The Geological Story Briefly Told 79 



uplift occur during Monterey time. The net result was deposi- 

 tion of sediments to a total thickness of more than 6,000 feet. 

 The most characteristic feature of the Monterey group of rocks 

 is its content of bituminous shale. Nearly all the oil in Cali- 

 fornia is directly or indirectly associated with this shale rock. 

 Monterey time was brought to a close by uplift and disturbance 

 of the Coast Range region. The deposits of the San Pablo epoch 

 (the next following the Monterey) rest upon the worn and 

 eroded edges of the upturned Monterey rocks, showing that an 

 interval of erosion followed the uplift and contortion which 

 closed the Monterey epoch. 



During Miocene and Pliocene time a large part of the Great 

 Valley of California was under water, a great gulf connected 

 by sounds with the ocean. Along the eastern shore of the gulf 

 the Sierra Nevada formed a low range along the western side 

 of which was deposited a series of clays and sands, the lone for- 

 mation. These sediments came from streams which flowed 

 down the western slope of the Sierra. In these streams beds of 

 auriferous (gold-bearing) gravels accumulated. 



Tertiary Time Marked by Earth 

 Disturbances 



During the Tertiary period events of great importance oc- 

 curred in California. In Cretaceous time the Sierra region had 

 been uplifted, warped and wrinkled, and during Tertiary time 

 was repeatedly disturbed by uplift and mountain-building. 

 Volcanic activity and faulting of the crust of the earth added 

 to the changes that occurred. Throughout the first half of 

 Tertiary time the Sierra region remained a land traversed by 

 mountain ridges of low altitude. Gravel, sand, and clay 

 were brought down by rivers of the western Sierra slope and 

 deposited in the shallow sea which during most of Tertiary time 

 occupied the basin of the Great Valley of California. Toward 

 the end of the Eocene epoch (estimated to have been about 

 40,000,000 years ago) occurred a series of volcanic outbursts 



