Hedin, Sven. “Some physico-geographical indications of post-Pluvial climatic changes in Persia.” Internat. Geol. Congr., Stockholm, 1911. “Die Veränderung des Klimas.”
CHAPTER VIII
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.