284 HISTORICAL GEOLOGY 



the United States, there are, in general, four regions : Atlantic and 

 Gulf Coastal Plain; western interior; Pacific Coast; and Alaska. 

 The discontinuity of the areas on the Atlantic and Gulf Plain, 

 especially the former, is due to the fact that later deposits overlap 

 and conceal the Lower Tertiary strata in places. The Lower 

 Tertiary strata extend oceanward or Gulfward under much or all 

 of the Coastal Plain. In the western interior the numerous dis- 

 connected areas are chiefly due either to deposition in separate 

 basins or removal of the strata from some places by erosion. On 

 the Pacific Coast, Lower Tertiary strata appear mostly as small, 

 narrow belts, because only the eroded edges of the upturned and 

 folded rocks are visible in the mountains. Such strata are in reality 

 much more extensively developed than these surface areas seem to 

 indicate. There is no evidence that Lower Tertiary strata were 

 deposited over any other parts of the continent than those above 

 mentioned. 



Upper Tertiary (Miocene and Pliocene) strata show a surface 

 distribution as indicated on map Fig. 176. In general this dis- 

 tribution is much like that of the Lower Tertiary. On the Atlantic 

 and Gulf Coastal Plain, it is quite the rule that the Upper Tertiary 

 beds form a somewhat discontinuous belt between the continental 

 margin and the belt of Lower Tertiary beds. Upper Tertiary 

 strata are more extensive at the surface than Lower Tertiary on 

 the Atlantic Coast and less extensive on the Gulf Coast. The 

 margin of the continent, as well as much of Florida, are occupied 

 by Quaternary deposits which are, mostly at least, underlain by 

 the Upper Tertiary. In the western interior a large, nearly continu- 

 ous area extends from northern Texas into South Dakota. The 

 comparatively small, disconnected areas in the northwestern 

 United States mostly represent deposition in separate basins. 

 On the Pacific border of the United States the Upper Tertiary 

 outcrops extensively as long, narrow bands, due to the fact that 

 usually only the edges of the upturned strata are exposed. In 

 British Columbia and Alaska Upper Tertiary rocks are only 

 slightly developed. It is not known that late Tertiary strata ever 

 occupied any other portions of the United States or Canada than 

 those aW . mentioned. 



Atlantic Coastal Plain Strata. — Lower Tertian (Eocene) 

 strata are only slightly exposed to view, while those of L T pper 

 Tertiary (Miocene and Pliocene) age are extensively exposed in the 



