POTASH SALTS AND OTHER SALINES IN THE GREAT BASIN REGION. 37 
is applied. The sink serves as a reservoir for the basin which it occupies. Naturally, 
it also serves to receive whatever soluble material is brought to it by surface or under- 
ground waters. As a consequence, playas may be looked upon as favorable places 
for the accumulation of salines. In order to present more clearly the nature of playa 
deposits, I have sketched the structural development of a desert basin. 
STRUCTURAL DEVELOPMENT OF A DESERT BASIN. 
The adjustment of the fault blocks of the Great Basin initiated a period of erosion, 
with its consequent deposition of detrital material in the intermountam areas. Flank- 
ing the mountains first appeared steep sloping aprons, or talus slopes. The finer 
sands and gravels were deposited in a wider zone at the foot of the apron area, and 
in the central portions the finest silts and clays were deposited. During this period 
shallow lakes no doubt occupied many of the basins. As erosion proceeded finer 
material was brought down and the mountain aprons assumed a less taluslike appear- 
ance. The sand and gravel aprons extended out and encroached upon the silt area 
and sufficient silt was brought down to fill the central basin and in some cases to 
obliterate the shallow lakes occupying them. Were lateral streams the only agents of 
transport, we should have a very definite structure revealed by the basin. A cross 
section of such an ideal basin would show a wide, level, and relatively thin body of 
fine silts and clays, flanked on either side with masses of detrital material rising in 
long, sweeping grades to the steeper slopes of the mountains. A gradation from fine 
sands to coarse angular debris mixed with material of all sizes would be noticed in 
passing from the central portions of the playa to the steeper mountain slopes. Figure 
3 illustrates the section described. Under the conditions indicated above we should 
expect the central mass A to be characterized by little movement of ground water, 
while the flanking masses B and B would be zones in which ground waters could 
Fig. 3. — Ideal cross section of a desert basin. 
actively circulate. With the progress of time, we should expect the apron slopes tx> 
flatten and the silt and clay area to extend laterally until a shallow, panlike basin 
would result. Surface waters would accumulate and shallow lakes would form in 
the rainy season. The impervious bottom of this lake would protect the waters from 
loss by seepage. The evaporation of these lakes would leave surface accumulations 
of salts which would receive fresh accessions each year until deposits of appreciable 
thickness would make their appearance. 
Water is not the only agent which acts. Wind erosion and deposition also plays an 
important part. Under the influence of this agent the central portion A of the basin 
would not be composed of a homogeneous mass of clay and silts, but we should 
expect to find its homogeneity disturbed by layers of sand, volcanic ash, or other 
wind-blown material. Such beds would be continuous in a large measure over basin 
areas occupied by shallow lakes, but in the case of the ordinary^ playas conditions would 
not be favorable for the deposition of wind-blown material in thin beds. Upon the 
dry playas we should expect to find the fine silts of the central portion more or less 
eroded by the wind and deposited over the outwash slopes, or in certain portions of 
the basin as dunes. It is not an uncommon thing to find in a playa area one or more 
portions occupied by sand dunes of considerable extent. Another point should be 
mentioned, and that is that in periods of excessive rainfall or during cloud-bursts the 
streams would be so increased in volume as to carry out upon the central silt area 
sand and gravel. In this way tongues of coarse material would be formed in the silt 
portions and would form beds in which ground water could circulate. A perennial 
stream entering one end of a basin would effect a somewhat similar structure, but 
on a much greater scale. 
The width of the intermountain spaces and the height of the inclosing mountain 
ranges would determine the proportion between the silt and detrital masses. With 
narrow valleys and high mountain ranges we should expect the outwash slopes to meet 
or even overlap at the center. As the valley filled a mass of silt of triangular section 
would be formed in the central part. Figure 4 illustrates this structure. 
