46 e. a. McGregor 



came entirely cut off by the Colorado delta from the present gulf 

 and existed, no doubt, for centuries as a large inland salt or 

 brackish water sea with an initial area of about 2100 square 

 miles. Eventually the sea water was displaced and the valley 

 was occupied by fresh water. 



Geologists tell us that after this stage of the formation of the 

 Salton Basin had been reached there occurred a long succession 

 of inundations and desiccations. Owing to the relatively steep 

 slope or "fall" of the delta northwestward there was a constant 

 tendency for the Colorado River to abandon its course to the gulf 

 and break through the delta barrier to the recently formed Salton 

 Basin. This, we are told, happened repeatedly with the result 

 at times that the Salton Basin became filled to the level of the 

 rim of the delta. Thereupon the river again changed its course 

 back to its seaward channel, leaving this great inland body of 

 water slowly to dry up through the agency of excessive evapora- 

 tion. All persons who have travelled much about the present 

 Imperial Valley have observed at various points the ancient 

 shore line which marks the periphery of the prehistoric inland 

 sea. Untold centuries were doubtless occupied by these alter- 

 nate inundations and desiccations until, finally, man took pos- 

 session of the valley and constructed barriers to prevent the 

 further breaking in of the river. 



In this manner the Imperial Valley was formed. It is easy to 

 understand, then how it happens that the floor of the valley within 

 the old beach lines is covered with a thick deposit of silt. Obvi- 

 ously, this material was carried in by the successive floodings of 

 the Colorado River until, at present, we have the large fertile, 

 flourishing region, reaching from a point in Mexico 20 miles south 

 of Mexicali to Indio and from Dixieland to Holtville. Surround- 

 ing the Salton Basin on the west and north are fans of alluvial 

 material which has been washed down from the arid mountain 

 slopes for past ages. The transition on these two sides of the 

 valley from silt to alluvium is very abrupt. The Salton Basin 

 is bordered on the east by a low range of drifting sand hills. 

 Southward the Imperial Valley opens, with practically no bar- 

 riers, to the head of the Gulf of California. The former inland 



