36 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
The desert gravels on the surface are without question still accu- 
mulating and are in part of Recent age. Their deposition, however, 
is slow, and from their thickness it is probable that the lower portions 
of the formation are of Pleistocene age. The length of time re- 
quired for their accumulation is indicated not only by the present in- 
frequency of cloudbursts. Embedded in the gravels are pebbles 
deeply scored by wind erosion on the exposed side, while the under 
surface is untouched. Such pebbles must have been stationary for 
long periods. The alluvial fa us, with the exception of a single chan- 
nel, are superficially stained by the black " desert varnish," a process 
also requiring a considerable period. 
PLAYA DEPOSITS. 
Playa deposits composed of light-gray or white fine clay, in many 
places arenaceous, occupy the bottoms of nearly all the inclosed val- 
leys. Grotesque concretions of calcium carbonate from 1 inch to 8 
inches in length are locally embedded in the clay, while some strata 
are lime cemented. Salt Flat and other areas mapped as playa de- 
posits in Death Valley differ from those just described in the fact 
that sonic water continually stands in depressions and that consider- 
able amounts of saline material are being deposited. For convenience 
several areas in ()a>is Valley have been mapped as playa deposits. 
These areas are occupied by clays, loams, and sands of complex origin, 
partly playa deposits, partly stream alluvium, and partly wind- 
deposited sand. The thickness of the playa deposits is unknown, 
although from their mode of origin it is believed that they are thinner 
than the desert gravels in the same basin. 
During periods of unusual precipitation the playas are wholly or 
partially covered by water, and at such periods some fine detrital 
material is washed into them. More or less wind-bloWn sand becomes 
mixed with the clay, and on desiccation of the temporary lakes chem- 
ical deposits are formed. The playa deposits are of late Pleistocene 
and Recent age. 
SAND DINES. 
Large sand dunes are confined to the southern portion of the area 
mapped, although smaller dunes border all the playas. Big Dune, in 
the Amargosa Desert, i- 800 feet high, and lower ones, similarly per- 
fect in contour, occur in Death Valley. These are said to retain their 
general position from year to year, their situation apparently beinj 
determined by wind eddies. Some of the smaller dunes are directlj 
dependent on the desert shrubbery for their existence (PI. II. B% 
while the position of those on higher ground is evidently determined 
by the protecting influence of surrounding hills. 
