234 E. C. ANDREWS. 



follow the line of greatest velocity taken by the stream. 

 The basin of corrasion will here be formed in much the 

 same manner as has been shown in the previous chapter, 

 but it will not now be symmetrically situated along the 

 axis of the channel constriction, but will be situated closer 

 to one side than to the other. The valley wall associated 

 with this deepest point of corrasion will be selected for 

 more severe attack than the opposite one. [Fig. 5 (b)]. 



This side then becomes the main portion of the cutting 

 curve. This form will receive attention later. This and 

 the following ideal forms are, of course, adduced on the 

 assumption of homogeneous structures not large as com- 

 pared with the stream volumes. In nature the cup-shaped 

 head is not always formed owing to the heterogeneous 

 character of the structures acted upon. Again, angular V 

 shaped heads (as also channel bases) are frequently formed 

 because of incipient corrasive action only being set up, and 

 because of the presence of strong joint systems arranged 

 at acute angles with each other. (This is well shown in 

 Yosemite valley details as will be mentioned later). 



Let us consider the cutting curve a little in detail other- 

 wise misconception may arise as to our meaning of the term 

 "resultant direction" of the stream's velocity. In the figure 

 (Fig. 5) the stream is shown as acting along a definite line, 

 and operating at one definite point. From the nature of 

 the case the stream cannot act at one definite point only, 

 but will act upon a whole bend of its channel while leaving 

 the opposite side comparatively protected. Nevertheless 

 there must be a point of maximum — as also one of minimum 

 — velocity and corrasion in every such bend of the channel, 

 and the location of such point of maximum corrasion along 

 a cutting curve it is that is indicated by R (Fig. 5). Thus 

 a small tributary stream may formerly have entered the 

 main channel approximately with accordant grade at the 



