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University of California Publications in Geology [Vol. 9 



region. The rain wash co-operates with gravity and transportation is 

 far more efficient. The rock slopes are generally much less than the 

 angle of repose of loose material, the talus is replaced by broad alluvial 

 fans, and for every enclosed basin there is the aggradational flat of 

 the playa. For mountains of approximately homogeneous rock the 

 epigene profile thus comprises three elements: (1) The rock slopes, 

 having an angle of less than 35°; (2) the alluvial fan slope rarely 

 exceeding 5°; and, (3) the flat of the playa. 



In general these three elements of the profile are easily discrimin- 

 ated one from the other ; but in certain cases the angle of the rock slope 

 approaches that of the alluvial fan and the two may be confused. 

 Similarly where the alluvial fan is very flat it may not easily be dis- 

 tinguished in its lower part from the playa. In many desert valleys 

 the playa is located at one end, toward which the drainage flows, so 

 that the fans from the two sides meet in the central part and there is 

 no playa element in the profile for long stretches. 



The proportion between the length of the rock slope and that of 

 the fan slope varies greatly. The fans are composed of the detritus 

 washed down from the rock and, as they grow, they rise about the 

 mountain flanks and steadily diminish the extent of the rock slopes. 

 Ultimately the upper edge of the fan reaches the crest of the mountain 

 and there is no farther addition to its surface. A small alluvial 

 embankment on the flank of a mountain ridge is, therefore, significant 

 of an early stage of the degradational process; and a large embank- 

 ment extending nearly or quite to the crest indicates a late stage. 

 In the deserts of the Great Basin both extremes may be observed. 

 At Genoa, for example, there is a bold, high mountain front of bare 

 rock with a small embankment at its base, and in places none at all. 3 

 At Cima in Southern California the mountain ridge is almost com- 

 pletely buried in its own alluvial waste. From this variable quanti- 

 tative relation of rock slope and alluvial slope it may be safely 

 inferred that the mountains of the region, considered as features of 

 the relief, are of diverse age. On the basis of this relation it is 

 possible to classify the desert mountains chronologically. Some came 

 into existence so long ago that their rock slopes have been almost 

 entirely buried in their own waste ; while others have had their origin 

 so recently that the effect of aggradation about their flanks is relatively 

 slight. Between these extremes a graded series may be recognized 

 significant of a time-sequence. This criterion for the chronological 



s Bull. Seism. Soc. Am., Vol. II. No. 3, Sept., 1012; II. S. G. S. Prof. Paper 73, 

 p. 18f>, 1911. 



