290 E. C. ANDREWS. 



to readjust its channel grade and to establish itself again as a 

 corrasive agent by gradually ridding itself of the surplusage 

 of infra-glacial load. The stagnant morainic mass now 

 presents a consequent surface to the reduced ice sheet. 

 Its basal portions will therefore find the lines of quickest 

 descent, and the lines of least resistance in the ground 

 moraine, and a relatively more rapid basal ice motion will 

 ensue locally with differential corrasion of the ground 

 moraine. Channels will be cut into it, and as these channels 

 are deepened the reduced mass will again attack the earth 

 structures, not by moving the ground moraine as a whole 

 over the fresh rock, this being a mechanical impossibility 

 in the stage under consideration, but by excavating trenches 

 in the morainic mass and accomplishing corrasion by local 

 concentration of force at the bases of such trenches ; [the 

 same force spread out over the whole area would produce 

 only negligible results. All streams act alike in this con- 

 nection, whether they represent extremely mobile, partially 

 plastic or partially solid types]. 



Thus in one place the reduced ice sheet attacks its 

 moraine, in another it overrides it, aggrades it, or moulds 

 it into long lenticular masses. These peculiar lenticular 

 forms are partly the result of corrasion of the ground 

 moraine, but are often in great measure a result of accre- 

 tions brought from regions higher upstream. Thus the 

 ground moraine belonging to the flood period may be 

 steadily incised in one place, and its material gradually 

 deposited on the lee seit of an obstacle downstream. Over- 

 lapping accretions drawn out axially will thus originate. 

 All sorts of conditions must be expected. 



But the aim of the reduced ice mass is to move the mor- 

 aines out of its way and to set up corrasion of the rock 

 structures afresh, and if the time factor be greatly in- 

 creased, it will succeed in its aim and commence to substitute 



