CORRASION BY GRAVITY STREAMS. 233 



It is also evident that with increased stream velocity, 

 all other things being equal, the slope A O will tend to be 

 increasingly flattened. Sapping is also set up, and the path 

 of a stream particle hints also at the profile thus obtained. 

 Information is also at hand to ascertain the nature of the 

 side and bottom profiles of the excavation. Of course, in 

 the ideal case under consideration, the profile O M is a 

 duplicate of the profile O M'. [Fig. 4 (c)]. As to the 

 angular values of the slopes M O and MO our slight excur- 

 sion into the consideration of the conditions favouring 

 increase of stream velocity has prepared us for the dis- 

 cussion of such. Thus, to take an extreme case, namely, 

 that of a relatively small cross-section of a profound canon 

 when a stream occupies the whole valley. Here we have 

 the conditions for the formation of great basin depths along 

 the channel bottom or at least for the formation of marked 

 over deepening of the valleys. The stream now seeks the 

 lines of least resistance, and owing to its enormous thick- 

 ness it delivers (near its base) great lateral, as well as 

 great vertical and longitudinal pressures. This great 

 lateral pressure is productive of steep cliffs by means of 

 under cutting and consequent sapping action. In the 

 extreme case immense vertical cliffs may be formed by such 

 action. 



Briefly then, excavations suchas we have been considering 

 are more or less cup-shaped at their heads. The conditions 

 favouring the formation of deep basins (below the associ- 

 ated local base levels) with flattish floors and steep sides 

 are : — 



(a) Great stream depth as compared with the width 



of the channel base. 



(b) Great local increase of stream velocity. 



Constrictions along a Crooked Channel. — If bends occur 

 in the channel constriction then the greatest corrasion will 



