THE TERTIARY PERIOD 289 



Pliocene deposits formed in many parts of the western interior, 

 but for most part they are difficult to separate from the later 

 (Pleistocene) deposits. They are mostly of terrestrial origin, 

 though probably with some lake deposits. Two formations which 

 have been described as Pliocene east of the Rockies are the Repub- 

 lican River of Kansas and Nebraska, and the Blanco of northern 

 Texas and Nebraska. Other Pliocene deposits quite certainly 

 occur west of the main axis of the Rockies. 



In addition to the Tertiary beds above described in the western 

 interior, there are also many small to large deposits, especially of 

 Miocene and Pliocene ages, in the northwestern United States 

 and the Great Basin between the Rocky and Cascade Mountains 

 (see map Fig. 176). For most part these formations have not been 

 carefully studied, though it is known that they represent all types 

 of continental deposits. Perhaps the best known area is the John 

 Day basin of eastern Oregon, where various continental deposits, 

 including volcanic ash, attain a thickness of several thousand feet 

 and are particularly rich in Eocene and Oligocene fossils. 



Pacific Coast Strata. — Tertiary marine strata, together with 

 some brackish and fresh water deposits, are extensively developed 

 west of the Sierras and Cascades and along the southern coast of 

 Alaska. 



Eocene strata are prominently developed in California, Oregon, 

 Washington, and Alaska. They are mostly of marine and brackish 

 water origin and very thick (maximum 8000 to 12,000 feet). 

 They are chiefly sandstones and shales, but with locally developed 

 tuffs, conglomerates, and diatomaceous shales. Some Eocene 

 strata of Alaska and Washington are of palustrine origin and 

 contain coal. 



Oligocene strata are much less widely distributed than the 

 Eocene. The deposits are mostly sandstones and shales of marine 

 origin in western Washington and Oregon, and north of Los 

 Angeles in California. Similar beds are known on the Alaskan 

 coast. 



Miocene marine strata are almost as prominently represented 

 on the Pacific Coast as the Eocene, the beds being very largely 

 sandstones and shales, often with much diatomaceous earth or 

 shale, especially in the Monterey formation (Fig. 179). The 

 Miocene strata exhibit a maximum thickness of from 14,000 to 

 16,000 feet, the Monterey alone reaching a thickness of 5000 feet. 



