THE WORK OF SNOW AND ICE 



139 



across areas where there had been no valleys in others. In choosing 

 their new courses, the streams sometimes ran down steep slopes 

 or fell over cliffs. Thus arose rapids and falls, which, on the whole, 

 are rather common in the glaciated area. 



Lakes, as well as rapids and falls, are marks of topographic 

 youth. Rivers are, on the whole, hostile to lakes, for the inflowing 

 streams bring in sediment which tends to fill the basins, and out- 



Fig. 147. A still later stage of the Great Lakes. The sea is thought to have 

 covered the area shaded by lines at the east. (Taylor.) 



flowing streams cut down their outlets. Many small lakes have 

 already become extinct by these processes, and many others have 

 been made smaller. The fact that so many falls, rapids, and lakes 

 still remain within the glaciated area shows that the time since the 

 melting of the last ice-sheet has not been long enough for these 

 features to be destroyed. 



Lakes, ponds, marshes, falls, rapids, etc., are much more abun- 

 dant in the area of the last ice-sheet than in the area of drift outside 

 of the last ice-sheet. This is largely because the older drift, where 

 now exposed, has been subject to rain and river erosion long enough 



