CHAPTER XXIX 



THE PLEISTOCENE OR GLACIAL PERIOD 

 FORMATIONS AND PHYSICAL HISTORY 



The distinguishing feature of this period is its extensive glacia- 

 tion. Thick sheets of ice, having the slow movement of glaciers, 

 covered six or eight million square miles of the earth's surface where 

 climates had been mild not long before. 



More than half the area known to have been glaciated during 

 this period was in North America, and more than half of the re- 

 mainder in Europe. 



North America. Nearly half of North America was covered by 

 ice (Fig. 504), and strangely enough it was the plain, rather than 

 the mountainous part, which had most ice. Three principal centers 

 whence ice moved have been recognized on the continent, l the 

 Labradorean, the Keewatin, and the Cordilleran. Spreading from 

 these centers, ice-sheets covered some 4,000,000 square miles. 

 From the Labradorean center, the extension was notably greatest 

 to the southwest, and in this direction the limit is some 1,600 miles 

 from the center of dispersion, in latitude about 37 30'. The exten- 

 sion of the Keewatin ice-sheet to the southward was scarcely less. 

 It found its limit in Kansas and Missouri, about 1,500 miles from 

 its center, while to the west and southwest it extended 800 to 1,000 

 miles toward the Rocky Mountains. One of the notable features 

 of the ice dispersion was the great extension of the Keewatin sheet 

 westward and southwest ward over what is now a semi-arid plain, 

 rising in the direction toward which the ice moved, while glaciers 

 from the mountains on the west pushed eastward but little beyond 

 the foothills. 



The Cordilleran ice-sheet is less simply denned. Much of it 

 occupied a plateau hemmed in by mountains; but plateau glaciation 

 was complicated by extensive mountain glaciation of alpine type. 



1 A fourth center (Patrician) has been suggested by Tyrrell, southwest of 

 Hudson Bay, and still another by Wilson, in the extreme East. Wilson, The 

 Glacial History of Nantucket and Cape Cod. 



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