cenozoic time — quaternary. 943 



1. Glacial Period. 



AMERICAN. 



Three subdivisions or epoclis, of the Glacial period, are recognized: 

 (1) the Early Glacial Epoch, or that of the Advance of the Ice and 

 its maximum extension ; (2) the Middle Glacial Epoch, or that of the 

 First Eetreat of the ice ; (3) the Later Glacial Epoch, or that of the 

 Final Eetreat. 



1. Epoch of the Advance. 



General Condition of the Continent during the Advance. 



Topographical and fluvial conditions. — The continent, when the Ice age 

 began, had its high mountains and full-grown rivers. The elevating of the 

 continental surface that was begun in the Tertiary had covered the land 

 with running waters, and the new and vigorous streams made erosion their 

 first work. The older streams, also, that had reached a level of no work, 

 received new energy and were set to work deepening their channels, leaving 

 the old flood grounds as terraces to mark progress. The time was especially 

 favorable for pre-glacial erosion. In addition to this growth of rivers, 

 forests took rapid possession of the continent, and faunas and floras greatly 

 widened their range. 



As the cold and precipitation increased, the time finally came when the 

 heat of summer was not suflicient to melt all the snows of the colder season, 

 and then began glacial accumulation. For a while glaciers were confined to 

 the higher mountains ; but gradually all glacier areas became united in one 

 great continental ice-sheet, Greenland-like, with local glaciers only along 

 some of the deeper terminal valleys. 



While thus spreading over the land, there were oscillations in the progress 

 of the ice-sheet, as in modern glacier regions, determined by meteorological 

 cycles, — the 11-year cycle dependent on the cycle of the sun's spots, and a 

 longer cycle of 35 to 50 years, as now in the Alps. And besides, there were 

 other sources of meteorological change, causing longer halts and recessions 

 in the ice-sheet, for which no explanation can yet be given. 



A large ice-sheet gives a temperature of 32° F. to the air above it, and this 

 favors its perpetuity. But the southern margin, at the time of maximum 

 advance, was in middle temperate latitudes with the tropics not far away ; 

 and warm or hot winds, therefore, were at hand to produce large fluctuations 

 in the extension of the ice with the changing seasons. 



Causes determining places of the first ice and of greatest accumidation. — 

 Since the ice would have accumulated most rapidly where abundant pre- 

 cipitation and low temperature were combined, the region of earliest com- 

 mencement and maximum accumulation would have been over the eastern 

 portion of the continent toward the Atlantic. Along the coast region of 



