CORRASION BY GRAVITY STREAMS. 291 



similar lenticular masses down stream derived from its 

 own corrasive products upstream. 



In this way some drumlin occurrences may possibly be 

 explained. They may exist by reason of a smaller re- 

 advance, or by a waning stage, of the ice floods (Ice Sheets). 

 Drumlins characteristically mark decadent, or relatively 

 weak ice action. In proportion to the length of time 

 occupied by this weaker stage, will the drumlins so formed 

 dwindle in height and decrease in number. The re-advance 

 of one of the larger ice sheets in all its greatness — the 

 time factor being of measurable or appreciable duration 

 and no movements of land subsidence intervening — would 

 clear away the present numerous forms and substitute for 

 them several large and gently arching morainic masses. 



It does not necessarily follow that drumlins have origin- 

 ated in this manner, but if we grant the existence of a 

 recent ice flood with its characteristic ground moraine, 

 then it does follow that the early action of a succeeding 

 decadent or weaker ice sheet will be in the nature of 

 channel readjustment, and this must result in the produc- 

 tion of various channels with inter-channel drumlin forms. 

 Any consideration of the peculiar relation existing between 

 stream velocity and stream energy will show this to be 

 evident: — the stream volume decreases; the transportation 

 power is greatly lowered; the burden falls out in great 

 measure; the stream overrides it; its irregularities of 

 ground moraine slope furnish opportunity for local acceler- 

 ation of stream velocity; distributaries are formed; corra- 

 sion is emphasised here ; gradually the mass is cut up and 

 finally moved away. 



S— Nov. 3, 1909. 



