GLACIATED AREAS 



379 



other known force besides moving ice could have carried 

 them. These irregularly distributed perched bowlders 

 are called erratics. 



170. Glaciated Areas. Over large areas of what are 

 now the most thickly populated regions of North America 

 and Europe are found 

 widespread formations 

 similar to those de- 

 scribed in the preceding 

 paragraphs. The soil 

 throughout is not like 

 that of the underlying 

 rock ; it must have been 

 transported. Careful 

 examination of all the 

 surface formations has 

 led geologists to believe 

 that at a former period 

 in the earth's history, 

 perhaps not more than 

 a few thousand years 

 ago, the northern part 

 both of North America 

 and Europe was covered 

 with a thick layer of ice, which after several advances and 

 retreats finally disappeared, leaving the country as we now 

 find it. 



Although the border to which the ice extended and 

 many of the changes which the ice made in the surface of 

 the country have been carefully studied and mapped, yet 

 the cause of this extension of the ice and the exact time 

 at which it occurred have not yet been determined. Many 

 theories have been brought forward to account for it, but 

 none of them explains all the facts'. 



A HANGING VALLEY. 



