THE GLACIERS OF THE TEMPERATE ZONES.
Having asserted that during the culmination of a frigid period the
ice-sheets spread over a portion of the lands of the tropical zone,
I will give my views, with those of several writers, on the spread
of ice-sheets within the now temperate latitudes; and meanwhile I
will repeat a portion of my former essays on the subject. Professor
Hitchcock, in his lectures on the early history of North America,
says that “the history opens with igneous agency in the ascendant,
aqueous and organic forces become conspicuous later on, and ice has
put on the finishing touches to the terrestrial contours.” But there
appear to be various opinions held by geologists respecting the changes
brought about on the earth’s surface during the glacial period. Some
think that glaciers have never been an important geological agent,
while others assert that during the glacial epoch heavy ice-sheets
covered the elevated portions of Western North America as far south
as the thirty-sixth parallel of latitude, and Eastern North America
was overspread with ice-sheets, which attained a depth of five or six
thousand feet, and were able to move their débris over wide lands of
little declivity toward the sea, their immense deposits forming the
lands of Cape Cod, and also the islands of Nantucket and Martha’s
Vineyard.
But it is now said that this implied magnitude of the glacial deposits
on the lands skirting the New England coast is without foundation,
since the larger bulk of these islands consists of upturned Cretaceous
and Tertiary strata, which are only thinly covered with glacial débris,
such as bowlders, gravel, clay, and sand, from the eroded shores of the
mainland of New England. But it appears that the dislocated and folded
cretaceous strata which underlie the glacial drift of Nantucket and
Martha’s Vineyard were during an early period deposited on the bottom
of a shallow sea, which then covered the Vineyard Sound, Buzzard’s Bay,
and their surrounding lowlands. Thus the ice-sheets of the frigid age
which moved over New England displaced the yielding stratified deposits
of the shallow sea, and forced them southward in a disturbed condition
to the position which they now occupy.
Still, it is apparent that only a small portion of the glacial drift is
found on these islands, which, according to appearances, must have been
eroded and moved southward from the rocky lands of New England during
the ice age; but there is sufficient to show that large quantities
of such débris were carried over the islands into the Atlantic. And,
judging from the eroded rocky New England lands, there must have been
sufficient glacial drift moved over Nantucket and Martha’s Vineyard
into the ocean beyond to far exceed in bulk the deranged Tertiary and
Cretaceous deposits which now form so large a portion of the islands.
For, when we look over lands bearing traces of the ice age, where
the glaciers did not move their drift into the sea, so the terminal
moraines of such glaciers can be better estimated, we can realize the
great work that has been performed by the ice-sheet that overran New
England during a frigid age.
Professor James Geikie states, in his discussion on the glacial
deposits of Northern Italy, that the deposits from Alpine glaciers of a
frigid period “rise out of the plains of Piedmont as steep hills to a
height of fifteen hundred feet, and in one place to nearly two thousand
feet. Measured along its outer circumference, this great morainic mass
is found to have a frontage of fifty miles, while the plain which it
encloses extends some fifteen miles from Andrate southward.” And it
is reported that there are found on the southern flank of the Jura
numerous scattered bowlders, all of which have been carried from the
Alps across the intervening plains, and left where they now rest. Many
contain thousands of cubic feet, and not a few are quite as large as
cottages.
Such blocks are found on the Jura, at a height of no less than two
thousand feet above the Lake of Neuchâtel. The Jura Mountains being
formed of limestone, it is easy to distinguish the débris deposited
by Alpine glaciers; and, from what I can learn of extensive glacial
work, it appears that intervening plains, lakes, and sounds are so
often found separating the source of ancient glaciers from their
deposits that their existence becomes almost necessary to represent
the general outlines of disturbance performed during an ice period. In
consideration of such facts and the foregoing statements of reliable
observers, I am prompted to offer my views on glacial work performed on
a portion of the Pacific shores of North America, which seems to me to
be much more extensive than hitherto supposed.
Professor Whitney describes the coast mountains of California as being
made up of great disturbances, which have been brought about within
geologically recent times; and this statement I found to be so obvious
in my travels over that region that it appears to me that the coast
ranges originated in a different manner from the older Sierras. The
western sides of the latter mountains everywhere show the great eroding
power of ancient glaciers; and, when I considered their favorable
position for the accumulation of snow during a glacial period, I was
led to seek for the glacial deposits adequate to represent the great
gathering of ice which an age of frigid temperature would produce.
But it seemed to me that such deposits could not be found in the
foot-hills of the Sierras, which contain the moraine of inferior
ice-sheets that terminated at the base of the mountains.
Under these conditions I came to the conclusion that during the earlier
ice period the immense glaciers which must have formed on the western
slopes of the Sierra range moved their gigantic accumulation of débris
so far seaward as to form the range of hills now existing next the
coast line, and perhaps the islands abreast the Santa Barbara coast,
the Contra Costa, or eastern range, being formed during a subsequent
ice period, in the same manner as the hills next the coast line.
Still, it may be that neither of the coast ranges was the work of a
single cold epoch; but the western range must necessarily have been the
earliest deposit. Although the coast ranges differ from the Sierras
in their make up, yet it does not disagree with the glacial origin
of the former inferior mountains, from the fact that the ice-sheets,
while moving their bulk westward, displaced the deposits of such bays,
lakes, rivers, and marshes as lay abreast of the Sierra slopes. The
advancing ice-sheets, thousands of feet in depth, moving from a lofty
and steep incline, pressed and ploughed below the somewhat superficial
cretaceous and alluvial strata which lay in their course. The disturbed
strata, while forced along in confused heaps in front of the ice, were
amassed in ridges sufficient to form the hills of the coast ranges.
The bowlders found imbedded in several of the coast hills must have
been moved by the ice from the Sierras on account of the coast ranges
not having a rocky core of sufficient firmness to give shape to such
bowlders. Moreover, the temperature of the Pacific waters would not be
favorable for glaciers to form on the coast ranges, with the ice-sheets
of the Sierras terminating at the foot-hills.
The Sacramento and San Joaquin Valleys are now covered by recent river
deposits. Therefore, the glacial drift which should be traced from the
Sierras to the coast ranges is concealed.
Yet the abraded appearance of exposed solid rocks at the base of the
foot-hills, and also the scattered bowlders which gradually disappear
beneath the diluvial deposits of the plains, indicate that the Sierra
ice-sheets could not have ended at the foot-hills, but must have moved
further westward, while gathering immense accumulations in their front,
sufficient to form the coast hills, the débris thus amassed being able
to arrest the further movement of the ice seaward.
The coast ranges in several places have been subject to igneous action,
which may have been brought about through heat generated from pressure
exerted on the interior masses after the ice had melted away, the heat
thus produced being sufficient to cause outbursts of lava, where the
nature of the material favored combustion. The low plains, lakes, and
bays which separate the Sierras from the coast hills are in a position
similar to the shallow sounds which separate Nantucket, Martha’s
Vineyard, and Long Island from the inferior slopes of the mountains of
New England. Therefore, while agreeing with glacialists, who believe
that great geological changes have been wrought by ice-sheets in Italy
and New England, it appears to me that the ancient glaciers of the
Sierra Nevada have accomplished more extensive work, owing to the
Sierras being situated in a more favorable position to receive the
humidity of the ocean.
Hence, with a low temperature, vast quantities of snow must have
collected on their lofty sides; and at the same time their great
height and declivity would cause the ice to move down their steeps
with greater force than the glaciers which passed over New England.
Writers who have given the subject considerable study think that the
deep valleys of the Sierra Nevada were produced by disruptive rather
than erosive agencies. This conclusion has been formed from the lack
of large accumulations of débris about their lower extremities, which
would not be the case if such valleys were the result of glacial
erosion. But, should the coast ranges be attributed to glacial action,
as has been stated, we can well account for the débris that should
accumulate from the erosion of the deep valleys.
The only thing that could prevent the ice from gathering on the Sierra
Nevada range during an ice period in greater masses than on any
mountains in the northern hemisphere would be the lack of cold; for,
with a low temperature, the fall of snow would be enormous. This is
shown by the great snow-fall during the short mild winters of to-day.
Therefore, with ice-sheets covering a large portion of the lands of
the high northern latitudes, and with the Japanese current which
tempers the north Pacific waters made cold in the manner described
in the foregoing pages, and while the sea along the north-west coast
of America was strewn with icebergs launched from Alaska and British
Columbia, it seems that California must also have obtained a frigid
climate during the ice age. Therefore, on account of its exposure to
the ocean winds, and the consequent heavy snow-fall, the accumulation
of ice on its lands must have been immense. For, when it is considered
that the glaciers of North America extended southward even into the
torrid zone sufficient to cover a large portion of Central America,
it is unreasonable to suppose that any portion of California could
escape being covered by heavy ice-sheets during the glacial epoch. The
comparatively scant fall of rain and snow over Greenland is known to
form ice-sheets hundreds of feet in thickness.
Therefore, what must have been the depth of ice over the high lands of
the Pacific coast north of California at the culmination of a frigid
period? The descriptions given by Dr. Dawson and others, of glacial
phenomena along that coast, favor the impression that an immense
ice-sheet at one time deeply covered the whole region from the top of
the mountain range to the ocean.
Thus all the deep channels were filled and all the islands deeply
overrun with ice, while the immense bergs launched from the shore
and carried by the winds and currents southward were probably not
melted until they reached the tropical latitudes. Thus, when the
whole circulation of the Pacific waters are taken into account, it
will be seen that their temperature during the ice age must have been
considerably lowered. The movement of ice-sheets on the Pacific slope
was probably local in character, and not connected with the movement of
ice on the eastern sides of the mountains.
From what I have seen of the vast territory lying between the Sierra
Nevada and the Rocky Mountains it appears that it obtained much heavier
ice-fields than generally supposed. Professor Geikie in his lectures
says of this region that during the glacial age, “in the Second
Colorado Canyon, the sides were completely glaciated from bottom to
top. These walls are from 800 to 1,000 feet high, and at the thickest
point the glacier was 1,700 feet thick”; and he says that “the country
around Salt Lake was covered with ice, for the rocks about there show
the action of ice, and that the bones of the musk-ox are found there.”
This vast area of ancient ice, although subject to little movement in
its interior basin, still, in whatever movement it may have had, must
have found its main outlet through the Grand Canyon of the Colorado.
For in no other way can we account for the erosive forces necessary to
excavate that immense chasm. Not even the mighty torrent that carried
off the waters of the melting ice-sheets that covered the interior
portion of the continent could accomplish work of such magnitude.
According to Professor Geikie’s observations the Second Colorado Canyon
was filled with glaciers during the ice age. Therefore, it seems that
these glaciers must have flowed down into the Grand Canyon, and there
united with glaciers flowing from more northern regions.
An account of a collecting expedition to Lower California by G. Eison,
in 1895, describes ancient moraines at the extremity of the peninsula
as being prominent, large, and steep. This region lies under the tropic
of Cancer, and 8° south of the mouth of the Colorado River where
it empties into the Gulf of California. Hence it appears that the
temperature of that portion of North America during the ice age was
favorable for the great glacier of the Colorado Canyon to have flowed
into the Gulf of California.
The wide, shallow basins of Utah and Nevada were filled with the water
from the melting ice-sheet on the breaking up of the ice period,
and the lakes so caused remained for a considerable time after the
disappearance of the ice. But, owing to the great evaporation and light
rain-fall of that region, the lakes gradually shrank away, the filling
and emptying of the lake basins being governed by the cold and mild
epochs.
The conglomerate deposits in the Appalachian district of North America
are known as occurring on a large scale. Professor Shaler is inclined
to attribute them to glacial action, because he knows of no other force
that could bring together such masses of pebbles from a wide-spread
surface. In Eastern Kentucky and East Tennessee these deposits are
found to be several hundred feet in thickness. Such accumulations of
apparent glacial origin are to be found from New Brunswick to Alabama.
Hence it seems that the ice during a frigid period followed down the
Alleghany range even so far south as Georgia and Alabama; and for a
time, when the ice attained its greatest spread, it flowed over the
central portion of the Gulf States. For how else can we account for the
clay mixed with gravel and pebbles and stony fragments being spread
broadcast over that region?
I know that such statements do not agree with the views of glacialists
who have written on the subject, and have drawn the glacial boundary
from seven to ten degrees further north, where a line of bowlders with
other glacial débris is plainly traced. Still, it appears to me that a
line of bowlders deposited by an ice-sheet spreading over a continent
and across many degrees of latitude cannot be compared to the moraines
of inferior mountain glaciers of the temperate latitudes of the present
age.
An ice-sheet moving from a high latitude to a lower would, while in the
colder latitude, freeze firmly to the rocky ledges, and hold them so
strong in its frigid grasp as to break off the weaker portions of the
rocks, and drag them toward a milder region, as far as the freezing
grip of the ice-sheet would permit; but, on gaining lower and milder
latitudes, the holding and dragging power of the ice would be lost on
account of the increased warmth of the earth over which the glacier
must pass, and also because of the ice-sheet having lost a portion of
the low temperature acquired in the higher latitudes. Therefore, on
such lines the bowlders would be released, while the ice-sheet would
still move on, although largely deprived of its eroding power.
This is the probable reason why a line of glacial débris, largely
composed of bowlders, is found to extend across the Middle and Western
States, and so generally supposed to be the glacial boundary of a
frigid period. But there is no reason to suppose that an ice-sheet,
although deprived of its eroding power, was arrested in its southern
movement on the line of its stony débris, because there could be no
sudden change of temperature in a particular latitude on the eastern
lands of North America to cause an abrupt ending of the ice-sheets.
And there appears to be nothing to hinder the ice from gathering and
flowing over lands warm enough to loosen its implements of erosion;
for there is much to show that the ice-sheets flowed much further
southward, even into the middle portion of the Gulf States, and there
spread the clay mixed with gravel and pebbles, with now and then a
bowlder, over the land. The scattered bowlders, found in numerous
instances many miles south of the bowlder line, were so deeply imbedded
in the ice-sheet that they could not be dropped on the usual releasing
ground. The ice-sheet, when deprived of its rocky, eroding implements,
would, while flowing over the land, leave few or no imprints on the
rocks; but it would probably move and spread a large amount of clay,
gravel, pebbles, and sand over its wide course, especially if the ice
moved from a region abounding with such material.
Should we place the glacial boundary on the line of the rocky débris,
how could we account for the glaciated stones found on the hills and
plains situated far southward of the bowlder-strewn regions of the
Middle and Western States? The clay mixed with gravel and sand, and
spread so broadcast over a large portion of Georgia and even into
Northern Florida, makes it appear that the ice of a cold period must
have covered that southern region.
Moreover, it seems to have been through the great abrasion which only
ice-sheets could perform that the sands of the Florida peninsula were
produced; for on examination they seem to have resulted from the
abrasion and weathering of crystalline rocks.
The worn remnants of such rocks are now found in the southern
Appalachian range. In fact, the hills and mountains of that region at
the present time are supposed to be a small remnant of the ancient
highlands. Thus, on consideration, it appears that the sands caused by
the action of glaciers were, on the disappearance of ice-sheets, blown
by the strong north-west winds toward the Florida peninsula as fast
as the receding waters of the ocean which flowed the lowlands on the
breaking up of the ice age would permit; and in this way the sand was
spread over the lowland region, which was largely composed of coral
sea shells and other marine matter. And it seems that the sand must
have been blown over large areas in Florida soon after the ending of
the frigid period, because the sand, in order to be moved by the winds,
must have spread over a country nearly destitute of vegetation; and
such would be the condition of that region during times which succeeded
the ice period and the subsequent brief flowage of the lowlands on the
ending of the frigid age, which would not be the case if such sands
resulted entirely from water erosion and weathering, because with
such a state of things the country would be covered with forests and
grasses, which would prevent the sand from being moved by the winds to
any great extent.
This goes to show that the region of the Gulf States was so much
affected by the cold of the glacial period, together with the
submergence of the lowlands at its close, its flora and also its
animals were exterminated; for how else can we account for the abundant
fossil remains of animals now found buried in the Florida sands? It
appears also that, when Florida was being covered with drifting sands,
many of the lake basins now formed did not exist, as the wind-blown
sand could not have crossed a continuous chain of lakes like the St.
John’s River; and it is an easy matter to-day to trace the beds of the
ancient lakes that prevented the sands from drifting over certain lands
now nearly destitute of it. And it is probable that the sea flowed the
lowest lands during the period when the winds were drifting the greater
portion of the sands over the peninsula. Therefore, regions which
embrace the Everglades and portions of the Indian River territory are
quite free from heavy sand deposits, and so also are the extensive flat
woods of the peninsula.
Since the sands blew over the ancient desert of Florida, many lake
basins have been formed because of the sinking of the ground. This
sinking of the ground is a common occurrence in limestone regions,
where a great amount of material is moved in solution, leaving caverns
whose roofs often fall in. The great amount of sand blown upon Florida
caused the marine strata to give way in the weaker places under its
burden. The sinks thus formed, probably of frequent occurrence at one
time, have now nearly ceased. Still, there are depressions to be seen
to-day where the tops of large pine-trees, which grew on dry, sandy
land, are barely above the surface of the water which partly fills
the basins so recently formed. Yet I would not assert that all of the
depressions where Florida lakes exist were caused by the sinking of the
ground; for the winds may have caused shallow basins in the sand, where
the decayed vegetation has formed mud sufficient to hold the water
which now partly fills such basins.
The mobility of Florida sands can be seen to good advantage when
exposed to a strong, dry north-west wind, where the ground happens
to be destitute of vegetation. An observer can then realize what the
result would be, should the whole land be deprived of vegetation and
laid bare to the action of the winds.
Under such conditions, not only would the winds be much stronger than
now, but the air near the ground would be filled with sand, moving like
drifting snow in a Dakota blizzard. And, furthermore, it is probable
that the rainfall was very light while Florida was void of vegetation;
and, even if shallow basins were formed, there would be a lack of rain
to supply them with water.
The wide plains west of the Mississippi River, extending southward into
Texas, during the frigid period must have been covered with a sheet
of ice and snow. And it is probable that it was not wholly a product
of more northern latitudes, but was mostly produced by the snow which
fell on the plains during the long winters of that period, which could
not be melted away during the cold summers of an ice age, when it is
considered that an ice-sheet, with a temperature sufficiently low as to
carry glacial drift, covered the lands of Missouri as far as latitude
38° south; and it may have been through the pressure from an ice-sheet
in its south-eastern movement that we are to account for the numerous
ore-bearing faulting fissures traversing the limestone strata.
The ice-sheet was also the probable cause of the erosion of the
horizontal bedded stones, yet it appears that the ice did not greatly
change the contour of the ground; for it is well known that glaciers
do move over lands that are not frozen to the ice without causing much
disturbance, especially where the gradient is small, and this was the
probable condition of the Western plains during the ice age. Thus it
seems that whatever disturbance this region has undergone could be
partly attributed to ice-sheets without the presence of bowlder drift,
because the temperature and texture of the ground in the limestone
region were unfavorable for such accumulations; yet it may be owing to
the action of ice that minerals once diffused are now found collected
in fissures. The deep valleys through which the large rivers now pass
on their way toward the sea were once filled with glaciers which flowed
into them from their tributaries. Thus the deep trenches of the plains
are largely the work of glaciers. It is generally supposed that the
driftless region of Wisconsin was free from ice during the frigid
period. But it seems impossible for this region to have escaped being
covered by ice and snow, with the great lakes filled with glaciers, and
the regions on all sides of the driftless area covered with ice.
The reason why this territory escaped the drift from the north was on
account of the hindrance which the drift-bearing ice-sheet encountered
in the deep basin of Lake Superior. In this great depression the
ice-sheet from the north was relieved of bowlders and other glacial
drift, as well as obstructed in its southern movement.
Therefore, the snow and ice which gathered on the driftless region had
little movement in any direction, while the temperature and consistency
of the ground under the ice were not favorable for the production
of bowlder drift; and, when we consider that the Mississippi valley
was deprived of great sources of warmth during the culmination of a
glacial period, we are forced to the conclusion that its wide lands
were also covered with snow and ice.
The tropical waters of the North Atlantic were so much chilled by the
floating icebergs of North-eastern America, Greenland, Iceland, and
Northern Europe that the Caribbean Sea, its warmest reservoir, was
reduced to a temperature so low that the easterly winds which blew over
its waters were unable to prevent ice-sheets from gathering on Eastern
Nicaragua.
Therefore, during such frigid times it appears that, with the waters of
the Caribbean Sea and Gulf of Mexico reduced to a low temperature, it
was impossible for the great Mississippi valley to escape glaciation,
while being surrounded by cold seas and glaciated lands which extended
even into the tropical latitudes. The broad, level lands of British
America and Siberia during the ice age must have been thickly covered
by the snow which fell on the deeply frozen plains, besides the large
amount of snow that the cold westerly winds must have drifted over
their icy surface from lands of greater snow-fall on their western
borders. This snow during such freezing times could not be melted away.
The great ice-sheets thus formed over wide, level lands could have
but little motion in any direction, certainly not sufficient to cause
glacial drift of much magnitude; yet the ice-sheet, at one stage of
its existence, probably served to widen and deepen the channels of the
great rivers which empty into the Arctic Ocean from these vast regions,
and the glacial débris from such erosion was deposited in the arctic
seas.