246 HISTORICAL GEOLOGY 



marine waters spread over the region. During all of Fredericks- 

 burg time a clear and unusually deep epicontinental sea occupied 

 much of Mexico and Texas and immediately adjoining regions. 

 Great chalk deposits were formed in this clear sea. Perhaps the 

 maximum northward extension of the Lower Cretaceous sea took 

 place during Washita time, when marine waters probably reached 

 as far northward as Colorado (see map Fig. 150). 



Throughout Upper Cretaceous time, from Woodbine to Na- 

 varro inclusive, marine waters appear to have persisted over the 

 Texas area, having been particularly clear during the deposition 

 of the Austin chalk. 



Western Interior Region. — The physical history of this area 

 during Lower Cretaceous time is still somewhat problematical, 

 but the best evidence seems to show that, just east of the site of 

 the Rockies, deposits of continental origin (Morrison and Kootenai) 

 were forming very much like those of the Potomac on the Atlantic 

 Coast. 



Pretty early in the Upper Cretaceous, the western interior 

 region witnessed a very extensive marine transgression beginning 

 during Dakota time and probably reaching a maximum during 

 Colorado time. In the comparatively clear waters of this Colorado 

 sea were accumulated the chalk and other marine deposits. This 

 great marine invasion must take rank as one of the most extensive 

 in the history of the continent, the marine waters having spread 

 from the Gulf of Mexico northward over the western interior 

 region to the Arctic Ocean by way of what is now the Mackenzie 

 River Valley (see map Fig. 151). There is no good evidence that 

 this vast western interior sea had direct connection with the Pacific 

 Ocean. In the latter portion of the period (Laramie) marine 

 waters did not prevail over the immediate Rocky Mountain dis- 

 trict through the United States and Alberta. Sufficient emergence 

 " formed a coastal plain, extensive marshes prevailed, and the 

 marsh deposits became coal beds. Sea, marshes, and river plains 

 alternated in sequence till near the close of the Cretaceous period " 

 (Bailey Willis). 



Pacific Coast. — Rather remarkable physical conditions must 

 have obtained in California, especially in the north, to give rise to 

 such a phenomenal thickness of sediments during this one period. 

 Apparently the explanation is not far to seek, because the newly 

 upraised Sierras must have undergone vigorous erosion with rapid 



