The Colorado Desert 113 



desert areas. The subsidence was so great that many old valley 

 bottoms are now hundreds of feet below sea level. 



Tertiary Time Was Long 



The Tertiary period lasted about 5 8 million years. During 

 Tertiary time the Colorado River discharged into the Gulf of 

 California upon its own delta. It has been stated that the back- 

 bone or axis of the delta was built as a ridge across the gulf from 

 Yuma to the Cocopah Mountains, and that then for a long time 

 the river discharged its waters both ways from the axis of the 

 delta. That much filling of the basin to the north occurred 

 is shown by the deposits that are revealed in borings for wells. 

 Alluvial deposits in Lake Cahuilla, washed in from the shores, 

 and silt carried into the lake by the river, make possible the 

 flowing and other artesian wells which today mark oases in the 

 great desert. 



Wells, some of them 1,000 feet deep, penetrate alluvial 

 clays, sands, and gravels. None has reached the valley bot- 

 tom however. No bed-rock was encountered at depths of 600 

 to 700 feet below sea level near Imperial. Wells in the Coa- 

 chella Valley, in the vicinity of Indio, in the narrow part of the 

 trough between the San Jacinto and San Bernardino ranges, 

 and at Salton station, indicate that bed-rock is at least 1,000 

 feet below sea level. Thus is seen how great was the down- 

 sinking or subsidence of the bottom of the basin, and what a 

 tremendous amount of earth has been borne into the basin by 

 the Colorado River and by in-wash from the surrounding 

 mountain slopes. 



The problem of watering the desert was one to be solved. 

 Irrigation is almost as old as civilization itself. The use of 

 water from wells for irrigation dates back far into antiquity. 

 In the Colorado Desert it has caused oases to appear where 

 thrive the most luxuriant of gardens. A grove of palms in the 

 midst of the arid desert surprises the visitor. The date gardens 



