a great lobe of retreating ice, to
find its present course many miles farther west.
How long ago did the Ice Age end? In seeking an answer to this question
we should bear in mind not only the fact that the Ice Age ended at
different times, according to latitude, the more southern districts
having been first freed from ice, but also the fact that approximately
4,000,000 square miles of the polar regions are now ice-covered, so
that in a real sense those portions of the earth are still in an Ice
Age. Some of the best estimates of the length of postglacial time for
a given place are based upon the rate of recession of Niagara Falls,
the average of the estimates being about 25,000 years. The evidence for
this conclusion is briefly set forth in Chapter III. A careful study of
the rate of recession of St. Anthony Falls, Minnesota, has led to the
conclusion that the last retreat of the ice occurred there from 10,000
to 16,000 years ago. Certain clays deposited under tidewater since
the last withdrawal of ice in Sweden show a remarkable succession of
alternating layers thought to represent seasonal changes. By counting
the layers it has been estimated that Stockholm was freed from ice only
9,000 years ago.
Although the actual duration of the Ice Age is by no means accurately
known, we can be quite sure that the time represented is far longer
than that of postglacial time. That it must have lasted fully 500,000
years seems certain when due consideration is given to amount of time
necessary to bring about the repeated changes of climate between the
glacial and interglacial stages; the amount of plant accumulation
during the interglacial stages; the amount of weathering and erosion
of the various layers of glacial deposits. Some estimates run as high
as 1,500,000 years for the duration of the Ice Age, and an average is
about 1,000,000 years, which probably indicates, at least roughly, the
order of magnitude of the time involved.
When it is considered not only that the fact of the great Ice Age
was not even thought of until 1837, but also that many factors enter
into the general problem of the climate of geologic time, it is not
surprising that the cause (or causes) of the glacial climate is still
not definitely known. A few of the various hypotheses which have been
advocated to account for the glacial climate will now be very briefly
referred to. One is that the increased cold (not more than 10 to 15
degrees for the yearly average) was brought about by the notably
increased altitudes of late Tertiary and early Quaternary times in
northern North America and Europe. In this connection it is interesting
to note that the four times of real glaciation during geologic time
(mid-Proterozoic, early Paleozoic, late Paleozoic, and early Cenozoic)
did occur directly after great crustal disturbances and notable uplifts
of land. According to this hypothesis the interglacial stages would
have to be explained by a rather unreasonable assumption of repeated
rising and sinking of the glaciated lands.
Another hypothesis, long held in favor, is based upon certain
astronomical considerations. Thus we now have winter in the northern
hemisphere when the earth is nearest the sun, but in about 10,500
years, due to wobbling of the earth on its axis, our winter will occur
when the earth is farthest from the sun, thus making the winters longer
and colder, and the summers shorter and hotter. After a much longer
period of time the earth will be millions of miles farther from the
sun in winter than in summer and this would still further accentuate
the length and coldness of the winters. The interglacial stages
represent the 10,500 year periods when the earth in winter (northern
hemisphere) is nearest the sun. A difficulty in the way of accepting
this hypothesis is that it is inconceivable that each glacial and
interglacial stage lasted only 10,500 years. Another objection to the
hypothesis as an explanation of Ice Ages is that it is directly opposed
by the fact of widespread glaciation at low latitudes either side of
the equator during the late Paleozoic Ice Age.
Another hypothesis is based upon variations in quantity of carbonic
acid gas and water vapor in the air. Increase or decrease of these
constituents causes increase or decrease of temperature because they
have high capacities for absorbing heat. "The great elevation of the
land at the close of the Tertiary seems to afford conditions favorable
both for the consumption of carbon dioxide in large quantities (by
weathering of rocks) and for the reduction of the water content of
the air. Depletion of these heat-absorbing elements was equivalent
to the thinning of the thermal blanket which they constitute. If it
was thinned, the temperature was reduced.... By variations in the
consumption of carbon dioxide, especially in its absorption and escape
from the ocean, the hypothesis attempts to explain the periodicity of
glaciation (i.e., glacial and interglacial stages)." (Chamberlin and
Salisbury.)
Still another suggested explanation is based upon variability of amount
of heat radiated by the sun. Slight variations are now known to take
place, and possibly in the past during certain periods of time these
variations may have been sufficiently great to cause a glacial climate
with interglacial stages.
Here, as in the case of so many other great natural phenomena, a
single, simple explanation does not seem sufficient to account for all
the features of the several well-known glacial epochs of geologic time.
Two or more hypotheses, or parts of hypotheses, must more than likely
be combined to explain a particular Ice Age.