The evolution of climate

by C. E. P. Brooks

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.