8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



ments were derived seems to have been relatively high, but the char- 

 acter of the rocks indicates that the mountains were lower than they 

 had been in Permian time. It is important in our present study to 

 note that the mountains had been reduced to such a condition that 

 they furnished little coarse material for the beds to the east, although 

 in western Colorado, eastern Utah and elsewhere they are con- 

 glomeratic. It is not certain, however, that the material of the 

 Shinarump conglomerate came from the ancient Rocky Mountains. 

 It may have come from lands farther to the west or south. The 

 sedimentary rocks are relatively thin and probably represent only 

 a small part of the Triassic system. The period seems to have been 

 chiefly one of erosion, not only in the mountain region, but over 

 most of the North American continent. 



More is known of the Triassic rocks west of the Rocky Mountains 

 than east of them and these have a significant bearing both on the 

 Triassic physiography of the mountain region and on the changes 

 which closed the Triassic period. These rocks, 1,700 feet thick in 

 western Colorado (Dolores formation), thin to 1,000 feet or less in 

 northern Arizona (Chinle formation), and still farther to the west 

 the Upper Triassic (equivalents of Chinle formation) are only a few 

 hundred feet thick. (See fig. 2, p. 13.) Although the differences 

 in thickness may be due in some measure to post-Triassic erosion, 

 the differences in thickness suggest derivation of the sediments from 

 the east. Also the occurrence of marine Triassic rocks farther to 

 the west and north (Moenkopi formation, classed by some as Perm- 

 ian (?)), seems to strengthen the belief that a large volume of 

 Triassic sediments moved in late Triassic time from the Southern 

 Rocky Mountain region westward to the sea across a low-lying plain 

 on which the sand and gravel of the Shinarump conglomerate, and 

 Chinle formation were laid down. 



A search through geologic literature shows that Triassic rocks 

 have been found in relatively few places in North America. Areas 

 occupied by sedimentary rocks of this age are found only along the 

 Pacific Coast and in the western interior of the continent. An inspec- 

 tion of existing maps and descriptions shows that certain " Red 

 Beds " in the mountain region are regarded as Triassic by some geo- 

 logists and as Permian or Pennsylvanian by others. The scarcity 

 of fossils in the " Red Beds " renders it difficult in many places to 

 distinguish between Triassic and older rocks. In large part, at least, 

 the Triassic sedimentaries of the mountain region represent upland 

 accumulation. Some of the beds east of the mountains which have 



