38 



BULLETIN 61, U. S. DEPAKTMENT OF AGEICULTUEE. 



Some basins are characterized by having the silt area on one side of the intermonn- 

 tain space. This is caused by a difference in elevation of the mountain ranges and a 

 difference in climatic conditions. The higher mountain ranges command a greater 

 rainfall. We would therefore expect that greater erosion would result from the higher 

 mountainb than from the lower. This would result in apron slopes of gi-eater extent 

 from the sides of the higher mountains than from the lower. This would place the 

 6ilt area close to the lower mountain range. Figure 5 illustrates the structure. 



The ground- water conditions in basins of the types illustrated are worthy of further 

 comment. In the case of figure 3, and assuming a region of inconsiderable rainfall, we 

 should expect that the ground-water accumulations in B portions would be smaller 

 and found at relatively great depths. The silt mass would retain its moisture close to 

 the surface and lose it by evaporation. Capillarity would draw up the moisture to- 

 gether with any soluble salts for a depth of 10 or possibly more feet from the surface. 

 At the central mass, built up by silt, or wind deposition, we would expect the soluble 



Fig. 4.— Cross section of a basin occupying a narrow valley between high mountains. 



salts to be drawn upward and to remain as a conspicuous surface feature. Only on the 

 edges of the silt portions would we expect to find any buried salt crusts and these would 

 be distributed irregularly and be com-paratively thin. In the case of a moderate rain- 

 fall we should expect an accumulation of ground water in B portions and the level of 

 this ground water would reach the surface on the periphery of the silt mass. It would 

 appear as spring water in the coarser material and as seepage water in the finer and more 

 compact marginal portions. Under conditions of this kind we should expect the 

 marginal portions of the silt mass, if not the entire silt mass, to be saturated with mois- 

 ture. If the amount of this water, plus the rainfall on the playa area, would be suffi- 

 cient to replace the evaporation losses, a lake mi»ht be expected to form and to remain 

 as a more or less permanent feature. If insufficient, we should expect a lake to form 

 during part of the year, and in the summer months the standing water would evaporate, 

 leaving a mud flat. Another case might be mentioned, and that is where the ground 

 water plus the rainfall would be insufficient to form standing water and only just suffi- 

 cient to maintain a mud flat during a part of the year. 



Fig. 5. — Cross section of a basin having higher mountains on one side than on the side opposite. 



In the case of figure 4 ground water would be present beneath the entire basin and 

 this would be in some cases artesian water. If the silt portion A maintained its homo- 

 geneity ground-water accessions could come only from the marginal portions of the 

 playa area. That this is not an ideal condition may be concluded from the fact that a 

 spring, or springs, is not infrequently found in the central portion of the playa. In 

 some cases this spring is sufficient to supply a small pond and in others the water flows 

 out and evaporates. An excellent example of this kind is found in Rhodes Marsh. In 

 almost the center a spring makes its appearance and a flow of some magnitude runs out 

 and meanders on the flat to be lost by seepage and evaporation. During some years 

 the rainfall plus this flow is sufficient to form a shallow lake. 



A conspicuous feature of many of the playas is the marginal rim of mud about the 

 relatively dry central portion. Silver Peak, Rhodes, Death Valley, and Teels Marsh 

 are examples. 



Another condition merits mention here. Often a playa will be in the focus of some 

 dominant surface drainage or ground-water drainage. These combined are sufficient 



