Page:Popular Science Monthly Volume 49.djvu/382

364 consider to have been formed rapidly while the ice was retreating from its Iowan stage, with no important general readvance dividing the Iowan from the Wisconsin or moraine-forming stage.

Not only are the Kansan and Iowan stages of culmination of the ice sheets closely alike for North America and Europe, but also the land depression of the Champlain epoch in both these widely separated great areas brought marine submergence of coastal tracts, and caused rapid disappearance of the ice sheets, with the formation of their drumlins and marginal moraines. These two continents were included in the portion of the earth's crust which twice experienced far-extended epeirogenic movements, first of high uplift, bringing the cold climate and snow and ice accumulation of the Glacial period, and afterward of depression somewhat lower than now, whereby the vast ice fields were melted away.

The accompanying maps show the area of the North American and European ice sheets in their maximum extension, and at definite times in their recession, as known by their areas of drift and belts of marginal moraines, and by the beaches of glacial lakes formed between the present watersheds and the northwardly retreating ice border. These maps give the boundaries of the Kansan, Iowan, and Wisconsin formations, adopting these names, according to the law of priority, for both continents, and add for the northeastern United States and Canada the subsequent Warren, Toronto, Iroquois, and St. Lawrence stages in the glacial retreat.

The culmination of the great epeirogenic uplift, which had been in progress through the preceding Lafayette period, raised the glaciated areas, both in North America and Europe, to so high altitudes that they received snow throughout the year, and became deeply ice-enveloped. Submerged valleys and fiords show that this elevation was one thousand to four thousand feet above the present height. The accumulation of the ice sheets, due to snowfall upon their entire areas, was attended by fluctuations of their gradually extending boundaries, giving the Scanian and Norfolkian stages in Europe, and an early glacial recession and readvance in the region of the Moose and Albany Rivers, southwest of James Bay.

During the Kansan stage the ice sheet attained its farthest extent in the Missouri and Mississippi River basins and in northern New Jersey, this being probably at the same time with the Saxonian stage, as later named by Geikie, of maximum glaciation in Europe.

In the Aftonian stage the ice sheet receded from its Kansan