SKETCH OF THE GEOLOGY AND SOILS OF THE CAHUILLA BASIN. 



27 



and it is certain that the Colorado flowed into Blake Sea and overflowed out of it, whether 

 or not the river stood to the Sea in a genetic relationship. If Blake Sea was originally a 

 marine water-body, it was soon and effectually freshened by the inflow of the Colorado. 



As stated, decision is now impossible between the two variant hypotheses noted. 

 The topography of the Delta and the Delta dam strongly suggests that it was built in 

 a water-body and at water-level, hence, that the current hypothesis is the correct one. 

 However, an opposite implication of even greater strength is furnished lay the high-level 

 beach of Blake Sea. If Blake Sea were marine and were cut off from the ocean by the build- 

 ing of the Delta, this process must have required a considerable time. But the high beach- 

 line does not indicate long-continued wave-erosion. Where the beach crosses gravels or 

 unconsolidated strata it is deeply cut, but where it crosses rock walls there is almost no 

 cutting, even on exposed points which would be attacked strongly by the waves. On such 

 rock faces the ancient water-line is marked only by coatings of travertine or accumula- 

 tions of loose stones. Persistent shore-lines are usually cut markedly even into the hardest 

 rocks, and even the comparatively transient shore-lines of the Quaternary lakes of Nevada 

 and Utah are frequently deeply graved. Absolutely nothing of the sort occurs on the high 

 strand of Blake Sea. Any cutting which the writer has observed along it could have been 

 done in a few score years. Indeed, some of the lower strands of the Blake Sea series, or 

 the high-level strand of the present Salton, are sometimes cut almost as deeply as the high 

 beach of the ancient Sea. It is difficult to reconcile this shallow cutting of its ancient- 

 beach with the long duration of Blake Sea required on the hypothesis that it was originally 

 marine. Collateral suggestion is carried by the absence of known marine fossils from the 

 deposits of Blake Sea, but this is far from conclusive, since such beds would belong to the 

 earlier portion of the Sea's history and would be buried by later deposits. (See p. 46.) 



In the writer's opinion, the most probable harmonization of these variant hypotheses 

 concerning the origin of the present depression lies in a slight modification of the second 

 hypothesis above suggested. That is, that the rise of sea-level relative to the basin has 

 been slow and gradual and that the growth of the Delta has proceeded and kept pace 

 with it. On this assumption the post-Tertiary Colorado is supposed flowing into the side 

 of, and through the lower part of, a trough freely open to the sea. The sea is supposed 

 to have advanced very slowly up this trough, the river meanwhile accommodating itself 

 to the changing conditions by a change of grade and the resultant deposition of alluvium. 

 It is conceivable that before the advancing sea-line reached the point where the river 

 entered the side of the trough, the river would have built across the trough a dam sufficient 

 to prevent further advance of the sea, especially if the displacement of sea-level were 

 becoming less rapid— as was no doubt the truth. Further vertical displacement might 

 produce no horizontal displacement, the deposition of alluvium by the river being rapid 

 enough to keep the Delta dam always above the rising sea. In this way there might be 

 formed behind the dam a dry depression such as actually exists. On this hypothesis the 

 formation of Blake Sea would be regarded as a recent and unimportant incident due to 

 the accidental diversion of the Colorado into the depression behind the dam. The writer 

 regards this hypothesis as being rather more in accord with known facts than is the theory 

 currently held. However, he has no wish to urge its acceptance. With the facts at present 

 available, that particular period in the basin's history must be regarded as quite unknown. 



With the existence of Blake Sea we leave speculation behind and return to history 

 for which there is better evidence. Whatever its origin, Blake Sea was ultimately fresh 

 and its disappearance was almost certainly due to evaporation following the cutting off 

 of water supply from the Colorado. Probably this stoppage was due to a natural return 

 of the river to its gulfward channel, though this may have been complicated by slight 

 orographic movement or by climatic or other changes affecting the regimen of the Colorado. 

 The fall of Blake Sea must have been by annual or semi-annual stages, much as the Salton 



