38 



E. C. ANDREWS 



they rise above the canyon rims, the stronger the resultant thrusts 

 along the central channel, as in the case of ordinary stream floods. 

 Along the lower spur ends the maximum force will thus be early 

 expended. The upper portions of the spurs will experience great 

 wear and tear, but the amount will be trifling as compared with that 

 felt by the spur portions near the base of the central channel. The 

 rock load, as also bottom friction, will cause reduction of speed 

 along the lowest portions of the glaciers, and, as with streams, we 



Fig. 13. — Sketch of Moonan Brook (New England, New South Wales). A typi 

 cal stream-developed valley. Note the overlapping spurs, the spur-cutting and aggra 

 dation by floods. Canyon about 3,000 feet deep. 



would expect the maximum velocity to occur along the central channel 

 at some point intermediate between the surface and base of the 

 moving ice-mass. Under the tremendous scouring action of the 

 armed glacier, we would then expect the lower spur ends to suffer 

 first. Fig. 7 appears to illustrate this stage of planation. Here the 

 floating spur ends have vanished, and precipitous facets to the sound 

 are presented by the truncated masses. 



All this time the glacier is increasing its load, and, as with streams, 

 the strong cutting action may have to be suspended (during local 

 slackness in ice-supply) until the excess of load has been removed. 

 Also as with streams, the degree of spur planation will depend upon 

 their arrangement and development. Small spurs in the direct line 

 of flow will early suffer complete removal, while large ones, favor- 

 ably situated as regards their preservation (points opposite the cut- 



