CORBASION BY GRAVITY STREAMS. 251 



utmost strength of that stream locally as the basin or 

 cirque head recedes. Thus when Z O marks the greatest 

 basin depth, the stream keeps the channel head A O clean, 

 but it may not have strength enough to utilize O B in its 

 entirety as a bridge for the transportation of material. It 

 must in such a case actually dump some of its debris in 

 the neighbourhood of B or along O B. 



Now suppose the stream volume to be reduced to one 

 half or a quarter of its former volume. In such a case 

 the stream velocity is considerably decreased, and on this 

 account alone the measure of vertical strength of the 

 stream will be less than Z O. But with decrease of stream 

 volume comes a much more pronounced decrease in 

 corrasive strength, and the vertical measure of stream 

 strength is therefore greatly lessened. 



Let Z'O' in Fig. 10 (b) represent the depth below which 

 the reduced stream cannot cut or transport its debris as a 

 whole. What then is the stream method here? It will be 

 forced to drop its burden on to the basin base and sides 

 A O B (until it can lower the point B). But the problem 

 is also a geometrical one from the nature of the dynamic 

 principles involved, and A Z' will be to Z'O' in the new 

 basin as A Z is to Z O when all other things are equal. The 

 new appearance will be much as in A O'B', Fig. 1 (b). 



Now under the new conditions when once the form A O'B' 

 is made, it will be maintained until the recession of A is 

 again carried on, at which stage Z'O' will progress upstream 

 and some aggradation will ensue along O'B'; that is, if the 

 stream volume is unaltered. 



Fig. 10 (b) shows then that upon the setting in of con- 

 ditions of great stream volume a great interruption of the 

 old channel bottom A Z B was set up and a basin A O B 

 was excavated. Upon the reduction of stream volume this 

 became the place to be aggraded, and in proportion to the 



