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HISTORICAL GEOLOGY 



Physical Geography of the Tertiary. Eocene 



The deformations that raised the Rocky Mountains and drained 

 the western interior of North America apparently affected the conti- 

 nent as a whole, and for a time the Atlantic and Pacific coasts were 

 farther out than in the period under discussion (Fig. 529) ; thus por- 

 tions of the Cretaceous sea bottom were exposed to erosion. This is 

 shown by the old land surfaces (unconformities) — not, however, 

 universal — between the Eocene and the underlying formations, on 

 both the Atlantic and Pacific borders of the continent. Since, when 

 traced eastward, the Cretaceous peneplain disappears beneath Eocene 

 deposits, we know that the beginning of the latter epoch was marked 

 by submergence. An important point to be kept in mind in our dis- 

 cussion of the physical geography of the Tertiary is that North 

 America has been a relatively stable continent since the close of 

 the Cretaceous. 



Atlantic and Gulf Coasts. — On the Atlantic coast, deposits occur 

 on Marthas Vineyard island, but not on the mainland of New England 

 or Canada (Newfoundland was probably a part of the continent at 

 this time), and extend from New Jersey into Texas, by way of Ala- 

 bama and Mississippi, then up to the mouth of the Ohio River and 

 thence southwest. The Atlantic deposits of this period are usually 

 loose and incoherent sands, clays, and green-sand marls, derived 

 largely from the Cretaceous formations but also to some extent from 

 older formations. In the Gulf regions the rocks are more consolidated, 

 sandstones, limestones, and shales being common. Extensive lignite 

 deposits occur in Texas and Louisiana, which may become valuable 

 at some future day when bituminous coal is more costly than now. 

 These lignite beds were formed from the peat bogs that existed on 

 poorly drained portions of the low-lying coast, just as peat is being 

 formed in similar regions to-day. 



Pacific Coast. — In the western portions of the continent the rocks 

 of the period are, for the most part, sandstones and shales, with oc- 

 casional conglomerates and tuffs, which rest unconformably on the 

 older rocks in many places, but in others are conformable, the divi- 

 sion being determined by the change in the fauna. The diatoma- 

 ceous shales which occur at the top of the series (in the vicinity of 

 Coalinga, California) should be mentioned, since they are believed 

 CO be the source of important deposits of petroleum. 



During the early part of the Eocene, marine conditions prevailed 



