THE WORK OF GLACIERS 



177 



(B) Ground Moraine. — The till which covers regions between 

 moraines and which constitutes by far the largest area of the gla- 

 ciated surface is called the 

 ground moraine. Its topog- 

 raphy varies widely, being 

 usually rolling and inter- 

 spersed with swamps, but 

 sometimes nearly flat over 

 large areas. The ground mo- 

 raine is of variable thickness, 

 being thinner in Canada 

 (Fig. 161) than in the United 

 States, since the ice in its 

 movement to the south car- 

 ried away much of the ma- 

 terial derived from the under- 

 lying rock, leaving little to 

 be deposited on the melting 

 of the ice. 



Drumlins, where they 

 occur, are conspicuous fea- 

 tures of the ground moraine. 

 These are smooth, elliptical hills composed of till and have their 

 longest axes parallel to the movement of the ice (Fig. 162). They 

 are not by any means found everywhere in the ground moraine, but 



Fig. 161. — Map showing the effect of glacia- 

 tion in different parts of North America. The 

 area of maximum deposition lies chiefly south 

 of the Great Lakes, where the ice was relatively- 

 thin and its erosive power was generally feeble. 

 (Modified after Dryer.) 



Fig. 162. — Drumlins. Wayne County, New York. (Photo. H. L. Fairchild.) 



are abundant in portions of central and western New York, Wis- 

 consin, eastern Massachusetts, and elsewhere. The islands in Boston 

 Harbor are drumlins. There is still much doubt as to the precise 



