180 GEOLOGY 



tween talus cones and alluvial cones there are, however, all gra- 

 dations. 



Conspicuous alluvial cones are common at the bases of steep 

 slopes in semi-arid regions; for in such regions the rainfall is fitful, 

 and the occasional heavy showers, which give rise to temporary 

 and powerful torrents, favor the development of cones of great 

 size. At the bases of the mountain ranges in the Great Basin, 



Fig. 148. Valley showing deposition at the bases of valley slopes, tending 

 to give the valley a U-shaped base. Unaweep Canyon, Colorado. 

 (Cross, U. S. Geol. Surv.) 



the talus and alluvial cones from the mountains are sometimes 

 2,000 or 3,000 feet high. 



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

 has a lower angle of slope. The term fan is indeed more appropriate 

 than cone for most alluvial accumulations at the bases of slopes. 

 The lower angle of the fan may be due to the less abrupt change 

 of slope where it is developed, to the larger quantity of water 

 concerned in its deposition, to the smaller amount of detritus, or 

 to its greater fineness. Less change of slope, more water, and less 

 and finer material, all favor the wider distribution of the sediment . 

 and so the development of fans rather than cones. Nearly all 

 young rivers descending from mountains build fans where they 

 leave the mountains. Thus, the rivers descending from the Sierras 

 to the great valley of California build great fans at the base of the 



