EROSION AND ITS SIGNIFICANCE. 



123 



the heaviest flooding the debris will be arranged on the 

 senkungsfeld area in a continuous curve convex to the sky. 

 Smaller floods will destroy the uniformity of this convex 

 curve. Such a phase of stream action is usually very short 

 lived. 



Thus the term "flood" is seen to possess a dynamical 

 significance for both degrading and protective stages. 



It is necessary thus to carefully examine present day 

 stream channels to understand whether they become flooded 

 or not at periodical intervals. For only by appreciating 

 the action of a flood can the formation of a stream channel 

 be understood. To understand the work of the Upper 

 Amazon in flood, one should have knowledge of it during 

 such period or at least one should have knowledge of other 

 large streams when in flood. Again, if glacial cirques are 

 stream channels, then they in turn must have been formed 

 during periods of greater ice volume than at present, 

 because the "Ice Age" has only just gone, and the high 

 level glacier marks may be seen on the cirque walls. But 

 to-day the ice in such situations is relatively meagre in 

 volume ; it is in a state of tension, whereas in channels 

 formed by streams, the streams themselves should have 

 been in a condition of compression when accomplishing 

 their task. This arises from the conception of flowage by 

 pressure as weight. 



All the foregoing account of flood action is dependent on 

 the condition that floods are of such frequent occurrence 

 that weathering has no opportunity to obliterate such flood 

 profiles by its action during interflood periods. 



Form of channel. — It is evident that the stream corrades 

 not so much by its own material as by the load of earth 

 debris it transports, unless indeed its own mass is so great 

 as to exceed the ultimate crushing strength of the rock 

 structures it traverses or so as to be enabled to detach. 



