TERTIARY FORMATIONS. 203 



now existing are not known with any great degree of certainty, must have 

 been of extremely local character. The aggregate thickness of these Eocene 

 formations is estimated at 7,500 feet, and they have been separated into 

 three groups, the Vermillion Creek, Green River, and Bridger, so named 

 from the localities in which they find their most characteristic development. 

 They are remarkable for the abundant remains of vertebrates found in 

 their beds, which, though distinct in these subdivisions, yet are considered 

 as representing Lower Eocene types. 



The beds of the Vermillion Creek series, which are chiefly arenaceous 

 in character, were deposited in greater thickness than either of the other 

 groups, and extended from the base of the Park Range to the flanks of the 

 Wahsatch Mountains. They consist in the lower part of coarse sandstones, 

 gravelly beds, and conglomerates, which, in the upper portion, pass into 

 finer sands, with an increasing proportion of fine argillaceous material, the 

 whole characterized by prevailing red and chocolate colors. Their fauna 

 indicates a warm climate, the general coarseness of their material a troubled, 

 stormy sea, while the time of their deposition, immediately subsequent to 

 the folding of the strata, the absence of calcareous material, and their pecu- 

 liar color suggest that they are composed of comminuted debris of the Meso- 

 zoic rocks, which are also characterized by an absence of calcareous mate- 

 rial, and in which the decomposition of the iron, found in considerable pro- 

 portion in the Cretaceous beds, would color the lower beds, while the red 

 sandstones and clays of the Triassic would, by rearrangement, produce the 

 peculiar clayey sandstones of the upper beds. At the close of the Vermillion 

 Creek period, there was a slight movement in these beds, in general not 

 sufficient to produce sharp flexures, but causing gentle undulations, and a 

 local non-conformity of deposition with the beds of the succeeding group. 

 This movement was greatest along the flanks of the Uinta Range, notably 

 at Henry's Fork Valley, a slight movement of uplift probably continuing 

 till near the close of the Bridger period. 



The beds of the Green River series contrast with those of the other two 

 groups by the relative prevalence of calcareous material, and the fineness 

 of their sediments. They were deposited in quiet, probably deeper waters, 

 and perhaps during the time that erosion was wearing away the limestones 



