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TEXTBOOK OF BOTANY 



Fig. 367. Centers of ice accumulation in North America during the Pleistocene. 



There seems to have been no permanent ice on many of the islands, 

 peninsulas, and headlands along the northern coasts, and much of 

 Alaska was unglaciated. Moreover, the ice of each glacial invasion was 

 not continuous from the Pacific to the Atlantic. Ice developed inde- 

 pendently in the western mountains (Cordilleran Center), on the land 

 west of Hudson's Bay ( Keewatin Center ) , and on the Laurentian high- 

 lands (Laurentian Center). One of these glaciers was retreating when 

 another was advancing; consequently there were occasional local areas 

 between the ice fields where the flora spread following the retreat of 



