SKETCH OF THE GEOLOGY AND SOILS OF THE CAHUILLA BASIN. 25 



"Blake Sea" : in his honor. (See p. 4.) The present Salton Sea has left precisely similar 

 records. Its high- water terrace corresponds in everything but depth of cutting to the ancient 

 high beach-line, and the present sea in its recession has left a series of semi-annual strands, 

 illustrated in Plates 1G, 21 , 25, and 26, which are precisely similar in every respect to the strands 

 of the ancient lake. Plate 6 d shows the strand system on one of the islands in the present sea. 

 The upper two-thirds of this system belong to the strand series of the ancient lake; the 

 lower one-third consists of the strands of the present Salton Sea. Without actual leveling 

 it is impossible to distinguish one system from the other or to tell where the latter begins 

 to be superposed on the former. It seems not improbable that the strands of the ancient 

 series are semi-annual, as are their analogues in the records of the present sea. 



HISTORICAL GEOLOGY. 



The decipherable history of the Cahuilla Basin begins with the uplift of the San Ber- 

 nardino and Santa Rosa Mountains. There is no means of dating this directly, but it 

 seems probable on general grounds that it was associated with the uplift of the Sierra 

 Nevada at the close of the Jurassic. It seems useless to speculate concerning the condi- 

 tion of the country prior to this uplift, as also concerning the cause, duration, and character 

 of the uplift and its relation to the granitic intrusives of apparently about the same age. 



Following or during the genesis of the mountains there must have been a period of 

 considerable erosion. The earlier portions of this period are represented by no known 

 deposits, and the material removed from the mountains either was carried entirely outside 

 the area or is buried in the lower and unexposed portions of the alluvial fill of the trough. 

 A little later the discharge of the products of erosion was not so free, and there were laid 

 down on the mountain slopes the conglomerates, grits, and sandstones forming the Mud 

 Hills Series. At the beginning of this period the sea must have occupied the southern 

 portion of the trough, at least as far north as the locus of the Carrizo Creek Miocene. 

 Evidently it did not fill the whole trough, since the basal member of the Mud Hills Series is 

 well exposed at Mecca and is a true basal conglomerate of strong continental facies. It is 

 quite possible that this period was marked by the gradual retreat of the sea from the trough, 

 with the formation behind it of the alluvial strata characteristic of the Mud Hill Series. 



The deposition of these alluvial strata was interrupted, probably in the very late 

 Tertiary or the early Pleistocene, by the violent and general movement which broke and 

 bent the Mud Hills strata into their present fragmentary and distorted form. While the 

 causal displacement was perhaps relatively simple, the resultant movement of the Mud 

 Hills Series was exceedingly complex and has not yet been unraveled in detail. This 

 second uplift seems also to have been followed by a period of considerable erosion. The 

 newly exposed blocks of soft clays and sands were carved into a very rugged topography. 

 The products of this erosion doubtless went to fill the trough still further, and with time 

 this sea of mountain waste has gradually risen until it has partially submerged the hills 

 from which it came. At the present time each hill of the Tertiary scries has its apron of 

 alluvium, the canyons which cut it are filled with tongues of debris, and many half-buried 

 spurs which front the major ridges indicate that we see only the top of a rugged bad-land, 

 the larger portion of which lies submerged in its own waste. This gradual submergence 

 of the foot-hills brings us to the present time, and the process is still at work. 



Two phases of this history have not been discussed : the relations of the trough to the 

 ocean and the Colorado, and the history of Blake Sea. The hypothesis regarding these 



1 Two other names have been suggested for this lake: Lake LeConte by Bailey (Bull. Cal. State Mining Bureau, 

 24 : 12 (1902), and Lake Cahuilla by Blake (Nat. Geog. Mag., 18 : S30 (Dec., 1907). Neither has taken root in 

 the literature. The name Blake Sea is due to Mendcnhall, who, however, abandoned it before publication in 

 favor of Lake Cahuilla, as used by Blake (Mendenhall, loc. cil., p. 20, footnote). In the present volume Cahuilla has 

 been reserved for the whole basin for reasons stated elsewhere, and Blake Sea is adopted as being obviously more 

 suitable, in the light of the history of discovery in the basin, than is the alternative name, Lake LeConte. 



