Geology of West Coast Region of United States 

 California is generally dated as middle Miocene and 

 was accompanied by a partial elevation of the 

 region affected. However, local but pronounced 

 folding occurred in the late Miocene or early Plio- 

 cene east of San Francisco Bay. Important dis- 

 turbances of probable Miocene age also took place 

 in northern Washington, accompanied by the in- 

 trusion of a granite batholith. Thick upper Miocene 

 marine strata occur in central and southern Cali- 

 fornia. In the vicinity of San Francisco Bay thick 

 lacustrine deposits of upper Miocene and lower 

 Pliocene age appear. 



In the Pliocene and Pleistocene periods lacus- 

 trine and terrestrial deposition has been active, 

 resulting in the filling of the larger valleys to a 

 depth of several thousand feet. During this time 

 marine deposition was confined to a narrow coastal 

 strip. From Miocene times on, northeastern Cali- 

 fornia, eastern Oregon and Washington have been 

 affected by enormous volcanic eruptions, which 

 have resulted in the construction of a great plateau 

 composed of from 3000 to 5000 feet of lava flows, 

 and covering an area exceeding 200,000 square 

 miles. This plateau slopes towards the northwest 

 from an elevation of about 5000 feet in southeast- 

 ern Oregon to 1500 feet in central Washington. It 

 is sentineled on the west by lofty volcanoes, such 

 as Mount Shasta, Mount Hood, Lassen Peak, etc. 

 The last mentioned is now in a state of mild ac- 

 tivity, which, according to Mr. Diller, may prove 

 to be preliminary to more violent eruptions. 



In southeastern California deposition was ter- 

 restrial and lacustrine throughout all the Tertiary 

 and Quaternary. In the Quarternary period the 

 continental ice sheet covered the northern portion 

 of Washington, an important lobe invading the 

 Puget Sound region. Mountain glaciers occupied 

 the higher portion of the Sierra Nevada and Cas- 

 cade ranges, remnants of which are preserved as 

 the small mountain glaciers of today. The com- 

 plete study of the glaciation of the Sierra Nevada^ 

 Cascade ranges has not as yet l^een undertaken. 

 It is an important and much needed work. 



The dynamic history of this great region since 

 Jurassic times is varied and complicated, and does 

 not lend itself to simple summary statement. Dur- 

 ing the Cretaceous, the Sierra Nevada-Cascade 

 ranges underwent erosion, and at the close of the 

 period, portions at least were reduced to an ap- 

 proximate base level as in the Klamath Mountains. 

 In the Sierra Nevada, base leveling was delayed 

 by a number of elevations of the mountain region, 



49 



