204 HISTORICAL GEOLOGY 



of these two systems are not at all sharply separated. All the 

 known Triassic rocks of the western interior are located within the 

 shaded area, indicating continental deposits on map Fig. 123. Red 

 Beds (sandstones and shales), similar to those of the underlying 

 Permian and often with salt and gypsum beds, are the most com- 

 mon rocks (Fig. 122). Thej^ are certainly very largely of conti- 

 nental origin. Their thickness varies from 100 to 200 feet in the 



Fig. 121 



Tilted and faulted Triassic sandstone in the Connecticut 



Valley near Northampton, Mass. (W. J. Miller, photo.) 



eastern part of the western interior to 2000 or more feet in Utah. 

 In the Rocky Mountains the strata of this age are often highly 

 tilted or folded, while just east of the Rockies they are generally 

 nearly horizontal. 



Rocks of the Pacific Coast. — These are the only true Marine 

 Triassic rocks of North America, and they are there extensively 

 developed with practically all portions of the system from oldest 

 to youngest well represented, particularly in California and 

 Nevada. The rocks consist mostly of shales, slates, limestones, 

 conglomerates, and sandstones, usually several thousand feet thick 

 and with a maximum thickness of 17,000 feet in Nevada and 13,000 

 feet in British Columbia. In Alaska and British Columbia the 

 system contains much igneous material. 



