28 P. B. KING 



but in later Eocene time some of them on the west became the sites 

 of great lakes in which fine-grained, thin-bedded, water-laid sedi- 

 ments accumulated, which were in part calcareous or petroliferous 

 (Bradley, 1948, pp. 640-647). Lacustrine deposition had already 

 begun during Paleocene time in the western part of the Colorado 

 Plateau area, where the Flagstaff limestone was laid down widely, 

 but during the Eocene the center shifted eastward and a great 

 confluent body of water spread north and south of the Uinta Moun- 

 tains, in which the Green River formation was deposited ; a smaller 

 lake also existed in the Bighorn basin, represented by the Tatman 

 formation (Fig. 7). 



Ranges between the basins doubtless continued to rise during 

 Eocene time and to shed their detritus into the basins, but the over- 

 lap of the Eocene strata along their edges indicates that uplift was 

 less active than earlier. The ranges could not have projected more 

 than a few thousand feet above their surroundings, else they would 

 have created a rain shadow to modify the prevailing humid climate. 

 Presence of the same species of mammals in more than one basin 

 indicates that the ranges were not barriers to migration. 



These conclusions are incompatible with reports of Eocene glacial 

 tills at various places in the Rocky Mountains, especially near Ridge- 

 way, Colorado (Atwood and Atwood, 1938, p. 961). On this basis, 

 some geologists have assumed that the ranges of the Rocky Moun- 

 tains projected to alpine heights at the time. These far-reaching 

 interpretations have been built on very local, very dubious evidence, 

 which may well be otherwise interpreted (Van Houten, 1957). 



The extensive intermontane and lacustrine deposits of Eocene 

 time might suggest that the Rocky Mountains and their environs 

 were then a region of interior drainage — that not only were the seas 

 driven from the region by Laramide orogeny but also, for some time 

 thereafter, no rivers flowed from it to the sea. A little reflection sug- 

 gests that this is implausible. Although the region was not lofty, its 

 summits must have projected to some height above the lowlands on 

 the east and formed a drainage divide. The probable rainfall was 

 greater than could have been trapped entirely by basins without 

 outlets. The subsiding intermontane basins caught the detritus 

 which was being washed down from the mountains, but during most 

 of the time the streams which entered them probably flowed on to 

 the sea. When subsidence of the basins was excessive, runoff was 



