Throughout the Glacial period of growth, culmination, and decline of the North American and European ice-sheets, the climate causing the snowfall and ice accumulation fluctuated to such an extent that the boundaries of the continental glaciation were alternately advanced and checked or drawn back.
North America had five stages of widely extended icefields. First was the long time of Nebraskan glaciation, followed by a great recession of the ice-borders during an interglacial stage named Aftonian, from its stratified beds and fossils near Afton, in southwestern Iowa. The second and maximum extension of ice accumulation is named, in the Keewatin area of its outflow west of the Mississippi, the Kansan stage, which was broken by the Yarmouth interval of ice melting and retreat. With the Kansan glaciation, but in part spreading across the eastern edge of its drift sheet and melting later from the Mississippi, . . .