CORRASION BY GRAVITY STREAMS. 235 



point A [Fig. 5 (c)] on a cutting curve. Under the later 

 conditions of greater stream volume for the main channel 

 such as we have just been describing, this tributary mouth 

 may be cut back to the point B Fig. 5 (d). Nevertheless 

 this may not mark the maximum point of corrasion along 

 such a cutting curve, the maximum corrasion being located 

 at O in Fig. 5 (e). It is evident indeed that with each 

 alteration of volume the point of maximum corrasion must 

 vary along any cutting curve. This is a point needing 

 special attention when studying the causes leading to the 

 formation of Hanging Valleys in glaciated regions. This 

 subject is treated more fully in Part II of this Series. 



A channel confluence forming a constriction. — Here the 

 sum of the cross-sections of the tributaries is larger than 

 the cross-section of the main stream at or near the stream 

 confluence. We will suppose the channel grade to be very 

 slight or almost negligible. The case is now similar to 

 that of corrasion in a channel constriction. The more 

 pronounced the constriction of the main stream the more 

 accentuated will be the increase of stream velocity at that 

 point. This again implies greatly increased stream energy. 

 So long as the stream has power to utilise its basin bed as 

 a bridge for the transportation of its load proper as a whole, 

 so long will corrasion have a vertical component there. 



The whole valley sides also will (in the extreme case 

 where a channel forms a whole valley) be called on to 

 accommodate themselves to the basin profiles. Powerful 

 under cutting action takes place with the removal of spurs 

 and the cutting back of tributary mouths. In proportion 

 as these tributaries are weak so will the basining action 

 of the main stream in the constriction cause the mouths 

 of the minor side valleys to be left perched high above the 

 floor of the channel. 



