CORRASION BY GRAVITY STREAMS. 267 



the walls as though the central ice pressure had thrust the 

 lateral portions upwards and outwards. 



5. On the shoulders of spurs standing in the way of the 

 heavy glaciation much abrasion was noted on the upstream 

 side, and much quarrying on the downstream side, the 

 quarrying often producing incipient cirque forms. 



In all these Sierran observations one fact stood out 

 plainly, to wit, that the ice locally had followed the lines 

 of least resistance ; that it would not ascend a steep slope 

 if opportunity offered at all for horizontal or less steeply 

 inclined motion; nevertheless if no such opportunity offered 

 it would ascend the steepest slope and even corrade such 

 in its ascent, provided that the obstacle face was small as 

 compared with the glacier volume, whereas on a down- 

 stream obstacle slope its abrasive action weakened in pro- 

 portion to the verticality assumed by such downstream 

 slope until in cases of actual verticality no polishing or 

 smoothing was observed, although the ice-scratchings 

 were strongly marked over all the associated forms. 



And this is just what we should expect from a stream 

 flowing under gravity because of the tendency to form a 

 parabolic path. For the downstream slope of a small 

 obstacle to experience the full stress of the glacier there 

 must be no motion. But a stream passing over a small 

 obstacle has a velocity, and this must be taken into con- 

 sideration as well as the velocity due to gravity. This 

 will give a direction approximating locally to the vertical 

 in very sluggish streams, and this vertical motion is less 

 marked in proportion to the increase of stream velocity. 

 Hence, all other things being equal, on the downstream 

 slope of small obstacles — slopes of which the stream motion 

 is independent — we would expect to find abrasion less 

 marked in proportion to the verticality of such slope. On 

 the other hand we would expect heavy abrasion on the 

 upstream slope. 



