864 



GEOLOGY 



was more effective at some times and places than at others. On 

 the whole, the topographic effect of glacial erosion was probably to 

 soften the surface contours, without noticeably diminishing the 

 relief. 



The second great result of the ice-sheets was the deposition of 

 the drift. Some of it was deposited while the ice-sheets were grow- 



1/2 



Fig. 575. One phase of ground moraine topography; elongated hills of 

 drift of the type shown here are called drumlins; southeastern Wiscon- 

 sin. (U. S. Geol. Surv. ) 



ing, some of it after they had attained their growth, and some of 

 it while they were declining. Some of it was deposited beneath 

 the body of the ice, and some of it at its edge. The drift altered 

 the topography notably, especially where it was thick and the relief 

 of the underlying rock slight. 



Formations Made by the Ice-sheets 1 



The drift formations fall chiefly into three categories, namely 

 (1) those made directly by the ice (unst ratified), (2) those m;i<lo 



1 Jour. Geol., Vol. II, pp. 517-538, and International Geol. Congr., .~>th 

 Session, 1893; also McGee, idem. 



