84 C. K. KEYES INTERMONT PLAINS OF THE ARID REGION 



Playas and similar mud-flats of the arid plains must be regarded as 

 areas of great degradation as well as areas of aggradation. It is doubtful 

 whether in the great majority of cases the former lags little or any behind 

 the latter. When the waters are finally all evaporated the playas are 

 veritable "mud-flats." This bottom mud as it dries curls up into leaves 

 a millimeter or two in thickness and several centimeters across. The 

 first strong wind that comes along blows these leaves away like the fallen 

 foliage of the trees after the first blast of winter. Much of this material 

 is carried bodily away, miles out of the playa area, or gathers in huge 

 windrows about its margins. A large part is ground to dust in the mov- 

 ing about and is then carried off like other dusts of the plains. With 

 every shower that falls on the playa, there is the same mud layer again 

 'formed and further exportation of the material. 



When old playas have been cut through by recent torrential or stream 

 corrasion the soft deposits appear, in a number of observed instances at 

 least, to have no very great thickness. In the Meadow valley, in south- 

 eastern Nevada, for example, 100 feet beneath the surface of the old 

 playa, the hard rock-floor of the plains is in evidence — ancient lime- 

 stones, sandstones, and shales highly inclined and horizontally and evenly 

 beveled. In the Amargosa valley of southeastern California similar 

 phenomena are presented. In the remnants of the old bolson surfaces 

 along the Eio Grande there is often displayed the ancient rock-floor sur- 

 face high above the present level of the existing channel. 



Many salinas exhibit like conditions. In the great Hueco bolson, in 

 central New Mexico, are shallow alkaline lakes which are dry for the 

 greater part of the year. As the waters dry up, a thin sheet of various 

 salts, chiefly gypsum, is left on the bottom. This soon curls up into thin 

 leaves, which when blown together are quickly broken up into gypsum 

 sands and salt sands. The winds carry the white sand out on the plain, 

 where it gathers into immense dunes, 60 to 100 feet in height, miles wide 

 and a score of miles in length ( see plate 5 ) . 



The central flats of the intermont plains of the arid districts are, then, 

 not alwa3''s areas of constant aggradation, as has been commonly inferred, 

 but are areas of most rapid degradation as well. In some of the central 

 flats a rock-floor is known to exist at shallow depths. In the large major- 

 ity of them nothing is as yet deflnitely known regarding the character or 

 the depth of the substructural surface. Whenever any of the last men- 

 tioned class have been examined carefully as to the depth of their rock- 

 floors the mantle of detritus has been found to be of no great thickness. 

 In a few cases great thicknesses of surface materials no doubt exist, but 

 most of those which have been so reported do not appear to be thus deeply 



