THE WORK OF STREAMS 175 



The structure of alluvial fans is characteristic, and results 

 from the method of their growth. The coarsest material is 

 dropped at the apex of the fan, where the current is first 

 checked, and the deposit made at any given time becomes 

 progressively finer toward the margin. This does not mean 

 that the material in a vertical section through an alluvial 

 fan, all parts of which are at the same distance from the 



FIG. 181. Section of an alluvial fan, Owens Valley, Cal. (Trowbridge.) 



apex, is all of the same degree of coarseness. On the con- 

 trary, the material would probably change frequently, 

 both horizontally and vertically, for the volume (and so the 

 carrying power) of different distributaries would vary at 

 the same time, and that of any given distributary at differ- 

 ent times. Such variations in the tops of fans may often 

 be seen in the sides of the channels which trench them (Fig. 

 181). 



The angle of slope of a fan depends upon how suddenly 

 and how much the velocity of the depositing waters was 

 diminished, and upon the kind and amount of material 

 they carried. A sudden and great reduction in the velocity 

 of a stream heavily loaded with coarse material, gives a 

 relatively steep slope ; the opposite combination a gentle 

 one. The profile of a fan along any radius, like the profiles 

 of other depositional slopes, is a curve concave upwards 



