GEOLOGIC BACKGROUND 



A resume of the geologic conditions between middle Eocene and middle Pliocene in western 

 North America will enable us better to understand the habitats of the merycoidodonts. This gross 

 area at present extends from the Pacific Coast to the Mississippi River and from New Mexico to 

 the Cypress Hills in Saskatchewan, Canada. Idaho and Washington have not so far shown any 

 members of this family. 



The exact horizons whence came some of the older types are not definitely known, but from 

 subsequent collecting and more exact geologic correlations we can approximate these horizons, in 

 many cases with a high degree of exactitude. 



The earliest horizon in which the merycoidodonts have been found is the upper Eocene Uinta 

 formation in western Wyoming, the remains occurring to some degree in divisions A and B but 

 mainly in C. The typical Uinta of Marsh, King, and White has a thickness of about 600 feet and 

 is composed chiefly of altered eruptives, presumably dacite tuffs, containing much glass. These 

 fine-grained rocks are in general gray to pale-green in color, but they have also a reddish tinge, 

 due to the iron content. Horizon C passes upward into the Duchesne River, which in turn lies next 

 below the lower Oligocene Chadron horizon. The texture of all these sediments is lithologically 

 much alike. 



The entombed fauna is that of river and forest types, as well as of meadow and plains, and the 

 bulk of the species has been found in the first hundred feet above the base of Horizon C. Species 

 of Protoreodon, as well as cameloids and hypertragulids, were the earliest representatives of the 

 plains fauna to appear and probably were transitional to the plains fauna of the basal Oligocene of 

 the Great Plains. 



The Uinta formation was laid down in several independent intermontane synclinal basins, such 

 as the Uinta, the Bridger, and the Wind River, and the material is the clay, silt, and sand locally 

 washed down from the encircling mountains, although wind was probably an active, if less impor- 

 tant, agent. While these are mostly flood-plain deposits, with coarser fan material near the foothills, 

 yet there is no complete sedimentary record in any one of the basins. The mountainous rims were 

 probably a deterrent to faunal intermigrations. 



From the close of Fort Union time (Paleocene) volcanic eruptions, chiefly of an explosive 

 nature, though with progressively diminishing activity, continued throughout the Eocene, thus 

 accounting for the large and varying amount of ash and debris in these deposits. By late Eocene 

 the mountains had been much lowered by erosion and by differential warping, so that there was 

 once more a return to a mild, moist climate, with palms growing and crocodiles living as far north 

 as Montana and Idaho. 



Many of the modernized animals which appear in this epoch, including the merycoidodonts, 

 were probably migrants from an Eurasiatic source by way of the land bridge across Bering Strait. 



With the beginning of the Oligocene our major interest shifts to the east of the Rocky Moun- 

 tains, for it is here that we find the remains of oreodonts in great profusion. Doubtless these and 

 other Oligocene forms inhabited the Basin region also, but little evidence of them remains, as 

 Oligocene sediments for the most part have been eroded away. 



No true Eocene deposits, with one exception, have been discovered east of the Front Range, 

 although the warm and often moist climate of the late Eocene continued into the Oligocene, with a 

 prevalence of forests along the streams, similar to those of the former epoch, consisting of palms, 

 figs, planes, hickories, and hackberries; that is, the climate was probably like that of Louisiana to-day. 

 Yet toward the end of the epoch the climate became cooler. Between the streams were large areas 

 of open country, too dry for trees, supporting only herbaceous plants and grasses. The conditions in 

 western North America during late Eocene and Oligocene times have been likened to those of 

 modern equatorial Africa, with its high plateaus, forests, rivers, and savannahs. In fact, these 

 environmental conditions prevail to-day in large areas of the interior of all the great continents, as 

 witness a considerable portion of our own Southwest. 



