MESOZOIC ERA: THE AGE OF REPTILES 517 



Pacific Coast. — On the Pacific coast Upper Cretaceous sediments 

 occur in California and northward at points as far distant as Alaska, 

 where they are sometimes conformable and sometimes unconformable. 

 Usually they are not thick, but in one locality (California), at least, 

 they apparently reach the great thickness of 25,000 feet. 



Western Interior. — The geography of the western interior can 

 be roughly divided into (1) an epoch when the sea was excluded, and 

 beds, mainly of non-marine (Dakota) sediments, were laid down ; 

 (2) an epoch of pronounced extension of the sea during which a 

 great thickness of marine sediments (Colorado and Montana) ac- 

 cumulated ; and (3) an epoch during which the land was so low that 

 slight oscillations produced conditions which resulted in the forma- 

 tion of bodies of salt, brackish, or fresh water (Laramie). 



(1) The sediments of the first epoch (Dakota) probably covered 

 an area 2000 miles long by 1000 miles wide, which stretched from 

 Canada on the north into Texas on the south, and from Minnesota 

 and Iowa on the east to beyond the present site of the Rocky Moun- 

 tains on the west. The formation is largely sandstone, though it 

 contains much conglomerate and clay, and some lignite. At this 

 time marshes and lagoons existed near the shores, while inland slug- 

 gish streams were depositing fine sediment over the bottom. The 

 presence of brackish water fossils in beds of this age in Kansas in- 

 dicates marine conditions at certain times, or at least a low shore 

 on which fresh and salt water were mingled. The porous beds thus 

 formed are now the great water-bearing strata of the Great Plains, 

 the water of these porous sandstones being derived from the rains 

 which fall upon and the streams which flow over their upturned edges 

 in the mountainous regions. Although of such wide extent, the 

 Dakota sandstones have a fairly uniform thickness of only 200 to 

 300 feet. 



(2) This epoch (Dakota) was followed by one of extensive submer- 

 gence which resulted in the formation of a great sea (Colorado), 

 stretching from the Gulf of Mexico to the Arctic Ocean and covering 

 the Great Plains of Canada and the United States and the site of 

 the Rocky Mountains, with the possible exception of some large and 

 small islands. As the sea encroached upon the land, muds were 

 first deposited, but as it deepened and widened the waters became 

 clearer, and chalk and limestone were laid down locally, while in the 

 extensive bordering swamps peat accumulated to form important 

 beds of coal which are best developed in Wyoming and Utah. This 



