THE WORK OF SNOW AND ICE 



135 



sometimes happened, the unfilled part between became a basin fit 

 for a lake. Devil's Lake, in Wisconsin, is an example (Fig. 142). 

 The number of lake basins which arose by the filling of river valleys 



Fig. 142. Sketch showing a lake in a former river valley, held in by drift 

 dams. The dotted areas are terminal moraines. 



in one or more places by drift, is very large. The remarkable Finger 

 Lakes of New York are examples. Rock basins are often made 

 deeper by the deposition of drift about their rims. 



The ice-sheets gave rise to lakes and ponds in other ways also. 

 Many of them are in hollows in the surface of the drift. Nearly all 

 the numerous lakes of North America are in the area which was 

 covered by the ice-sheet or by mountain glaciers. 



Some lakes produced by the ice had but a short life. Such was 

 the history of those which came into existence along the margin of 



