132 REPORT UNITED STATES GEOLOGICAL SURVEY. 



feet of shales, and 350 to 400 feet for the upper sandstone series. At 

 many places this latter has been totally removed by erosion, leaving 

 nothing but the shales to occupy the highest points. 



Throughout the region the beds of the Green River series show a 

 characteristic style of weathering. Wherever the progress of erosion, 

 either fluyiatile or atmospheric, is very slow, there the outlines presented 

 by members of this group are gently rounded. If the erosive agents 

 are more active, steep, picturesque forms result. Tables and buttes, 

 resembling those near Green River City in general structure, are found 

 in the western portion of our district. From the lithological character 

 of the strata we may infer that the lower series was deposited into deep, 

 still water, while the upper essentially represents shore deposits. Very 

 few fossils were obtained, and these only fragmentary, for the reason 

 that the shales were too much decomposed and we could not spare any 

 time for the purpose of excavating. It seems, too, that the localities 

 farther west and southwest contained a much more extensive fauna than 

 the eastern ones. In Northern Colorado and other regions to the east- 

 ward, the Green River Group is almost entirely destitute of palseontolog- 

 ical remains. 



Bridger Group. 



In 1870 Dr. Hayden found a number of outcrops of the Bridger Group 

 on the drainage of the Sandies. From his description it appears that 

 they were superimposed upon the Green River series. Within our dis- 

 trict a number of exposures were observed that have been referred to 

 the Bridger. Forming the summit of Station 21 we found a series of 

 light-colored clays and shales, partly calcareous. In these a few leaves 

 occurred, numerous specimens of Lymncea, and remains of turtles. The 

 color of the beds containing them is either white, yellow, or very light 

 pink. Toward the west the summits of Steamboat Buttes are composed 

 of the same material. It was found that wherever the bluffs rose several 

 hundred feet above the general upper level of the western portion of 

 our district, the highest portions were found to be composed of Bridger 

 strata. An enormous amount of erosion has taken place in this region, 

 and has broken all continuity between the outcrops. Enough was seen, 

 however, to let it appear certain that the entire area was here at one 

 time covered with a sheet of Bridger beds extending nearly to the base 

 of the Wind River Mountains and reaching northeastward beyond Sta- 

 tion 21. Both to the north and east the beds appear to thin out. The 

 more northerly outcrops of both the Green River and Bridger Groups 

 participate in the prevailing slight southerly dip, amounting from 1° to 

 4°. Remains of turtles, probably Trionyx, were found at a number of 

 localities where the Bridger was exposed, and below them Mr. Howes 

 obtained a specimen apparently belonging to Crocodilus. Owing to the 

 erosion of the entire region we find only the lower portions of the Bridger 

 series. As a maximum thickness which this attains in our district we 

 may regard 500 feet. Toward the south and southwest it grows much 

 thicker, after passing beyond the limits of the area we surveyed. 



Pliocene Group. 



Within the narrow depression between the subsidiary Prozoic Range 

 and the western base of the Wind River Mountains, we find a local de- 

 posit which is probably of Pliocene age. The valley is of narrow ellip- 

 tic shape, trending about northwest to southeast. It is filled with a 

 loose white and yellow deposit of clays and marls. Some arenaceous beds 



