ALLUVIAL DEPOSITS 



109 



Fig. 108. An alluvial cone. (U. S. Geol. Surv.) 



of talus. In the latter, gravity brings the material down with little 

 aid from water, but between the two types of cones there are all 

 gradations. 



Conspicuous alluvial cones are common at the bases of steep 

 slopes in semi-arid regions. The rainfall there is fitful, and the 



I ML,'. IOQ. Deposition at the bases of valley slopes, tending to give the valley a 

 U-shaped base. Unaweep Canyon, Colorado. (Cross, U. S. Geol. Surv.) 



occasional heavy showers, which give rise to temporary and power- 

 ful torrents, favor the development of great cones. At the bases 

 of the mountain ranges in the Great Basin, some of the talus and 

 alluvial cones are 2,000 or 3,000 feet high. 



An alluvial Jan is the same as an alluvial cone, except that it 



