CORRASION BY GRAVITY STREAMS. 211 



base and sides at the points where the greater glacier had 

 exerted its maximum corrasive strength, and such aggra- 

 dation and relative glacial stagnation would ensue until 

 the channel grades had been readjusted to the new con- 

 ditions. 



The significance of this is briefly as follows : — Very large 

 glaciers have been acting along certain valleys; these 

 large ice masses have only just vacated the valleys, and 

 dwarfed glaciers now occupy tlieir places. The plasticity 

 of the present day glaciers is much less than that of the 

 preceding flood glaciers, and if we assume that glaciers are 

 capable of even slight corrasion, then the action of the 

 dwarfed glaciers of to day along the same valley is alto- 

 gether different to that of the recent glaciers. Instead of 

 studying present day glaciers as models of the corrasive 

 action of ice during the recent Ice Ages, we should rather 

 beware of pitfalls by such study just as students of water 

 streams should beware of attempting explanations of river 

 channel formations before they have observed the action of 

 a large flood, otherwise they may be tempted to deny the 

 power of the stream to excavate deep potholes or basins in 

 the channel base and sides because the normal stream is 

 practically inert at such spots. 



Furthermore, it will be seen that all streams must 

 conform to certain underlying principles from the very 

 conception of stream flow, seeing that gravity, the stream 

 maker, is a constant vertical force at the earth's surface, 

 ever compelling the stream to take the lines of least 

 resistance. 



In the case of a vertical force this implies the necessity 

 of the stream, as a unit, to take the line of quickest descent. 

 The stream may be conceived as a group of textural units 

 possessed of greater or less individual coherence. As 

 pressure (by volume or increased speed) asserts itself such 



N— Oct. 3, 1909. 



