CHAPTER XXX 

 THE PLEISTOCENE OR GLACIAL PERIOD 



The distinguishing feature of this period is its extensive glacia- 

 tion, ice-sheets covering six or eight million square miles of the 

 earth's surface where mild climates had prevailed not long before. 

 But for the ice-sheets and their effects, this period might properly 

 be joined to the Pliocene, making one period of high and extensive 

 lands and correspondingly restricted oceans. 



General Distribution of Glaciation 



More than half the area glaciated during the Pleistocene period 

 was in North America (Fig. 560), and more than half of the re- 

 mainder in Europe (Fig. 561). The glaciation was, therefore, 

 notably localized, though the whole world felt its effects. 



North America. Nearly half of North America was buried by 

 ice (Fig. 560), and strangely enough, it was the northeastern half 

 of the continent rather than the northern. More strangely still 

 it was the plain, rather than the mountainous part, which had most 

 ice. Alaska was largely free from ice except in the mountains, 

 and continuous glaciation did not extend as far south on the 

 mountain-girt plateaus of the Pacific border as on the smooth low 

 plains of the Mississippi basin. 



Three great centers of ice radiation, besides Greenland, have been 

 recognized. 1 These are the Labradorean, the Keewatin, and the 

 Cordilleran. From these centers, ice-sheets spread, covering some 

 4,000,000 square miles. The centers from which the movements 

 radiated are determined with certainty by glacial striae, and by the 

 direction of transportation of drift. 2 



From the Labradorean center, the extension was notably 



1 A fourth center has been suggested. Wilson, The Glacial History of 

 Nantucket and Cape Cod. 



2 This applies only to the radiations of the last glacial epoch. 



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