6i6 JOHN LYON RICH 



gravels to an arid climate is this — under such conditions whatever 

 rainfall there is usually comes in the form of torrents or cloud- 

 bursts. These give a very heavy precipitation over local areas for 

 a short time, with resulting temporary streams of great transporting 

 .power. These occasional torrential rains act on slopes unprotected 

 by vegetation and consequently pick up and carry forward a heavy 

 load of rock- waste. Just as soon as the streams leave the steeper 

 slopes much of this debris must be dropped, and all must come 

 to rest within a comparatively short distance, for the water either 

 sinks into the ground or rapidly evaporates. The consequence of 

 this process is the accumulation of piedmont gravel slopes round 

 the higher lands. Under a moist climate the streams are perma- 



Fig. 7. — Sketch showing conditions round the base of Aspen Mountain. A 

 lens-shaped debris fan mantles the lower slopes. 



nent, they work continuously; the slopes are protected by vegeta- 

 tion, hence furnish less debris to the waters of a passing storm. 

 Weathering reduces the rocks to finer particles, which are exported 

 by the ever-flowing streams, consequently degradation rather than 

 aggradation is the rule. It is believed that under these conditions 

 planation, with the removal of the resulting waste, would be 

 pushed close up to the base of the harder rocks. 



This, as an explanation of the conditions round Aspen Moun- 

 tain, is not without its uncertainties, for it might be argued that 

 conditions of extreme aridity might allow the development of a 

 flat or gently sloping plain close up to the base of the mountain 

 by wind erosion. In that case the efficiency of the wind in removing 

 loose debris from round the base of the mountain must have been 



