THE WORK OF RUNNING WATER, 



183 



tality. If many alluvial fans develop in promixity to one another, 

 as at the base of a mountain range, they may expand laterally until 

 they merge. A long succession of them may thus give rise to an ex- 

 tensive alluvial piedmont plain, or a compound alluvial fan. The lower 

 edge of such a fan is often somewhat lobate. Such plains exist along the 

 bases of many mountain ranges (PL VI), and may be seen in miniature 

 even along low ridges. 



A permanent stream, as well as a temporary one, may develop an 



Fig. 171.- 



-Miniature levees on an alluvial cone. Slope of Gray Peak, Colo. 

 Chamberlin.) 



(R. T. 



alluvial fan at the base of a mountain slope; but since the moimtain 

 course of the former is Hkely to be less steep than that of the latter, 

 its waters suffer a correspondingly less reduction of velocity at any 

 one point. The fan of the permanent stream is therefore hkely to be 

 relatively flat, and to stretch far down the valley. Such fans grade 

 into valley plains. From the general principles already discussed, it 

 is clear that well-developed fans go with relatively youthful stages of 

 erosion, and belong normally to the upper parts of drainage hnes. 



Ill-defined alluvium. — There is a Tvidespread mantle of alluvial 

 material deposited by running water which was not organized into 

 distinct streams. The water which runs down smooth slopes in sheets 

 during showers carries fine earthy matter, as well as some that is 



