380 



GEOLOGY. 



in the thousands of lakes which affect the surface of the later drift, 

 and to a less extent, the surface of the older. The basins of these 

 lakes or ponds arose in various ways. There are (1) rock basins pro- 

 duced by glacial erosion; (2) basins produced by the obstruction o f 

 river valleys by means of the drift; (3) depressions in the surface ol 

 the drift itself; and (4) basins produced by a combination of two or 

 more of the foregoing. The third class, as above, may be subdivided 

 into depressions in the surface of (a) the terminal moraine, (b) the 

 ground moraine, and (c) stratified drift. Since the stratified drift in 



Fig. 508. — Diagram illustrating normal drainage in the driftless area of 

 Wisconsin and Illinois. 



which the lakes of this last sub-class lie is largely in valleys, it would 

 not be altogether inappropriate to class some of them with group (2). 



In addition to the lakes and ponds now in existence, there have 

 been others of a more temporary character. Some of them have 

 already become extinct by reason of filling or by the lowering of their 

 outlets since the ice melted; others depended for their existence on the 

 presence of the ice, which often obstructed valleys, giving rise to basins. 1 

 The ice also developed basins outside of valleys, when the surface slope 

 was favorable. 



x For examples of such lakes, see Glacial Geology of N. J., pp. 151-159, and Fair- 

 child, Bull. Geol. Soc. Am., Vol. X, pp. 27-68, and Sta^e XII following. 



