THE QUATERNARY PERIOD 451 



ing the effects of weathering and erosion on the older and 

 younger sheets of drift it is possible to gain a rough idea as to 

 their relative ages. Estimates thus made of the length of 

 time since glaciation began range from 500,000 to 1,500,000 

 years. It is impossible to make a much closer calculation 

 than this because there are so many factors which vary from 

 time to time and in a way which cannot be predicted. But 

 the fact is clear that the period was many times as long as the 

 known part of human history. 



How the ice sheets changed the land surface. The work 

 of glaciers has already been discussed in Chapter VI. There it 

 was shown that the effects wrought by glaciers are very differ- 

 ent in different places. Thus the last Canadian ice sheets 

 produced varied changes according as the country they in- 

 vaded was flat, hilly, or mountainous. 



In the mountains of New York the ice scoured off the slopes 

 of the hills, and removed the crags and talus slopes, but did not 

 greatly change the general forms 

 (Fig. 468) . Preexisting valleys 

 were scoured out and deepened 

 where they ran parallel to the 

 ice movement, and were par- Fl " 4 68.-Low mountains which 



. . r have been scoured by an ice sheet, 



tially filled with drift where leaving the summits smooth and 



their courses lay across the u " ded ^ the valleys partly 



filled with drift. 



line of glacial movement. The 



so-called finger lakes of western New York are in valleys thus 



deepened and locally blockaded. 



Where the hills were lower and the ice thicker in proportion, 

 the effects of erosion by the ice sheet were more pronounced. 

 Not only was a vast amount of soil and rock ground from the 

 hills, but many of the preglacial valleys were completely 

 buried (Fig. 469). In such regions the present hills and hol- 

 lows are simply the irregularities of the drift itself, as it was 

 deposited. The older topography has thus been obliterated 

 over large areas of Illinois, Minnesota, and other northern 

 states. 



