294 HISTORICAL GEOLOGY 



restricted, but marine waters spread over the whole Gulf Coast 

 from Florida to southern Mexico, except for a small island in 

 Florida (see Fig. 181). 



The presence of some marine Pliocene strata in Florida and 

 along the Gulf Coastal Plain border shows those areas to have 

 been submerged during portions of Pliocene time at least. 



Western Interior. — The extensive folding, faulting, and lava 

 extrusions which marked the close of the Cretaceous period left 

 the western interior topographically rugged with conditions 

 favorable for rapid erosion of the mountains and deposition of 

 sediments in the intermontane basins. As the character of the 

 Tertiary sediments indicates, all sorts of continental deposits were 

 formed, that is in lakes, on river flood-plains, as alluvial fans, as 

 wind-blown deposits, and even as volcanic dust or tuff in many 

 places. Marine deposition was wholly lacking. As shown by the 

 above statements regarding the distribution of the various forma- 

 tions, it is apparent that the principal areas over which sediments 

 were being deposited must have shifted more or less. 



That there was very active vulcanism during Tertiary time in 

 this western interior region is proved by the presence of so much 

 volcanic dust and ash. Also in the great area between the Rockies 

 and Sierra-Cascade Ranges, there was tremendous volcanic activ- 

 ity, but this will be described under a separate heading below. 



Many of the Tertiary deposits of the western interior now lie 

 at altitudes of from 5000 to 10,000 feet above the sea, and they 

 have been sbmewhat deformed by tilting or warping. 



Pacific Coast. — Summarizing the physical history of the Pa- 

 cific Coast during Cenozoic time, Ralph Arnold says in part: 

 " Following the period of elevation and erosion at the close of the 

 Cretaceous, the Eocene was inaugurated by a subsidence below 

 sea-level of the greater part of western Washington and Oregon 

 and the western part of central and southern California. Volcanic 

 activity was pronounced in the early and middle Eocene. Later 

 in the Eocene brackish and freshwater conditions prevailed over 

 the same area, and extended over much of Alaska. . . . The 

 Oligocene was a period of elevation, with marine conditions re- 

 stricted to a much smaller area than in the Eocene. . . . The 

 lower Miocene marked a widespread subsidence in the Coastal belt 

 which was followed by a period of mountain building (Coast 

 Ranges) and great local deformation, vulcanism, etc. . . . The 



