TERTIARY PLAINS OF WYOMING. 67 



color, the beds are all of light shades ; the marls, which form the most promi- 

 nent feature in the cliffs, suggesting the name of Chalk Bluffs. 



From the exposures in the Chalk Bluffs, along the tributaries of Crow 

 and Little Crow Creeks, Prof O. C. Marsh has made large collections of 

 vertebrate fossils, now in the museum of Yale College, which abundantly 

 prove the Miocene age of the lower beds. Among the more important of 

 these vertebrate remains obtained from this locality may be mentioned the 



following: 



Brontotlierium ingens, Marsh ; 



Brontotherium gigas, Marsh ; 



two huge mammals, nearly as large as an elephant, and exceeding in size 

 any other extinct animal from this formation. These animals belong to an 

 entirely new family, which Professor Marsh has designated the Brontotlieridm, 

 They were as large as the Binocerata, which characterize the Eocene lake- 

 basins of Wyoming. They are most nearly related to the rhinoceros, but 

 show some characters that indicate close affinities with the elephant. 



Another large mammal, described from tins region by Professor Marsh, 

 was Elotherium crassum, also about the size of a rhinoceros. In these same 

 Miocene beds were also obtained many small mammals, carnivores, rodents, 

 insectivores, and a few fragments of bu'ds. 



NiOBKAKA Pliocene. — The upper 300 or 400 feet of the Chalk Bluffs 

 consist of Pliocene strata, which cover the entire area of our. map east of the 

 Laramie Hills. The surface of the country is undulating, but generally so 

 smooth and covered with loose soil as to present but few good exposures, 

 except along the valleys of numerous streams and on the benches high up 

 near the mountains. The valleys of Crow, Lodge Pole, Horse, and the 

 Chugwater Creeks cut deeply into the underlying strata; the bluffs upon 

 both sides of the streams presenting long continuous walls of bare rocks, 

 from 100 to 200 feet in height. These streams, whose valleys vary from 

 ^ to 4 miles in width, usually erode through the soft strata till they reach 

 some bed of hard, impervious clay, which forms their bottom. The surface 

 of the Pliocene lake-basin, along the immediate base of the range, had an 

 altitude above sea-level of at least 7,000 feet, and perhaps from 100 to 200 



