26 THE SALTON SEA. 



matters, which was advanced by Blake and is still current among geologists who have 

 examined the basin, is that the trough was originally open to and partly filled by the ocean 

 and was gradually cut off therefrom by the building of the Delta of the Colorado. The 

 growth of the Delta being complete, the Colorado might flow directly into the Gulf as 

 now, or might flow into the Cahuilla depression, fill it, and overflow into the Gulf, as it 

 would probably have done had the engineers failed to control it after the inbreak of 1905. 

 On topographic grounds alone Blake Sea might be regarded either as the final cut-off 

 remnant of the arm of the sea which once occupied the trough, or as a lake produced by 

 the incursion of the Colorado as noted. The present writer, while not venturing a vigorous 

 dissent from this hypothesis of the exclusion of the sea by delta-building, regards it as quite 

 unproven. The known facts of the matter are two: the strata of the Mud Hills Series are 

 of continental origin, and Blake Sea was fresh. The former fact has already been discussed, 

 and it has been pointed out not only that the strata in general resemble in every detail 

 those characteristic of mountain aprons in a desert valley, but that the occurrence of 

 gypsum and mirabilite in the series proves the existence of at least local areas from which 

 the drainage had no seaward egress. It can not be held certain that the basin as a whole 

 was undrained. The observed facts can be explained as well on the hypothesis of local 

 undrained areas cut off from the main drainage line of the valley by low structural or allu- 

 vial divides. Examples of this are numerous in the present desert valleys of North America. 

 It should be noted, however, that while the evidence does not necessarily indicate an entirely 

 inclosed basin during the late Tertiary, it does indicate arid conditions and the absence of 

 the sea from at least that part of the Sink which is now the lowest. 



The second fact, the freshness of Blake Sea, follows from three other facts. First, 

 shells characteristic of fresh or slightly brackish waters are scattered by millions over the 

 desert once occupied by the ancient lake and must have been plentiful in its waters. Second, 

 the coating of travertine deposited by the lake at its water-line, and as it underwent sub- 

 sidence, is such as would be deposited from a fresh or at least a continental water. These 

 coatings are discussed elsewhere in this volume by Professor Jones. Third, the amount 

 of salt present in the salt-body now covered by the Salton Sea was only a small fraction 

 of the salt which would have been deposited had Blake Sea been of ocean water. There 

 is no possibility of the escape of any salt, and burial by alluvium seems scarcely likely 

 when the strand-lines and terraces of Blake Sea are so excellently preserved. 



We know, therefore, that the ocean did not occupy the trough just previous to the 

 post-Tertiary uplift, and that it did not occupy the trough during the later history of 

 Blake Sea. This leaves a long intermediate period of which we know nothing at all. During 

 this period there may have been a marine occupation, the Delta may have been built, 

 and the sea shut out, as generally assumed. This is quite possible, but there is no evidence 

 directly indicating it, and it is pure speculation. It is quite as possible that the trough 

 remained above sea-level until after the Delta dam was built, and that the depression 

 behind the dam has never been occupied by the sea. During this doubtful period there 

 occurred unquestionably two events: (1) the general depression of the trough relative to 

 sea-level; and (2) the formation of the Delta dam. The varying hypotheses concerning 

 the period vary only in the relative order in which they view these events. The current 

 hypothesis regards the depression as prior to the building of the dam; the hypothesis 

 suggested by the writer regards the formation of the dam as prior to the depression. Our 

 outline of the history of Blake Sea will vary greatly according to which of these hypotheses 

 we accept. If the former, Blake Sea will be regarded as the cut-off portion of an arm of 

 the ocean; if the latter, it appears as a lake created by the accidental flow of the Colorado 

 into a previously existing depression behind an alluvial divide. In either case it is certain 

 that the formation of Blake Sea could not have been prior to the depression of the valley 

 relative to sea-level (the Delta dam would not withstand a steeply graded overflow), 



