684 GEOLOGY 



In the West 



The deposits of the western interior. 1 The interior area of sedi- 

 mentation, chiefly between the 100th and 113th meridians, had 

 its southern limit, so far as now known, near the southern boundary 

 of the United States, while at the north it extended into Canada. 

 This area is believed to have been cut off from the Gulf by land in 

 eastern Texas, or if it had connection with the sea, it was probably 

 slight. Into this interior area of sedimentation, detritus was borne 

 from the surrounding lands. Some of the deposits were probably 

 laid down by streams, some in fresh lakes, and some in bodies of 

 salt water, as in the Permian period. Fresh-water fossils (unios) 

 are found locally, as in New Mexico. The structure of some of i he 

 sandstone is such as to suggest an eolian origin. 



The deposits of the period are in large measure concealed by later 

 beds, but they are exposed at various points where the strata have 

 been warped, and the overlying beds removed by erosion. The 

 most easterly outcrops are in Texas, Oklahoma, and South Dakota. 

 The system may underlie later formations between these localities 

 and the Rocky Mountains. Red beds which are thought to be 

 Triassic outcrop interruptedly along the eastern base of the Rocky 

 Mountains from British America to New Mexico. These beds are 

 thin, and contain more or less gypsum and sometimes salt, ( )cca- 

 sionally they contain fossil leaves. 



Farther west, Red beds have representation among the surface 

 rocks. Some of these beds are perhaps Triassic; but in much of 

 the western interior, the undifferentiated Triassic and Permian 

 rest conformably on the Carboniferous (Pennsylvania!!), though 

 occasionally they overlap it and rest upon pre-Cambrian format ions. 

 In southwestern Colorado and eastern Utah, the Trias rests un- 

 conformably on older, deformed, unfossiliferous rod hods (pre- 

 sumably Permian) and on strata of Pennsylvanian age. 2 



Thickness. In the eastern part of this western area, t he Triassic 

 system is thin, sometimes no more than 100 feet. To the west it 



1 There is some doubt about the age of most of the beds formerly reiVnvil 

 to this system. The tendency of later st inly h.i- been in refer MKHV :md more 

 of them to the Permian. See references under P.M-mian, ;m<l Hill. Physical 

 Geography of the Texas Region, folio U. S. Geol. Surv. 



2 Cross; See footnote p. <;<;_>. 



