276 HISTORICAL GEOLOGY 



deposits were forming in the western interior of the United States; 

 and the Pacific border was mostly occupied by marine waters. 

 Later in the Triassic the same conditions prevailed in the west, 

 but long, narrow troughs were formed along the Atlantic side in 

 which were accumulated the thick continental and estuarine 

 (Newark) deposits. Map figure 123 shows the condition of the 

 continent at that time. At the close of the Triassic, or beginning 

 of the Jurassic, there was enough crustal movement to convert the 

 basins (Newark) of deposition in the east into dry land, while in the 

 west the conditions appear to have continued as during the period. 



During the earlier Jurassic the conditions just described in the 

 west still prevailed, but in the later Jurassic a transgression of the 

 sea took place from British Columbia southward over the Rocky 

 Mountain region as far as northern Arizona (see map Fig. 133). 

 During the whole Jurassic eastern North America was land under- 

 going erosion toward the peneplain condition. 



During the Lower Cretaceous there was enough subsidence of 

 the Atlantic and eastern Gulf borders to produce flood-plains, 

 lakes, and marshes in which were deposited the Potomac series of 

 sands, gravels, clays, etc. About the same time the continental 

 (Trinity) deposits, followed by the marine Fredericksburg and 

 Washita beds, were accumulating over the western Gulf (Texan) 

 regions and southern western interior regions, and continental 

 deposits were forming over the northern western interior region 

 just west of the site of the Rockies. During the Lower Creta- 

 ceous on the Pacific border there were accumulated very thick 

 marine deposits just west of the newly formed Sierras, especially in 

 the Great Valley of California. Marine deposition also took place 

 along some of the coast north of California. 



The Lower Cretaceous closed, or the Upper Cretaceous opened, 

 with the eastern part of the continent all undergoing erosion; a 

 continuance of marine waters over the western Gulf (Texan) and 

 southern western interior regions; and some deformation (folding 

 and faulting) of the strata in parts of the Coast Range district. 



At the opening of the Upper Cretaceous the condition of the 

 continent was essentially that just described for the close of the 

 Lower Cretaceous. Early in the Upper Cretaceous, marine waters 

 spread over practically all of the Atlantic and eastern Gulf Coastal 

 Plain areas. At the same time "Appalachia," which had been so 

 long persistent, became submerged, not again to reappear. The 



