CORRASION BY GRAVITY STREAMS. 255 



entirety, and the "broad" becomes in part an aggradation 

 spot, and is utilised as a bridge for the transportation of 

 lighter river material only. In times of heaviest volume, 

 the stream will develop velocity sufficient for the corrasion 

 of its base with the production of cutting curves at isolated 

 spots, but such basin development will be weak as compared 

 with that in the "narrows." Basins may often be de- 

 veloped here, however, by aggradation processes, as when 

 the heavy rush from the " narrow" carries the material a 

 certain distance only down the "broad" and allows of local 

 impounding of the stream. Such "broads" and constrictions 

 arise frequently as the result of stream action on associated 

 weak and hard rock structures. The stream excavates a 

 "broad" in weak structures while waiting for the channel 

 base to be lowered in the more resistant structures down- 

 stream. 



Corrasion of obstacles in channels. — Obstacles small as 

 compared ivith volume of stream. — Take the case of an 

 obstacle small as compared with the volume of the stream. 

 The obstacle has no appreciable effect on the stream motion 

 as the stream rises over and glides around it in taking the 

 lines of least resistance. Gravity is always tending to 

 impart a vertical direction to the stream flow and all other 

 things being equal, the obstacle on its upstream aspect 

 receives the greater part of the local abrasive action of 

 the stream. The result is much local corrasion, and such 

 force delivered is at the general expense of the stream 

 momentum and energy. 



But on the downstream aspect of the obstacle the case 

 is different. For although gravity is attempting to give a 

 vertical direction to the stream at this spot, the direction 

 of the stream velocity as it passes over the obstacle will 

 not allow the abrasive energy to take full effect here. 

 So long as the stream has any velocity whatever, so long 



