CHAPTER XXV 



The great ice sheets. During the later part of the Ter- 

 tiary period the climate of the northern regions was becom- 

 ing somewhat colder, so that palms no longer nourished in 

 Greenland, nor corals off the coast of Scotland, as they had in 

 the early Tertiary. In the Quaternary period, from causes not 

 yet understood, the temperature of the northern regions had 

 been lowered to such a degree that the snows of winter were 

 not melted off in summer. Thus glaciers came into existence, 

 not only in high mountains and polar regions where we have 

 them to-day, but over large regions which are now free from 

 them. Through the long accumulation of snows, thick ice 

 sheets, or continental glaciers, grew up in North America and 

 in Scandinavia and spread outward in all directions until 

 they covered Canada and much of Europe. 



In North America the ice sheets extended into the United 

 States as far south as southern Illinois and New Jersey. 

 Singularly enough, they did not cover much of Alaska, in 

 spite of the fact that it is farther north than some of the coun- 

 tries which were glaciated (Fig. 467). 



The fact that ice sheets did not cover Alaska and Siberia, 

 two of the coldest parts of the world, shows that low tempera- 

 ture was not the only condition needed to bring on glaciation. 

 Plenty of snow is likewise essential, and so in rather dry 

 regions or where there are short, hot summers, even where 

 there is great cold, we find no glaciers. 



Successive advances and retreats of the ice. The Glacial 

 epoch was marked by the growth and eventual melting off of 

 not merely one ice sheet but of several, one after the other. 

 This is true of both Europe and North America. In the 



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