PLAYA DEPOSITS ■ 83 



Much of the sujDposed general leveling effects ascribed to sheetflood ero- 

 sion should be probably attributed to eolian activity. 



PLAYA DEPOSITS 



In the Basin Kange region it has been customary to regard the inter- 

 ment plains on the one side as deeply covered by vs^aste which gradually 

 becomes thinner until finally the bare surface of the tilted block emerges 

 and passes into the mountain elevation (figure 6). Most of those per- 

 sons who have had to deal with the basin ranges of western America have 

 thus interpreted the underground structure of this region. It now seems 

 that this idea is due to a purely deductive conclusion based on wholly 

 "erroneous premises. 



Later observations recognize three distinct belts of relief: (1) The 

 dissected highlands, composed usually of bare rock-masses; (2) the 

 graded piedmont slopes, made up of more or less angular and coarse 

 rock-waste, and (3) the aggraded central plain, composed of fine clays 



Figure 6. — General Conception of Basin Range Structure 



and often occupied by lakes, salinas, or broad playas. The necessary 

 deduction is that eventually the central area of the plains is the final 

 resting place of all or nearly all of the materials brought down from the 

 peripheral highlands, and that the deposits thus formed are very deep. 

 This is McGee's view.^^ It is implied by Davis^^ when he classes them 

 as aggraded areas, at least during the earlier stages of the arid cycle. It 

 seems to be also the conclusion of every casual observer. It suggests 

 itself as the only possible alternative, especially when the valley plains 

 have no outlets, but are truly inclosed basins. 



There are many transitions, among these interment basin plains, from 

 those holding extensive and permanent lakes to those which are perfectly 

 drained. The stage known as the playa is one in which there is a broad 

 expanse of barren silt ; "dry lakes" they are usually aptly called. Waters 

 are only brought down once or twice a year from the surrounding moun- 

 tains, and cover the mud-flat to depths of a few inches or a foot or two. 

 Such lakes are very short-lived, lasting only a few weeks at most. Nine 

 or ten months out of the year the areas occupied by them are dry flats. 



=* Bulletin of the Geological Society of America, vol. 8, 1897, pp. 87-112. 

 " Journal of Geology, vol. xili, 1905, p. 387. 



