THE QUATERNARY PERIOD 



367 



Lake Lahontan, occupied some thousands of square miles of west- 

 ern Nevada, but it had no outlet. Since the lowering of the water 

 levels in these basins, crustal disturbances have caused a tilting of 

 the old shore lines some hundreds of feet. 



Locally, along the Pacific border, Quaternary fossiliferous 

 marine deposits occur up to altitudes of 200 or 300 feet or more. 

 " Important and more or less widespread periods of diastrophism 

 later than the one terminating the Monterey (middle Miocene) 

 period of deposition 

 occurred in the Pleis- 

 tocene. . . . Minor 

 movements produc- 

 ing local unconform- 

 ities took place in 

 central and southern 

 California at various 

 times during the 

 Pleistocene in addi- 

 tion to the more 

 far-reaching disturb- 

 ances in the same 

 epoch." l 



The islands off 

 the coast of southern 

 California were con- 

 nected with the 

 mainland late in the 

 Tertiary, or early in 

 the Quaternary, as shown by the flora, and in one case the remains 

 of a Mammoth. A subsidence, causing the separation of the islands 

 from the mainland, was followed by partial re-elevation to the ex- 

 tent of at least 1500 feet, as proved by the raised sea-beaches on 

 the mainland and on some of the islands. A remarkable fact is 

 that some of the adjacent islands were subsiding during this whole 

 time. Raised beaches near San Francisco testify to Quaternary 

 upward movements of 1500 to 1800 feet. Similar beaches on the 

 northern coast of Oregon lie at 200 feet or more above the sea. 



Fig. 227 

 Map showing extent of the extinct Lakes Bonne- 

 ville (a) and Lahontan (b) in the western United 

 States. Heavy black lines are axes of Sierra 

 Nevada and Wasatch Mountains respectively, 

 c, area of Great Salt Lake. (After Gilbert and 

 Russell, from Le Conte's "Geology," permission 

 of D. Appleton and Company.) 



1 Ralph Arnold: Outlines of Geologic History, by Willis and Salisbury. 



