232 



E. C. ANDREWS. 



increase of stream velocity. The law is easily grasped, but 

 its significance in nature is far reaching. Having elucidated 

 this point, we are now in a position to consider the general 

 shape of the excavation formed below base level. 



It is evident all other things being equal that the greatest 

 stream velocity and strength will be along the main valley 

 axis, and that the greatest basal stream strength will also 

 be along this main axis. (Johnson discusses a somewhat 

 similar action, pp, 576-577). The deepest point of the 

 excavation will therefore be along the valley axis. It is 

 clear also that this maximum corrasion will cause the point 

 of the excavation lying farthest upstream to lie along 

 the valley axis while the progressively weakening lateral 

 action will cause the upstream rim of the excavation to 

 present a concave 1 aspect downstream as MAM' [Fig. 4 

 (a) and (b). 



Fis. 4. 



Longitudinal sec- 

 tion of rock basin 

 excavated along a 

 former channel 

 grade of negligible 

 slope in a channel 

 constriction. 



Plan of such rock 

 basin. 



(c) Transverse section 

 of such rock basin. 



' The general case is considered in the appendix. 



