244 HISTORICAL GEOLOGY 



central western North America. The igneous activity represented 

 by these rocks is more fully described in succeeding pages. 



Physical History 



Atlantic and Eastern Gulf Coasts. — The Cretaceous period 

 opened with the coast line of the eastern United States somewhat 

 farther out than it now is, but, early in the period, there was enough 

 subsidence, or possibly warping, of the coastal lands to allow 

 deposition of sediments over much of what is now known as the 

 Atlantic and eastern Gulf Coastal Plain. That but little down- 

 warping of the surface was necessary in order to produce proper 

 conditions for this sedimentation is evident, because the coastal 

 lands just prior to the Cretaceous were already low-lying as a result 

 of the long Jurassic erosion interval. There was just enough 

 warping of the low coastal lands to produce wide flats, flood-plains, 

 shallow lakes, and marshes back from the real coast line. Over 

 such areas were deposited the sediments derived from the Pied- 

 mont Plateau and Appalachian areas. The very irregular arrange- 

 ment of the deposits (Potomac) and their rich content of fossil 

 land plants afford conclusive evidence that the sediments were 

 accumulated under continental conditions. 



The pretty widespread unconformity between the Lower and 

 Upper Cretaceous in these regions proves that, about the close of 

 the Lower Cretaceous, there must have been enough emergence of 

 the lands to convert the basins of deposition into areas of erosion. 

 Early in the Upper Cretaceous, however, a submergence of the 

 coastal lands took place, inaugurating the deposition of the Upper 

 Cretaceous strata. The general character, mostly marine 1 origin, 

 and present extent of these deposits prove that the submergence 

 allowed a shallow sea to spread over much of what is now called 

 the Atlantic and eastern Gulf Coastal Plain. 2 In this connection it 

 is very important to note that Appalachia, the great land-mass 

 which had persisted through the many millions of years of the 

 Paleozoic era as well as most of the Mesozoic era, disappeared 

 under the Cretaceous sea not again to reappear. 



Texas. — Early in Trinity time continental deposits (sands) 

 only were forming over the Texas region, but later in the epoch 



1 Some beds of continental origin occur in the L T pper Cretaceous of 

 Maryland and New Jersey. 



2 Certain minor oscillations of level are here disregarded. 



