QUATERNARY 647 



the Labradorean ice sheet was to the southeast, where it stretched 

 1600 miles south of the center. There was also a movement north 

 from this center, but it is not known to have been nearly so extensive. 

 This ice sheet, in its greatest expansion, crossed the Ohio River into 

 Kentucky, and extended into southern Illinois. 



The Keewatin ice sheet extended almost as far southward as the 

 Labradorean, its front at one time being in Kansas and Michigan, 

 about 1500 miles from its center. The movement of the Keewatin 

 ice sheet is remarkable since, beginning in a low, flat region, which 

 is now semiarid, the ice moved upgrade into the United States. 

 This is more astonishing when we find that the Cordilleran sheet, 

 starting from the lofty mountains of western North America, ap- 

 parently failed to move beyond their foothills. The Cordilleran ice 

 sheet should, perhaps, be considered as the product of the confluence 

 of mountain glaciers, spreading out as they reached lower and less 

 rugged ground, much as do some of the Alaskan glaciers to-day. 



Besides these great centers of ice movement, large local glaciers 

 accumulated on the mountains of the western United States, where 

 they were vigorous for many years, as is shown by the cirques (p. 143), 

 moraines, rock basins, and other evidences of glaciation common 

 throughout the high mountains of the west. This is well seen on the 

 topographic maps of Montana, Wyoming, Colorado, and neighboring 

 states. 



REFERENCES FOR GLACIATION IN NORTH AMERICA 



Chamberlin and Salisbury, — Geology, Vol. 3. 



Geikie, A., — Textbook of Geology. 



Geikie, J., — The Great Ice Age. 



Wright, G. F., — The Ice Age in North America. 



Wright 3 W. B., — The Quaternary Ice Age. 



Development of the Ice Sheets 



The great ice sheets (with the probable exception of the Cordilleran 

 center) did not begin as mountain glaciers which by their coalescence 

 became one great glacier, but were the result of the gradual accumu- 

 lation of snow in the north, due to the lowering of the temperature 

 (p. 660). 



Thickness of Ice Sheets at Center. — It has been held that the 

 great ice sheets were several miles thick at the various centers, from 

 which points they gradually thinned toward the margins. A study of 

 existing Greenland and Antarctic glaciers shows that such is not now 



