6 4 6 



HISTORICAL GEOLOGY 



ficient precipitation, which makes an accumulation of snow impos- 

 sible. 



2. North America. — North America (Fig. 565) was more exten- 

 sively affected by glaciation than any other part of the world, about 

 4,000,000 square miles being covered by ice at the time of its greatest 

 extension. Two peculiar features are especially striking in the dis- 

 tribution of the North American ice sheets : (1) the greatest extent 

 of ice was in the low regions of the northeast, instead of in the high 



Fig. 565. — Map of North America, showing the area covered by ice at the stage of 

 maximum glaciation, and the centers from which the ice moved. The arrows show the 

 directions of ice movement. 



mountains of the west and northwest; and (2) the northeastern 

 portion of the continent, rather than the northern, was the scene of 

 maximum glaciation, even Alaska being largely free from ice. 



The great ice sheets moved in all directions from three great centers 

 and probably to some extent from other smaller ones, as is proved by 

 the direction of the striations on the underlying rocks, and the courses 

 along which the bowlders were carried. These three great centers of 

 radiation were: (1) that situated in Labrador (the Labradorean) ; 

 (2) that just west of Hudson Bay (the Keewatin) ; and (3) that in 

 the western mountains (the Cordilleran). The greatest extent of 



