84 Adventures in Scenery 



and with them were borne clay, sands and gravels from the 

 higher lands. The waters of this lake were cut off from the 

 ocean by the earlier uplift of the Coast Ranges. A channel or 

 strait where now is the Bay of San Francisco allowed the waters 

 to escape to the ocean. Uplift of the floor of the Great Valley 

 in Quaternary time caused the waters to drain away. The 

 Quaternary deposits of clay, sand, and gravel have been buried 

 in the lower valley by recent river alluvium. The lowest part 

 of the Great Valley is now hardly above sea level. Boats pass 

 from the harbor of San Francisco Bay to Sacramento and 

 Stockton. This low part of the valley is filled with silt that 

 has been carried down the Sacramento and San Joaquin rivers 

 and deposited, building up the deltas of these rivers. Below the 

 river alluvium is clay, sand, and gravel carried into the valley 

 during Quaternary time. 



Southern California Covered by the Sea 



An arm of the ocean extended over southern California in 

 late Tertiary and Quaternary time. Where Los Angeles and 

 San Diego now stand rolled the great sea during later Tertiary, 

 becoming dry land at the time of the uplift in early Quaternary. 

 During late Tertiary the great Colorado Desert region was a 

 huge oyster bed. This arid land would be the last place one 

 would look for oysters, yet in the rocks at Coyote Wells, along 

 Carrizo Creek, and in the sides of the San Jacinto mountains 

 1,000 feet above sea level, oyster shells and those of other marine 

 mollusks occur in great numbers. The gulf or arm of the ocean, 

 after the early Quaternary uplift, extended over the region of 

 the Colorado Desert and the Salton Sea to the San Bernardino 

 and San Gabriel mountains on the north. The peaks of the San 

 Jacinto and Peninsular ranges were islands on the western side 

 of the gulf. After the great uplift which raised the Sierra Ne- 

 vada Range Mt. Whitney, monarch of the Sierras, looked out 

 over the shallow sea which covered the region of the Mojave 

 Desert. 



