RIVER TERRACES AND GLACIAL SERIES 455 



half feet of snow at an altitude of 4,300 feet. A yet greater 

 difference was observed in the melting of the snow. Near the 

 coast the atmosphere was uniformly cold and dense, the sun's 

 direct rays had comparatively little power, and there was no very 

 great difference in the rate of melting of the snow on opposite 

 sides of the valleys. Farther inland and at a higher altitude, the 

 atmosphere was clearer and lighter,- weakly absorbent of the 

 sun's rays and radiation rapid. The result was a great contrast 

 in the rate of melting of the snow on opposite sides of east-west 

 valleys. It is well known that it is the great characteristic of an 

 alpine climate that the general temperature of the air is low, but 

 that the sun's rays by direct impact on the earth are very power- 

 ful. There is, therefore, a great contrast between the heat in 

 light and shadow, a contrast which is much less at lower alti- 

 tudes. At this camp, the southern side of the valley will be 

 covered with snow down to the river level, while the sunny slope 

 will be completely bare to an altitude 2,500 feet higher. A 

 northerly facing gulch will be impassable from snow a month 

 after flowers have been in full bloom in a southerly facing gulch 

 at the same altitude. 



In my first paper on the ancient glaciers of this region I 

 called attention to the fact that in the last stage (the only one 

 there discussed) they were apparently very sensitive to light 

 and shadow, in their retreat creeping up close to precipitous 

 peaks. It has long been evident to me that the Salmon River 

 glacier in its last great stage was much higher on the west than 

 on the east side, because its chief gathering ground was in the 

 shadow of a high spur of Mt. Courtney, where there is even now 

 a small glacier. Blocks of granite fell from the saw-like crest 

 of the mountain and traveled across the surface of the glacier 

 so as to be distributed along the east slope of the valley. Under 

 these conditions serpentine from the east side of the valley 

 could not very well superglacially reach the gorge on the west 

 side and be carried down the old Salmon River valley. 



Under low altitude conditions, this glacier would not have 

 been so sensitive to sunlight and one side would not have greatly 

 predominated over the other. Indeed, it is probable that in 



