74 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



TRANSITIONAL LOWER TO MIDDLE EOCENE DEPOSITS 

 HUERFANO FORMATION OF COLORADO (LOWER AND MIDDLE EOCENE) 



While the lacustrine and flood-plain Green River 

 and Wind River formations were being deposited in 

 Wyoming there were accumulating in southeastern 

 Colorado the lower fossiliferous beds of the Huerfano 

 formation, described by Hills (1888.1), explored by 

 Osborn and Wortman in 1896 and by Granger and 

 Olsen in 1918, and now known as Huerfano A. The 

 deposition of this formation apparently began near 

 the end of Wasatch time and extended into early 



although part of its fauna is doubtless transitional 

 from the summit of the underlying lower Eocene. 



In the upper half of the Huerfano formation (Huer- 

 fano B) are found mammals that are also characteristic 

 of the lower Bridger (A) . The imperfectly known life 

 of the upper level includes the tillodont Trogosus and 

 two kinds of small titanotheres, one (Eometarhinus) 

 resembling Metarhinus and the other Palaeosyops fon- 

 tinalis of Bridger A; also a horse {OroMppus?) and ani- 

 mals resembling the Bridger genera Hyrachyus, Hyop- 

 sodus, Microsyops, as well as more ancient genera — 

 the creodonts Amhloctonus and Didymidis — which 



Figure 50. — Sketch map of the region of the Huerfano and Cuchara formations in southern Colorado 

 After Hayden (1880), Hills (1888.1), and Granger (1918). 



Bridger time. Among the mammals of the lower 

 Huerfano, which corresponds with the upper Wind 

 River (="Lost Cabin"), are the rare Coryphodon, the 

 small-limbed titanothere Lambdotherium, EoTiippus, 

 Oxyaena, Didymidis, and Heptodon, a purely upper 

 Wind River (="Lost Cabin") fauna. 



The whole Huerfano formation is 3,500 feet thick, 

 and a large part of it (see fig. 51) lies below horizon 

 A. (Granger, 1918.) Huerfano B, although it lies 

 immediately above Huerfano A, contains the genus 

 Palaeosyops, a distinctive middle Eocene form. Con- 

 sequently Huerfano B is placed at the base of the 

 middle Eocene and is correlated with Bridger A, 



suggest a fauna more ancient than that of Bridger B, 

 corresponding perhaps with the still unknown fauna 

 of Bridger A. It appears probable that the Huerfano 

 will give us a complete faimistic transition between the 

 end of Wasatch and the beginnuig of Bridger B (middle 

 Eocene) time. 



WIND RIVER BEDS AND THEIR FAUNA 



The discovery of the geologic section at Beaver 

 Divide, between Wind River and Sweetwater River, is 

 one of the most significant recently made in the study 

 of Rocky Mountain basin geology. Here deposition 

 without angular unconformity extends from the third 



