92 CARNEGIE INSTITUTION OF WASHINGTON. 



comparatively narrow valleys. These trough-like valleys have been 

 filled deeply with alluvial debris from the erosion of the bordering 

 mountains, and it is from these alluvial deposits that nearly all of the 

 underground waters of the region have been obtained. Thiere being 

 no outward drainage and very few through-flowing streams, the filling 

 of the valleys has been done entirely by local stonn-waters and by the 

 small streams of the mountain slopes. By erosion at higher levels and 

 deposition at lower, this process is gradually lowering the mountains 

 and filling the valleys. 



The occurrence of artesian and other underground waters in the 

 accumulations of alluvimn which fill the valley troughs appears to 

 depend entirely on the existence of buried stream-channels. The 

 streams which descend the mountain slopes and which have brought 

 about the alluvial filling are extremely variable both in volume and in 

 position. As they vary from flood to dryness, and as they migrate 

 back and forth over the slope which they are building, they leave 

 alternate and variable deposits of gravel, sand, and clay. Where a 

 stream is stationary long enough to form a channel this channel will be 

 marked by a hne of sand and gravel, and when the stream moves on, 

 floods or other streams may cover the sand-streak of this abandoned 

 channel with finer material, even with clay. In this way there are 

 built into the mass of alluvium many such abandoned stream-channels, 

 all leading from the mountain-slope toward the center of the basin. 

 Since these buried channels are of sandy and gravelly material, they 

 are easily pervdous to water. Many come to be sealed above and 

 below by clay or by the desert hardpan called caliche. The dish- 

 shaped or bowl-shaped contour of the valleys gives such buried channels 

 a considerable difference of elevation between the lower end at the 

 basin center and the upper end toward the rock wall. Usually this 

 upper end merges into the talus of gravel and boulders which borders 

 the mountain. It is obvious that this structure may result in artesian 

 conditions. Water which falls as rain on the mountains flows freely 

 through the bordering talus of coarse material and enters the upper 

 ends of the buried channels, which offer it easy passage down the 

 slope. Further down this slope the intercalated clay or caliche beds 

 confine the water of the channels and an artesian pressure is developed. 



It is possible to make three deductions of i)ractical value: First, 

 artesian water is to be expected only where this bowl-shaped structure 

 of the alluvial fill occurs. Second, artesian water will not occur 

 necessarily in well-marked strata or at definite depths as it does in rock 

 artesian areas, but may occur at different depths and pressures in 

 adjoining wells, depending upon which of the buried channels may have 

 been penetrated; it is possible even to have entirely drj^ wells and pro- 

 ductive wells side by side. Third, if a valley be divided into concentric 

 zones outward from the center, artesian conditions will be most likely 



