SKETCH OF THE GEOLOGY AND SOILS OP THE CAHUILLA BASIN. 23 



character, indicates that the Mud Hill Series is essentially late Tertiary. Very probably 

 it includes beds of all ages from the Carrizo Creek Miocene into the early Pleistocene. 



The origin of the series as a whole seems unmistakable. Its strata have every char- 

 acteristic of continental beds laid down on the bordering ddbris slopes of desert mountains 

 and composed of materials carried down and assorted by rain wash. Precisely similar beds 

 are now being deposited on the flanks of hundreds of the desert mountains of North 

 America. The prevalence of clay beds in the basin-ward part of the series suggests that 

 these portions were deposited in a marine estuary or in the central lake of an inclosed 

 basin which the mountains bordered, but no conclusive evidence of marine or lacustrine 

 deposition has been discovered in this part of the series. The presence of gypsum and 

 mirabilite, as above noted, indicates that there were at least local areas of inclosed drainage 

 and salt accumulation. There is also the implication, strongly confirmed by the general 

 nature of the series, that the climate under which it was deposited was arid and not essen- 

 tially dissimilar from that of the present. As a whole, the series must be regarded as 

 continental and alluvial. The marine strata of Carrizo Creek probably represent an 

 incursion of the sea into the southern portion of the trough only. 



Below the hills of the Mud Hills Series the floor of the valley is of typical alluvial 

 outwash, similar in every respect to that of other desert basins. Nearer the mountains 

 this is gravelly and sandy; toward the center clays and silts predominate. Always the 

 slopes are natural and gentle, the only cuttings being the shallow channels of the "washes," 

 always dry except for the sudden floods which follow occasional storms. This alluvial 

 outwash covers the whole central portion of the valley and has filled it to an unknown 

 depth. As usual, it is of interbedded sands and clays, the clay strata forming a seal for 

 the waters of the sandy strata and producing artesian potentialities recently much devel- 

 oped in what is known as the Coachella Valley between Indio and Mecca. (See Plate 15.) 

 Numerous shallow wells have been bored in this region, and several have been sunk to 1,000 

 feet or more. None of the wells penetrated anything but the usual alluvial succession of 

 clays, sands, and gravels. 



At present the deepest depression of the basin is occupied by the Salton Sea, the 

 origin and character of which are fully discussed elsewhere in this volume. Before the 

 incursion of the waters the deepest depression contained a body of crystalline salt, 

 located south of the station of Salton, and which was utilized commercially (See Plate 

 Ic.) It consisted of a crust of fairly pure sodium chloride under which was black mud 

 with disseminated crystals and crystalline masses of sodium chloride and other salts, 

 and known to extend to a depth of 25 feet below the surface. According to Bailey this 

 was underlaid by hard clay to 300 feet. Both mud and salt were saturated with a brine, 

 the main constituent of which was sodium chloride, but the exact composition of which 

 is unknown. 



In summary, the basin before the formation of the Salton Sea consisted of a pre- 

 sumably structural trough lying between mountain ranges essentially of Mesozoic or 

 earlier granites, gneisses, and schists, and deeply filled with the erosional debris of these 

 ranges. Bordering the ranges proper were foot-hills of Tertiary strata, apparently the 

 broken and tilted remnants of a former alluvial apron. Below these hills the surface was 

 entirely of recent and present alluvial fill, a typical bajada merging into a central sink 

 of comparatively small extent and which carried a small salt body of usual type. 



The extension of the Colorado Delta into tliis trough has already been described by 

 Mr. Sykes, and very little need be added from the geologic point of view. The superficial 

 layers of the Delta material, where exposed in the cuttings of the New and Alamo Rivers, 

 consist of the usual alternating beds of fine sand and silt. No deeper sections are available 

 and the depth of the Delta material, its relations to underlying rock or alluvium, etc., 

 remain quite unknown. At its edges the cone of Delta material merges so imperceptibly 



