558 



EOCENE AND OLIGOCENE PERIODS 



which they are unconformable (Fig. 446). Clays, sands, and green- 

 sand marls are the most common materials of the system, and the 

 conditions of sedimentation were much as in the preceding period. 



The system is thicker (1,700 feet maximum) in the Gulf region 

 than on the Atlantic coast. It contains much lignitic matter in 

 places, showing that marine conditions were not uninterrupted. 

 In Texas, gypsiferous and saliferous sediments recur at various 

 horizons, though most of the beds are of marine origin, and there are 



numerous local unconformities in 

 the system, suggesting repeated 

 changes in the conditions of sedi- 

 mentation. 



The Pacific coast. Marine and 

 brackish-water beds. 1 Marine 

 Eocene formations are widespread 

 west of the Sierra and Cascade 

 ranges (Fig. 472), and have con- 

 siderable development in Alaska. 

 Throughout Washington and 

 Oregon and in parts of California, 

 the Eocene is unconformable on 

 the Cretaceous (or Shastan), but 

 in much of California it is con- 

 formable on the Chico, the plane 

 between the two being defined by 

 fossils. These relations suggest 

 that just before the Eocene, all 

 of Washington, most of Oregon, 

 and parts of the coastal region of 

 California were land, over which 

 the sea advanced later. The 

 rocks are mostly clastic, sandstone 

 and shale predominating, but 

 there are conglomerates, tuffs, and 

 diatomaceous shales, the last 

 thought to be a source of oil. In 

 not a few places, marine beds are 

 succeeded by brackish-water de- 

 posits. 



Fig. 472. Map showing supposed 

 distribution of land and water on the 

 Pacific coast of the United States 

 during the Eocene period. (Ralph 

 Arnold.) 



1 Arnold, Jour. Geol., Vol. XVII. 



