428 BAILEY WILLIS 
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. 
