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 



actively circulate. With the progress of time, we should expect the apron slopes to 

 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. 



