CHAPTER XXIII 



THE JURASSIC PERIOD 

 FORMATIONS AND PHYSICAL HISTORY 



Eastern North America. Jurassic formations have not b'een 

 identified in the eastern half of the continent, where erosion seems 

 to have been the leading geologic process during the period. Its 

 effectiveness may be judged by the fact that both the uplifted and 



deformed Triassic system and 

 the Appalachian mountain re- 

 gion farther west were essen- 

 tially base-leveled before the 

 next period was far advanced. 

 Western interior. Deposi- 

 tion was in progress, probably, 

 in some parts of the western 

 interior, though early Jurassic 

 beds of this region have been 

 clearly differentiated from the 

 Trias in but few places. There 

 is perhaps room for doubt 

 whether the lower and middle 

 parts of the system have much 

 representation in this region. 



Late in the period, an arm 

 of the sea covered a large tract 

 in the western interior, cover- 

 ing much of Wyoming, Mon- 

 tana, Utah, and Colorado, and 

 parts of several other states 

 (Figs. 423 and 424). This is 



Fig. 423. Map showing the general 

 relations of land and water in the western 

 part of North America during the later 

 part of the Jurassic period. The black 

 areas represent known areas of Upper 

 Jurassic. The dotted line is the conjec- 

 tured outline of the bay. (After W. N. 

 Logan.) 



shown by the presence in these 

 states of sedimentary beds con- 

 taining marine fossils of late 

 Jurassic age. The avenue 



502 



