GEOGRAPHICAL FEATURES OF THE CAHUILLA BASIN. 17 



THE PATTIE BASIN BARRIER. 



The extensive alluvial region which forms the barrier between the Gulf and the Pattie 

 Basin is in reality a continuation, past the partial obstruction offered by the Cocopah 

 Mountains and sundry small isolated hills, of the main Delta slope until it has met and 

 become incorporated with the shore of the peninsula. In general character the country 

 is low and flat, cut into by numerous tidal channels from the Gulf side, and subject to 

 much tidal and river overflow. It is probably but few feet above sea-level at any point, 

 but as no detailed surveys or observations have as yet been made in this region, our laiowl- 

 edge of it is far from complete. 



THE CAHUILLA VALLEY. 

 THE SALTON SINK. 



The total area of the Cahuilla Valley is about 8,000 square miles, and out of this 

 total an area of approximately 2,200 square miles lies below sea-level and doubtless still 

 represents with fair accuracy the former limits of the upper extremity of the Gulf. This 

 depressed portion of the valley floor is nearly surrounded, at a slightly higher level, by a 

 conspicuous ancient shore-line; and as thus inclosed and defined is generally known as 

 the Salton Sink. Its total length is about 100 miles, its greatest width 35 miles. It is 

 roughly elliptical in form, with its major axis extending from 32° 35' N. and 115° 20' W., 

 to 33° 45' N. and 110° 15' W. There is a break in the inclosing shore or beach-line of 

 about 14 miles at the southeast end of the ellipse, and this space has been the entrant 

 point for an immense prism of sedimentary material, which has almost covered the floor 

 of the ancient gulf. A rough computation, based upon the assumption that this ancient 

 gulf-floor was approximately level, gives the contents of this prism at about 17 cubic 

 miles, this volume representing the amount of solid matter deposited by the Colorado 

 within the area since its isolation from the sea, and therefore further indicating the quantity 

 of river water which must have flowed into and been evaporated from the inclosed basin 

 since such isolation was accomplished. (See p. 6.) 



THE ANCIENT SHORE-LINE. 



The tides in the head of the Gulf of California rise to a great height and are at times 

 very violent,' and this energetic tidal action has been mainly instrumental in building 

 up along the surrounding shore-line strongly marked beach-ridges, raised in exposed situa- 

 tions to heights of 20 or 30 feet above mean sea-level. 



The extent and character of the beach-terraces surrounding the Salton Sink at once 

 suggest their common origin with the above — ^the same great tidal action. With the excep- 

 tion of the opening mentioned above the beach-line around the Sink is nearly complete, 

 generally conspicuous and discernible from a great distance, and about 235 miles in total 

 length. 



It should be borne in mind, however, that all this beach material must have been sub- 

 jected to much washing and rearrangement by lacustrine wave-action since the cutting 

 off of the basin from the sea, and it has probably been literally rolled and pushed up the 

 shore slope in the exposed situations in which this action has been most vigorous and as 

 the progressive building up of the Delta dam has raised the spillway toward its present 

 height above sea-level. The marine shore-line in its original location has thus been obliter- 

 ated. The prevailing winds in the Cahuilla Valley are from the west, and we may there- 



' The tide tables of the United States Coast and Geodetic Survey give the ratio of range between the entrance 

 to the Colorado River and the port of reference, San Diego, as 5.65. The extreme tidal range in San Diego is about 

 8.50 feet, so that under exceptional conditions a tidal range of over 50 feet may easily be exceeded at the mouth of 

 the river. This tallies with the writer's own observation, a range of over 45 feet having been measured upon more 

 than one occasion. (See p. 3.) 

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