GEOLOGY OF WEST COAST REGION OF UNITED STATES 

 land and water. The Great Basin Sea was drained, 

 the Pacific Ocean advanced eastward, and from now 

 on we have to do with the advances and retreats of 

 the epicontinental stages of the Pacific Ocean, the 

 records of which form the final volume of the geo- 

 logical history of the west coast. 



THE GEOLOGICAL HISTORY OF THE WEST COAST 

 SINCE THE CORDILLERAN REVOLUTION. The early 

 Cretaceous (the Shastan of the west coast) was 

 marked by the rapid advance of the Pacific Ocean 

 over the western portion of the Pacific States. The 

 recently folded Coast Range stood but a brief in- 

 terval above the ocean, which also crossed the site 

 of the present Cascade range in southern Oregon. 

 The enormous thickness of the combined Knox- 

 ville and Horsetown of the Shastan system (maxi- 

 mum thickness 25,000 feet) suggests that the high- 

 lands east of the Shastan trough were eroded 

 rapidly. There was considerable shifting of areas 

 of sedimentation between the Knoxville and the 

 Horsetown, and also between the latter and the 

 Chico (upper Cretaceous). The Chico sediments 

 register a second great advance of the Pacific Ocean 

 which occupied the western portion of the Pacific 

 States, and covered the bevelled Klamath Mountains, 

 bathed the western foothills of the Sierra Nevada, 

 and reached the Blue Mountains of eastern Oregon. 



No revolution comparable with that which af- 

 fected the Rocky Mountain region to the east, 

 separated the Cretaceous from the Tertiary. In 

 numerous localities, especially in southern Califor- 

 nia, no pronounced break can be recognized be- 

 tween the Chico and the Tejon (Eocene). 



The Eocene strata reach their greatest thickness 

 in the Puget Sound embayment, where at least 

 10,000 feet of sediments were deposited, including 

 the thickest and best coal strata of the Pacific Coast 

 region. Elsewhere along the coast local Eocene 

 basins were converted into swamps for brief in- 

 tervals, the organic accumulations of which are 

 preserved as lignite, or a poor grade of bituminous 

 coal. Oligocene strata have been recognized, es- 

 pecially in Washington and Oregon. 



The Miocene is supposed to represent a brief 

 period of geologic time compared with earlier geo- 

 logic divisions. In California it was marked by 

 the deposition of great thicknesses of sediments, 

 and embraces a mountain-making epoch of no mean 

 dimensions. The thick lower Miocene sediments 

 include many thousand feet of diatomaceous ma- 

 terial one of the principal sources of California 

 oil. The second folding of the Coast Ranges of 



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