512 



HISTORICAL GEOLOGY 



formation. This sea was later shut off from the ocean and soon be- 

 came a salt lake, as the deposits of gypsum and salt show. In the 

 Upper Triassic (Keuper), marine beds, thin coal seams, gypsum and 

 salt deposits, and in the last stage (Rhaetic) marine deposits again 

 show a few of the fluctuations of the period. 



Jurassic 



Atlantic and Gulf Coasts. — In eastern North America the emer- 

 gence of the continent seems to have continued from the Triassic, no 



trace of marine sedi- 

 ments being known 

 except in Mexico 

 where the Gulf of 

 Mexico extended 

 west of its present 

 position. Probably 

 near the close of the 

 Triassic the conti- 

 nent was warped in 

 such a way that the 

 Triassic sandstones 

 and shales of the 

 Connecticut valley 

 were tilted to the 

 east, and those of 

 New Jersey and far- 

 ther south to the 

 west. The Jurassic, 

 like the Triassic, ap- 

 pears to have been a 

 period of continued 

 erosion in the eastern 

 half of the continent. 

 Western Interior. 

 — No early Jurassic 

 rocks are known with 

 certainty to occur in the western interior, but later in the period an 

 arm of the sea of great width extended south (Fig. 484) from Alaska to 

 Wyoming, Utah, and the Black Hills of South Dakota. The preva- 



Fig. 484. — Map showing the probable outline of 

 North America during a portion of the Upper Jurassic. 

 (Modified after Schuchert.) 



