CORRASION BY GRAVITY STREAMS. 269 



declivities are working in the same corrasive direction as 

 flow action. This follows from an apprehension of the 

 mechanics involved. Both are due primarily to the same 

 vertical force; in both cases the bodies act on similar 

 declivities ; both follow the lines of least resistance, the 

 paths of quickest descent. One delivers a more sudden 

 blow; the other delivers a less sudden blow in a similarly 

 situated spot. 



(a) Heavy volume (Flood) stage. — As the volume increases 

 the glacier may swell over the valley or canon rims and 

 partly drown the higher topography. The few exposed 

 rocks now can shower very little debris on to the more 

 solid surface layers of the stream. Thus the stream at 

 this high stage is not heavily penalised by a superficial 

 burden. Moreover as the study of mechanics has shown 

 us, its speed in such case is much greater than at the 

 time when the stream mass was possessed of less 

 volume; its textural units now glide more rapidly on each 

 other; the units become smaller with increased volume, 

 and the surface also is cleaner by reason of this quicker 

 action in reaching stream destination. Of course, in this 

 case, it picks up a greater bottom load, but since its 

 accelerated velocity is not related in a simple ratio to its 

 increase of transporting power, its ability to handle its 

 bottom load increases in geometrical ratio to its increase 

 in velocity by volume. It can also (as we have shown in 

 Part I of this Series) drop its bottom load or override it as 

 occasion arises. 



Corrasive effects. — Now, what will be the general stream 

 action as it swells over the rims of the stream-developed 

 valleys ? These valleys, by whatever other stream action 

 determined primarily, mark the action of gravity in forcing 

 stream material to take the lines of least resistance, the 

 lines of quickest descent, to the local base levels. As such 



