THE GLACIAL HISTORY OF NORTH AMERICA
The glaciation of North America was even greater and more complicated
than was that of Europe. It spread from three main centres, the
Cordilleran or Rocky Mountain centre, the Keewatin centre west of
Hudson Bay, and the Labradorean centre. Vancouver Island in the west
and New Brunswick and Newfoundland in the east, were also independent
centres of glaciation, and ice from the latter may have reached the
coast of the United States in places. The ice covered an area of
about 4,000,000 square miles, and the main ice-sheet extended to 38°
N., or twelve degrees further south than the Scandinavian ice-sheet.
Nine stages are recognized by American geologists, though opinion is
divided as to whether all the stages of “deglaciation” represent real
interglacial periods. The sequence is as follows:
1. Nebraskan, Jerseyan or pre-Kansan glaciation.
2. Aftonian deglaciation.
3. Kansan glaciation.
4. Yarmouth deglaciation.
5. Illinoian glaciation.
6. Sangamon deglaciation.
7. Iowan glaciation.
8. Peorian deglaciation.
9. Wisconsin glaciation.
On the other hand, in the northern part of the Rocky Mountains there
is evidence of only two Glacial periods, separated by a single long
interglacial, though perhaps the second glaciation was double.
Further south, out of reach of the main ice-sheets, there are traces
of two and in places three separate developments of valley glaciers
resembling those of the Alps.
As in the case of Europe, the literature of the subject is extensive
and conflicting, but the following summary of the course of events
represents the views of most moderate American geologists.
The Quaternary period opened with extensive elevation of the whole
North American continent, which raised the Rocky Mountains several
thousand feet above their present level and extended the continental
area over much of the northern archipelago. In the east Newfoundland
is considered to have been raised at least 1000 feet, a movement
which converted the banks into dry land and interposed a large
cold area in the path of the moisture-bearing southerly winds. As
in northern Europe the high mountains of the west were the first
to develop large glaciers, which coalesced into an ice-sheet,
filling the valleys and rising up the slopes of the mountains until
it reached a thickness of 5000 feet. In Puget Sound the ice was
4000 feet thick, but seawards the slope is very rapid and the ice
was unable to extend far from the shore. This ice-sheet extended
south-eastwards some distance into the United States, forming
the first ground-moraine of that district. Probably while this
Cordilleran glaciation was still in progress ice began to spread
outwards also from the Labradorean centre, forming the oldest drift
of that region. These oldest deposits are, however, not yet well
understood.
This oldest boulder-clay is separated from the moraines of the main
glaciation near its southern limit by river gravels containing the
remains of mollusca and large herbivorous mammals—extinct species of
horse, the hairy mammoth of the old world (_Elephas primigenius_),
and two other extinct species of elephant, and also the true American
mammoth. This is the Aftonian fauna, which has been claimed as
evidence of an Interglacial period. That it evidences a retreat of
the ice-edge in that particular region is certain, but that the
climate became really temperate is very doubtful. More probably it
corresponds to the Gunz-Mindel “interglacial” of the Alps, and was
formed when the Cordilleran ice-sheet was retreating, but before the
Keewatin sheet had reached its maximum.
The Aftonian stage was followed by the Kansan glaciation, when the
ice-sheets reached their maximum area over the greater part of
North America. The chief centre of glaciation at this stage was the
Keewatin, west of Hudson Bay. While it is certain that the Keewatin
centre reached its maximum later than the Cordilleran, geological
opinion in America is divided as to whether or no the two ice-sheets
ever coalesced, but it is difficult to understand how an independent
ice-sheet could have grown up on the comparatively low ground of
the Keewatin centre. Most probably the course of events here was an
exact parallel of that in the better-known Scandinavian region—the
Cordilleran ice-sheet extended eastwards over the lower ground until
a glacial anticyclone developed east of the Rockies. When this
happened the supply of moisture to the western part of the ice-sheet
fell off somewhat, and the eastern part took on an independent life,
ultimately becoming the main centre of glaciation. It was while these
changes were in progress that the southern limit of the ice retreated
northwards and the “Aftonian” deposits were formed.
The next stage (Kansan) occurred when the ice from the Keewatin
centre spread outwards in all directions, and in the south reached
the maximum limits of glaciation in America. In the west this sheet
overlapped on to the ground-moraine of the former Cordilleran ice,
but the Rocky Mountains were too far away and too high for Keewatin
ice to dominate them and overflow them from east to west. Instead
these mountains must have maintained an extensive glaciation of
their own.
With the growth of the Keewatin centre the Labradorean also
decreased, but more slowly, and this change was not associated with a
retreat of the southern ice-edge, so that there was no corresponding
“interglacial” in the east of the United States. The moraines of
these older glaciations resemble those of the early ice-sheets of
Europe in presenting only featureless level surfaces of boulder-clay
without morainic ridges, lakes and the other characteristics of
ice-bearing surface detritus, and there is no doubt that conditions
at the southern edge were similar—the climate was severe in winter,
but not insupportable in summer. At the same time it was decidedly
more severe than the present, even as far south as Florida, where
there are colonies of northern plants, which migrated southwards
during the Ice Age, still living on local cold slopes with a
northerly aspect. After the maximum of glaciation the disappearance
of the ice took place gradually and chiefly by ablation, for there
are none of the extensive river gravels and flood terraces which we
should find had the melting been rapid. It is only in the valleys
of the Rocky Mountains that such deposits occur, testifying to
conditions such as obtained in the Alps.
The succeeding Yarmouth stage of deglaciation was very long,
corresponding in this respect to the Mindel-Riss interglacial of
Europe. The Kansan moraine was weathered to a depth of ten or twenty
feet, and four-fifths of its surface was removed by the erosion of
streams and rivers. In the mountain districts the side streams which
had been left occupying “hanging valleys” by the over-deepening of
the heavily glaciated main valleys, had time to cut out uniformly
graded broad V-shaped valleys descending to the level of the
main stream. In the Great Basin also, where the periods of high
water-level are considered to correspond to the main glaciations,
the interval of low water corresponding to the Yarmouth stage
was very long. A rough estimate of its length is about 200,000
years—somewhat shorter than the Mindel-Riss. Actually, though the
Kansan and Mindelian glaciations were approximately contemporaneous,
the subsequent recurrence of glaciation in America appears to have
preceded slightly that in Europe.
Of the climate of this stage we have unfortunately little evidence.
Old land surfaces of this age are known, containing deposits of peat
and bones of the wood rabbit and common skunk, but both of these
animals have a wide range. Perhaps the climate resembled the present
during most of the period; there is no evidence that it was ever
warmer, and it appears quite likely that ice-sheets maintained their
existence in the far north through the whole of this stage.
After this interglacial there set in a period of renewed elevation
in the Rocky Mountains and in the Labrador-Newfoundland centres,
which brought about a recurrence of the glaciation. In the Rocky
Mountains the ice was not so thick as in the preceding stage, but all
the valleys were occupied to a considerable depth and the ice spread
out to the eastward. The Labrador ice-sheets also developed again,
forming the Illinoian glaciation, the moraines of which are found as
far west as Illinois, but no moraines are known of this age due to
the Keewatin ice-sheet. The latter developed later, and is classed by
some American geologists as a separate glaciation, the Iowan, which
is only certainly found in northern Iowa, but may be represented
further east by a thin sheet of boulder-clay overlapping the
Illinoian moraine. The supposed interglacial between the Illinoian
and Iowan, the “Sangamon Stage,” is represented only by land surfaces
formed of the Illinoian moraine and covered by the loess or locally
by the equivalent of the Iowan moraine, and there is no evidence that
the ice-edge retreated far. Other American geologists, including
F. Leverett, do not recognize the existence of a separate Iowan
glaciation, and as the amount of weathering and denudation undergone
by the two moraines differs very little, this seems the more natural
view. The natural explanation seems to be that this was another case
of “glacial piracy,” the Keewatin ice-sheet, owing to its lesser
snowfall, developing more gradually, and finally diverting the supply
of moisture from the Labradorean ice-sheet, until it reached a
maximum after the latter was already on the wane. Both these sheets
of drift present similar flat features to the Kansan sheet, without
morainic ridges.
Leverett’s interpretation of the succession is as follows: The third
(Illinoian-Iowan) glaciation was followed by a period of moist
climate, when peat-bogs were formed on level poorly-drained surfaces,
while elsewhere coniferous forests developed. This was followed by a
period of dry steppe-like conditions with a cold temperate climate,
when the great American loess sheet was deposited. This loess sheet
extends northwards, overlapping the Iowan moraine, and in places
passing under the Wisconsin drift. The material has come from the
west, and probably most largely from the dry plains east of the
Rocky Mountains, from which it diminishes in thickness eastwards.
But unlike Europe this phase of steppe conditions was followed in
America by a definite interglacial, when the climate seems to have
become rather warmer than the present. In the northern States an old
land-surface formed on the loess, and, termed the Peorian stage, is
overlain by the Wisconsin drift; but near Toronto, on the shores
of Lake Ontario and in the Don valley, the gap represented by this
land-surface is partly filled by a remarkable series of lacustrine
deposits known as the Toronto stage. The Lake Ontario beds indicate
a climate slightly colder than the present, but the Don valley beds
contain plants and animals living in the central States, and refer to
conditions more favourable than those now found in the district.
The duration of this interglacial has been worked out in a
remarkable way by A. P. Coleman, who on the basis of wave-action
estimated it as 62,000 years, which agrees very closely with the
60,000 years found by Penck and Brückner in the Alps. This period
was not long enough for streams in the “hanging valleys” to cut
out uniformly graded valleys down to the main rivers, and was
consequently much shorter than the preceding interglacial.
The last glaciation of North America was the Wisconsin, which closely
resembles the Wurmian of Europe both in its relations to the older
glaciations and in the rough topography and unworn character of its
moraines. It extended within the limits of the Kansan drift across
fully two-thirds of the continent, from Nantucket and Cape Cod
through Long Island, northern New Jersey, Pennsylvania, southern New
York, Ohio, Indiana, Illinois, Michigan, Wisconsin, Minnesota, Iowa
and the Dakotas, Manitoba, Saskatchewan and Alberta. At the same time
the Cordilleran centre probably bore increased local valley glaciers.
Like the Wurm glaciation, the Wisconsin was double. The older
moraines are well-marked, and in places are covered by a foot or two
of loess, though this deposit reaches nothing like the thickness of
that overlying the moraines of the earlier glaciations. The moraine
under this loess is very little weathered, so that the time interval
was very short; possibly this loess is redistributed older loess
associated with glacial east winds. The ice of the first glaciation
melted very slowly and there is very little gravel outwash to the
moraines. But “after the Wisconsin ice-sheet had reached a position a
little outside the limits of the Great Lakes the retreat became much
more rapid, and large outwash aprons were formed from which valley
trains of gravel led far down the drainage lines. From this position
... the moraines are practically free from loess-like silts.”[4]
From this point onwards the glacial history of America is one
of irregular retreat, with occasional halts or even readvances
resembling those of the Scandinavian ice. Banded clays are found
similar to those used so successfully by Baron de Geer in dating
the retreat stages of Scandinavia, and this geologist has recently
been investigating them, but until his results are worked out no
correlation with Europe can be attempted.
A natural clock of another type is provided by Niagara Falls, which
are cutting their way back up the gorge at a rate which has been
definitely ascertained. Taking into account the varying amounts
of water which have passed over the falls at different stages of
post-glacial geography, the duration since the region became free of
ice has been calculated at about 20,000 years, which agrees closely
with the time elapsed since the Scandinavian ice-sheet left the North
German coast.
Before leaving North America it is necessary to give a brief
account of the phenomena outside the main centres of glaciation,
and especially of the history of the Great Basin between the Sierra
Nevada and Wasatch Mountains. The lowest levels of this basin are
at present occupied by several salt lakes without outflow, of
which the largest is the Great Salt Lake, the level of the water
being determined by the balance between inflow of the rivers and
evaporation from the surface. Twice in the past this balance has
been decidedly more favourable, and then the lakes grew to many
times their present size. The two greatest of these old lakes have
been fully described under the names of Lake Bonneville (of which
the Great Salt Lake is a vestige) and Lake Lahontan, further to
the west. The investigations have shown that before the Glacial
period, and extending back into an unknown past, there was a period
of great aridity. To this succeeded a long period of high water,
during which, however, neither of the lakes overflowed. This stage
was followed by a very long period of great aridity, during which
the lakes dried up completely, and all their soluble matter was
deposited and buried by alluvial material. This period was followed
by a return of moist conditions, during which the water reached a
higher level than before, and in the case of Lake Bonneville actually
overflowed into the Snake river, cutting a deep gorge. This period,
however, was shorter than the preceding moist period. It was followed
by an irregular fall interspersed with occasional slight rises,
but ultimately both lakes descended below their present level and
probably again dried up completely. Both lakes suggest that this low
level was followed by a third rise to a height very slightly above
the present level, followed by a slow fall in recent years.
The relations of the periods of high water to the glaciations are
not clear in these large lakes, but in the Mono Basin, a small
basin further west, there is no doubt that the two were almost
contemporaneous, high water accompanying the maxima of glaciation and
extending some way into the retreat phase. The very long interval
between the first and second period of high water, several times that
since the second period, agrees with this correlation. We find then
that south-west of the main glaciated area there was a district of
greater precipitation or less evaporation, or more probably both.
This is confirmed by the valley moraines of all this region—Sierra
Nevada, Uinta and Wasatch mountains, Medicine Bow Range of northern
Colorado, etc., all of which indicate two glaciations, of which the
first was the greater, separated by a very long interval. In several
ranges the moraines of the second glaciation are double, and some
geologists consider that there were three Glacial periods in these
regions.
In the extremely arid region of Arizona, on the other hand, which is
considerably further south, the evidence of the Gila conglomerates
indicates that while frost was very active, the increase of
precipitation, though undoubtedly present, was comparatively slight.
This shows that the climatic balance was not greatly disturbed, the
chief effect being an important lowering of temperature, probably
due to cold northerly winds. The Gila conglomerates are double,
separated by a period representing present-day conditions.
Summing up the evidences of glacial climate in North America, we find
a striking similarity to Europe. In the north elevation and increased
land area caused the development of large ice-sheets, which appeared
first in the mountainous regions with a heavy snowfall, and later
spread over the drier plains and plateaux of the interior. This first
glaciation was long and complex. Owing to the anticyclonic conditions
which formed over the ice, the rain- and snow-bearing depressions
were forced to pass further southward, causing greater snowfall on
the mountains and high water-level in the lake basins. This greater
snowfall, together with the cold conditions due to the existence
of the ice-sheets to the north, caused the development of mountain
glaciers south of the main glaciated region. In the east there were
cold northerly winds which carried a severe climate as far south
as Florida. This Glacial period was followed by subsidence, and a
long spell of dry, moderately warm climate lasting perhaps 200,000
years, after which elevation and glacial conditions again set in.
These conditions were not so severe as the first, and their duration
was much less, while they were broken up by several intervals of
temporary recession of the ice, one of which, corresponding to
the Riss-Wurm period, lasted for 60,000 years, and perhaps should
be considered as an “interglacial.” This period was marked in its
early stages by the deposition of the curious æolian deposit known
as “loess,” indicating steppe conditions. After the last glaciation
there set in a stage of irregular retreat.
BIBLIOGRAPHY
Leverett, F. “Comparison of North American and European glacial
deposits.” _Zs. f. Gletscherkünde_ 4, 1910, pp. 280, 323.
Wright, W. B. “The Quaternary Ice Age.” London, 1914, Chs. 8-9.
Attwood, W. W. “The glaciation of the Uinta Mountains.” _J. Geol._,
15, 1907, p. 790.
Henderson, J. “Extinct glaciers of Colorado.” _Colo. Univ.
Studies_, 3, 1905, p. 39.
Gilbert, G. K. “Lake Bonneville.” Washington, _U.S. Geol. Survey
Monograph I_, 1890.
Russell, I. C. “The geological history of Lake Lahontan.”
Washington, _U.S. Geol. Survey Monograph XI_, 1885.
Coleman, A. P. “An estimate of post-Glacial and interglacial time
in North America.” _Rep. 12 Internat. Congr. Geol._, 1913, p. 435.