CORKASION BY GRAVITY STREAMS. 253 



(c) Result of reducing the stream volume in great degree. Z 

 the variable now practically vanishes, corrasion is thereby 

 minimised ; the old cutting curves are filled with stream 

 debris ; and the rock basin A B is silted up almost entirely. 



(This is the point which apparently has been neglected hitherto 

 in glacial discussions, for at the present day the channels formed 

 by flood glaciers are now occupied by much smaller ice masses). 



In the case of a small stream such interruptions of 

 channel bed are of slight interest to the ordinary person, 

 but in the case of very large streams, since the length, 

 breadth and depth all increase in some surprisingly rapid 

 ratio, basins may be obtained of topographic importance. 

 Nevertheless the processes in the formation of each are 

 alike in the main. 



Subsequent stage of corrasion, — After the enforced read- 

 justment of grades the stream will now excavate similarly 

 situated forms through the flood debris, and inside of 

 the old flood forms. This has been fully set forth by 

 Gilbert for the case of water. The profiles at this 

 stage will again be those of headward recession as dis- 

 tinguished from those of downstream advance by the 

 readjustment of slope. Steep and narrow trenches will be 

 cut into the similarly shaped and similarly situated envelop- 

 ing forms. If the original flood were short lived its cor- 

 rasive forms would tend to be shallow, action not having 

 lasted long enough in which to accomplish its purpose. 

 Should the weaker stream be long lived then time becomes 

 an important factor, and the newer corrasion will express 

 itself as narrower and deeper trenches developed within 

 much broader and shallower examples. This has been 

 discussed before when considering the path of a stream. 



We say again, however, that until the traces of these 

 older enveloping forms are either obliterated, or made 

 subordinate features altogether upon a general view, they 



