The Great Valley 131 



deposited either in lakes or spread out as fans upon the flat plain. 

 Fresh-water lake sediments and fluviatile deposits, river-wash, 

 cover the axial part of the great basin. These materials merge 

 below into the loosely consolidated deposits of the Tertiary 

 sediments. 



The basin is surrounded by mountain ranges. The cores or 

 axes of the mountain ranges are granite. The mountain ranges 

 represent great upheavals of the crust of the earth. In late 

 Tertiary time the mountain ranges both east and west of the 

 Great Valley were further uplifted, and it is thought that the 

 axial portion of the basin was depressed. This is shown by the 

 fact that the Tertiary strata on either side of the valley are bent 

 upward so that they lap upon the mountain sides. 



The Great Valley Once an Arm of the Ocean 



A land mass that is now beneath the sea probably existed 

 farther west than the present coast line. The Farallone Islands 

 and Point Reyes peninsula are thought to be high points, moun- 

 tain tops, of that now submerged land. What is now the Great 

 Valley was then an arm of the sea or bay. Sediments were 

 probably washed from the western land and deposited in the 

 bay and now make up a part of the deep floor of the Great 

 Valley. 



Deposits of great thickness were laid down on the floor of 

 the Great Valley. It is thought that the floor of the bay sank 

 as the sediments continued to be laid down. At the same time 

 the mountain ranges were further uplifted. Tertiary shales, 

 sandstones and conglomerates outcrop now in the foothills that 

 rise abruptly on the flanks of the mountains that surround the 

 basin. That here was an inland sea for a long time, and that 

 sedimentation in deep still water was continuous for long is 

 attested by the occurrence of shale deposits many thousands of 

 feet in thickness. That the deposits were made in deep still 

 water is evidenced by the fact that the shales are organic shales, 

 that is, they are made up largely of the remains of organisms, 



