294 THE ICE AGE IN NORTH AMERICA. 



faster motion at one place cause an excess of erosion there, the 

 slower motion at another place may bring about an excess of 

 deposition. This difference of action is known to prevail be- 

 tween the central and marginal parts of glaciated areas, and 

 the local accumulation of drumlins in an intermediate region 

 gives a smaller example of these two parts played by the ice. 

 If the causes of the irregular motion of the ice lie in the gen- 

 eral form of the country, the location of faster and slower cur- 

 rents will be relatively permanent ; the districts of faster cur- 

 rents would be found where the greatest volume of ice is 

 allowed to pass, and some of the points of retardation may be 

 the seats of long-continued drumlin-growth. The drumlins 

 thus begun will depend less upon the immediately local form 

 of the ground than on the topography of a more considerable 

 district, and hence we need not suppose every drumlin to have 

 begun its growth upon a knob of rock, although the beginning 

 of many hills may have been thus determined. Once begun, 

 the drumlins will go on increasing in size as long as deposition 

 exceeds erosion, always maintaining an arched form of least 

 resistance until a maximum size is reached or until the ice 

 melts away ; and in their growth they will approach the form 

 to which rough, rocky hills would be reduced by the reverse 

 process of erosion if time enough were allowed. Under un- 

 ending glaciation the whole surface must be rubbed down 

 smooth.* 



As these theories relating to the formation of drumlins 

 involve the general principles upon which we are to explain 

 other evidences of the varying degrees of erosion effected by 

 moving ice, we may as well introduce here, as anywhere, Mr. 

 Geikie's general reply to the objections urged from the sup- 

 posed nature of the case : f 



Our ice-sheet flowed, we can not doubt, with a differential 

 motion : it must have moved faster in some places than in 

 others. In steep valleys and over a hilly country its course 



* "American Journal of Science," vol. cxxviii, pp. 413-416. 

 f " The Preservation of Deposits of Incoherent Materials under Till or 

 Bowlder Clay," in the "Geological Magazine;' February, 1878, pp. 2, 3, 6, 7. 



