THE COLORADO DESERT 



683 



inland 200 miles farther than at present, 

 so that its waves lapped the base of the 

 Santa Rosa Mountains and of San Jac- 

 quito Peak — physical features that are 

 now far inland. At that time the mouth 

 of the Colorado River was in the vicinity 

 of Yuma, Arizona, 60 miles in an air line 

 north of where it now is. Presumably, 

 then as now, the river discharged annu- 

 ally into the gulf sufficient silt to cover 

 one square mile to a depth of 53 feet. 

 This material represented the products 

 of the erosion of the great canyons in 

 Utah and Arizona that are properly re- 

 garded as among the wonders of the 

 western world. 



At the point where the river discharged 

 into the old gulf this silt was deposited 

 as a great delta which gradually extended 

 entirely across the gulf to the Cocopah 

 Mountains on its western shore. As a 

 result of this extension the water body 

 was divided into two parts, the one an 

 inland sea and the other the modern gulf, 

 with about the same dimensions and out- 

 lines that it has today. 



Delta growth, however, did not cease 

 with this separation ; silt continued to be 

 brought down by the Colorado and to be 

 deposited in its bed, along its banks, and 

 in the still waters at its mouth. By this 

 process a stream builds up its immediate 

 channel until this channel is higher than 

 the adjacent land on either side. It is 

 then in an unstable condition and will 

 shift to more favorable courses at times 

 when extreme floods breach its immedi- 

 ate banks. By this process continually 

 repeated it comes eventually to flow over 

 all parts of its delta, building up each 

 part in turn, until the whole stands well 

 above sea-level. By such a process the 

 Colorado River has built the famous 

 delta lands of the Imperial Valley, and 

 meanwhile has discharged alternately 

 into the Salton Sink and the Gulf of 

 California. 



During those periods when it dis- 

 charged into the sink this basin was filled 

 with water and became an inland lake. 

 During the other periods when it dis- 

 charged southward away from the lake 

 the supply of water which it contained 



quickly dried away and left the old lake 

 bottom as the Colorado De.sert. Doubt- 

 less this process was repeated many times, 

 but there exists clear evidence of only the 

 last occupancy. This evidence is in the 

 form of a remarkably well-preserved old 

 water line (see page 682) that rims the 

 desert from Indio to the Cerro Prieto 

 at a height of 40 feet above sea-level. On 

 the rocky points that projected into the 

 lake this shore line is indicated by thick 

 deposits of calcium carbonate, usually 

 spoken of as coral by the desert dwellers 

 because of a fancied resemblance to this 

 mineral. Where alluvial cones and the 

 sandy floor of the desert formed the 

 shore line, beaches have been developed, 

 and although of soft sand and easily 

 eroded, they are even now well pre- 

 served, thus testifying to the recency of 

 the action that produced them. Over the 

 floor of the desert and along the sandy 

 beaches are myriads of shells of brackish 

 water mollusks that lived in the lake. So 

 abundant are these tiny fossils in the 

 northern end of the desert that it has 

 been called, on account of their numbers, 

 the Conchilla (Little Shell) Valley. 



It is not possible to state the exact 

 period at which this lake disappeared. 

 The time units of geology are too large 

 and too indefinite to translate satisfac- 

 torily into years, so that when we say the 

 last existence of the lake and its disap- 

 pearance are the most recent of geologic 

 events, we still leave the mind groping 

 for a definite human standard of time. 

 It is the crudest of estimates, merely a 

 guess, indeed, to state that, reasoning 

 from geologic evidence alone, it may be 

 a thousand years since the lake vanished, 

 yet it puts in concrete form such a guess 

 as the geologist is able to make. 



When human records are studied, some 

 evidence on this point is found, but it is 

 almost as uncertain as to time as that 

 furnished by the physical features. The 

 Indians now living at Toro and Alamo 

 Bonito have distinct legends to the effect 

 that at some time in the past the valley 

 was occupied by a large body of water. 

 They record that this water contained 

 many fish, and that it disappeared grad- 



