NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 9 



been referred to the Triassic system contain salt, gypsum, and other 

 evidences of sea connection. It may not be out of place here to ques- 

 tion whether these beds do not really belong in some other system. 

 Similar deposits occur in this region in the Permian series and 

 accumulated at a time when marine waters had access freely to the 

 Southern Rocky Mountain Province. Also, it will be shown later 

 that certain younger beds of gypsum are probably Jurassic in age 

 and were derived from sea water late in Jurassic time. 



Inasmuch as the greater part of the continent was above sea-level 

 in Triassic time, it is not easy to understand how marine waters 

 could reach the Southern Rocky Mountain region and deposit the 

 gypsum. The reference of the gypsiferous beds to the Triassic in 

 the vicinity of the mountains does not harmonize with the evidence 

 which tends to prove that this region was one of erosion and of the 

 accumulation of sediments of continental type during the latter part 

 of the Triassic period. At present we are confronted with seeming 

 conflict of evidence. Apparently the Triassic rocks consist of debris 

 which came from the mountains situated in the midst of this region 

 of sedimentation ; that is, the site of the present Rocky Mountains. 

 Until more convincing evidence is brought forward than I have 

 found thus far, I prefer to think of the older salt and gypsum beds 

 as belonging in the Permian series of the Carboniferous system with 

 the other beds of marine origin, and of the younger gypsum as part 

 of the Jurassic system. (The gypsiferous Moenkopi formation is 

 not known to extend eastward to the mountains proper.) If this 

 relationship can be established, there is nothing that I know of in the 

 Rocky Mountain region to negative an orderly succession of events 

 such as follows: (i) The low-lying flats and shallow seas of Penn- 

 sylvanian time were disturbed by the uplift of mountains which rose 

 in the region of the present Southern Rocky Mountains. (2) There 

 followed a time in the Permian epoch during which detrital mat- 

 ter from the newly formed mountains gathered in the neighbor- 

 ing shallow seas and on gypsum flats and salt marshes. In many 

 places it gathered as upland deposits on the plains which sloped away 

 from the mountains, just as detrital matter is accumulating now in 

 the western interior in places which are thousands of feet above sea- 

 level. (3) There followed a time not well recorded in the mountain 

 region during which many events of importance occurred farther 

 west. An arm of the Pacific extended eastward into Colorado and 

 New Mexico in late Permian or early Triassic time ; was later 

 expelled; the rocks which formed in it (Moenkopi formation) 



