no WORK OF RUNNING WATER 



has a lower angle of slope. The term fan is 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 con- 

 cerned 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 

 range. Many rivers descending from the Rockies to the Great 

 Plains have done the same thing. The fans of some streams 

 descending from the mountains are many miles across. That 

 of the Merced River in California, for example, has a radius of about 

 40 miles. 



The fans made by neighboring streams may spread laterally 

 until they merge. The union of such fans makes a compound 

 alluvial fan, or a piedmont alluvial plain (PI. X). Such plains exist 

 at the bases of most considerable mountain ranges. Sheet wash, 

 as well as streams, contributes to them. The depth of alluvial 

 material in such plains is, in some cases, hundreds of feet. The 

 great spread of these land deposits is remarkable. East of the 

 Rocky Mountains they extend out more than a hundred miles in 

 some places. This wide spread appears to be the result of the long- 

 continued action of running water. The cone or fan, as first built, 



Fig. 1 10. Diagram to illustrate the spreading of alluvial deposits in a piedmont 

 position. The deposits may first take the position represented by the line i-i'. 

 At a later stage, as a result of erosion and redeposition, they take the position repre- 

 sented by the line 2-2', being spread farther from the mountain and having a lower 

 surface slope. At a still later time, they take the position 3-3', with a still lower 

 slope and a still wider spread. 



