THE WORK OF GLACIERS 



187 



Marjelen, Switzerland (Fig. 174), owes its existence to the dam 

 formed by the Aletsch Glacier. 



(2^ Rivers. — To form a conception of the effect of glaciation on a 

 well-drained region, imagine a mature country, such as portions of 

 West Virginia (Fig. 95, p. no), invaded by glaciers advancing from 

 the north. It is evident that the north-south valleys would be the 

 ones most likely to be deepened, since they are in the direction of 

 the movement of the ice, and that the east-west valleys would prob- 

 ably be entirely or partially filled with drift, leaving basins which 

 would be occupied 

 by lakes upon the 

 disappearance of the 

 ice. In many cases, 

 the streams would 

 keep their old, wide, 

 preglacial courses for 

 a portion of their 

 length, but in other 

 parts would occupy 

 deep, narrow, rock 

 gorges which they 

 had eroded after they 

 were forced out of 

 their old channels and 

 had cut down through 

 the drift (Fig. 175). 



Waterfalls and 

 rapids often occur at 

 the points where 

 streams have been 

 diverted from their 

 old channels by drift 

 (p. 163). Many of 

 the manufacturing centers of New England and other northern states 

 owe their establishment to the existence of such natural water power. 

 In portions of New York, Ohio (Fig. 176), Michigan, Indiana, 

 Minnesota, and other northern states, and in Canada, the preglacial 

 drainage has been greatly modified by glacial action. In certain 

 areas the streams have new courses ; the old valleys are so filled with 

 drift that no evidence of them is to be obtained except by borings. 



Fig. 176. — Map showing the present course of the 

 Ohio River and the course which it had previous to 

 glacial times (shown by arrows). Because of the deposi- 

 tion of drift the Ohio was forced to abandon its wide, 

 preglacial valley and to cut a new one. (Modified after 

 Fenneman.) 



