270 E. C. ANDREWS. 



the peculiar geometrical forms produced by corrasion dur- 

 ing the earlier period of stream transportation of debris 

 are those which will be more closely followed by any 

 immediately succeeding stream, however variable in texture 

 and composition. Therefore, at this early stage, however 

 high the stream rises, however much it disports itself above 

 the valley rims, its most intense action must lie along the 

 lower portions of the old stream developed channels which 

 it found ready to hand. 1 It may wear these barriers away 

 find modify its drainage later, but while it occupies that 

 topography, while those older stream developed valleys 

 remain, the new stream will exert its greatest corrasive 

 force along the deeper, steeper and more contracted points 

 of such valleys. And the more it rises above the valley 

 rims, and the deeper and more confined those valleys or 

 canons become, and the steeper the channel slopes, the more 

 strongly will the stream work along the lower portions of 

 the canons. Pressure and friction are opposed however ; 

 the reflection of bottom and side velocities has to be 

 accounted for and the actual maximum velocity of the ice 

 will be found not at the base but at a varying height above 

 it. 2 The greatest corrasive strength is however basal as 

 we have shown in Part I. 



At the summits, mighty as its volume is, the flood glacier 

 accomplishes but relatively little corrasion. There are not 

 the well defined channels here ; there are not the marked 

 declivities to act on ; there are not the valley constrictions 

 to induce increase of velocity. In other words the lines 

 of least resistance lie along the bases of the valleys which 

 are sunken deeply into the plateau, and along the latter 

 alone is seen the greatest increase of stream corrasive 

 power. Great as the flood volume may be away from the 

 channel proper, it acts somewhat similarly to the backwaters 



1 Andrews (a) pp 37-39. z Johnson, p. 576. 



