PALEOGEOGRAPHIC MAPS 199 



has been suggested except that a land mass diverted the ocean current. 

 The position of the supposed land was southwest of Mexico and is 

 indicated by the dotted area. 



North America was submerged over extensive areas during the 

 Upper Cretaceous. From Cape Cod to Texas the Atlantic and Gulf 

 coasts of the preceding period were transgressed by the sea. From 

 the Gulf to the Arctic marine waters spread over what is now the site 

 of the Great Plains and in the United States that of the Rocky Moun- 

 tains. The Pacific extended its limits in California and Oregon; 

 farther north, however, from British Columbia to Alaska the land 

 gained. 



In the central West, from New Mexico to Alberta the invasion of 

 the sea was followed by emergence of the area ruled on the map for 

 continental deposits. The surface of the area was built up by sedi- 

 ments which were derived from uplands west of it, and which accu- 

 mulated about as fast as the bottom sank. The area thus formed a 

 coastal plain, extensive marshes prevailed, and the marsh deposits 

 eventually became coal beds. Sea, marshes, and river plains alter- 

 nated in sequence till near the close of the Cretaceous period, when 

 in this Rocky Mountain area certain spots became mountains, the 

 forerunners of the Colorado Front Range, the Black Hills, and Big- 

 horn Mountains of today. 



East of the Rocky Mountain coastal plain the marine strait pre- 

 vailed to the end of the period. It divided the continent, reduced the 

 northern land area, and admitted warm waters to the Arctic. These 

 conditions favored the mild climate which the northern regions then 

 enjoyed. 



The eastern portion of the continent contrasted with the western. 

 Whereas in the west rising lands were eroded, carved into hilly or 

 mountainous landscapes, and yet became more elevated, in the east 

 the surface was a vast plain and remained a lowland. 



The close of the Cretaceous was marked by a general ebb of the 

 seas that had prevailed over continents, possibly because the ocean 

 basins deepened. In central western North America the land was 

 rising also, and the combined effect was to withdraw the waters of the 

 strait to the Gulf on the south and to the Arctic on the north. 



