LAND WATERS STREAMS 181 



range. Most of the rivers descending from the Rockier to the 

 plains to the east have done the same thing. The fans of streams 

 descending 'from the mountains are often many miles across. The 

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

 of about 40 miles. 



The fans made by neighboring streams may grow laterally until 

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

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

 exist at the bases of most considerable mountain ranges, and sheet 

 wash, as well as streams, contribute to them. The depth of alluvial 



Fig. 149. Diagram to illustrate the spreading of alluvial deposits in a pied- 

 mont position. The deposits may first take the position represented by 

 the line 1-1'. At a later stage, they take the position represented 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. 



material in such plains is often scores and sometimes hundreds 

 of feet. One of the most remarkable features of these land deposits 

 is their great spread. Thus 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 run- 

 ning water. The alluvial cone or steep fan, as first built, is de- 

 graded later, and its materials spread more widely. As the gradient 

 of the running water which makes the fan becomes lower, the 

 higher parts of the fan are spread out farther and farther, with 

 continually lower gradient, as suggested by Fig. 149. 



Deposits of this sort have probably been far more important in 

 the past than has been generally recognized. Much of the material 

 of the Coastal Plain of the Atlantic and Gulf slopes of the United 



