THE FEATURES IN DESERT LANDSCAPES 



213 



quake faults to indicate that the uplift is more accelerated than 

 the deposition along the mountain front. 



The zone of the dwindling river. The rapid uplift so generally 

 characteristic of desert margins gives to the torrential streams 

 which develop after each cloud- 

 burst such an unusual velocity 

 that when they emerge from the 

 mountain valleys on to the desert 

 floor, the current is suddenly 

 checked and the burden of sedi- 

 ment in large part deposited at 

 the mouth of the valley so as to 

 form a coarse delta deposit which 

 is described as a dry delta (Fig. 224). Dependent upon its steep- 

 ness of slope, this delta is variously referred to as an alluvial fan 

 or apron, or as an alluvial cone. Over the conical slopes of the 

 delta surface the stream is broken up into numerous distributaries 

 which divide and subdivide as do the roots of a tree. In the 

 Mohammedan countries described as wadi, these distributaries 



FIG. 224. Dry delta or alluvial fan at 

 the foot of a mountain range upon 

 the borders of a desert. 



FIG. 225. Map of the distributaries of neighboring streams which emerge at the 

 western base of the Sierra Nevadas in California (after W. D. Johnson). 



upon dry deltas are on the Pacific coast of the United States 

 referred to as " washes " (Fig. 225). 



Fast losing their velocity after emerging from the mountains, 

 the various distributaries drop first of all the heavy bowlders, 



