148 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Green River Group and the Wasatch Group. Bridger's Butte, as well 

 as the entire eastern portion of this divide fronting the valley of Black's 

 Fork, exhibit a large thickness of the somber, indurated sands, clays, 

 and sandstones of the Bridger Group, passing down into light, buff, 

 chalky layers, with Planorbis, Unio, Helix, Goniobasis, &c. Within 

 a distance of ten miles to the west of this butte the little streams cut 

 through the pinkish beds of the Wasatch Group, then pass up into 

 whiter, indurated, marly clays, with numerous concretionary layers, dif- 

 fering from the chalky beds of the Bridger and Green Eiver Basin. I 

 am inclined to the opinion that this divide forms the junction of the 

 shores of two great fresh- water lake basins which existed during the 

 upper miocene period ; that here the sediments of the western shore 

 of one were so mingled with those of the eastern shore of the other 

 that they cannot now be separated. 



The two great , basins may have been connected with each other at 

 different points at some stages of their growth, but there is an abrupt, 

 persistent, very marked difference in the character of the sediments of 

 the two basins. That the two great basins must have been synchronous 

 is inferred from the fact that the strata of both have been but slightly 

 disturbed by the elevation of the mountain ranges in the vicinity. The 

 want of conformity of the Wasatch Group with thecretaceous and eocene 

 beds will be shown hereafter to be well marked in a number of locali- 

 ties. Both the Bridger and Green River Groups have yielded many or- 

 ganic remains, but the Wasatch Group, although it occupies a very large 

 area, and has been excavated to a great extent along the line of the 

 railroad, has never, to my knowledge, afforded any distinct paleontolog- 

 ical evidence of its age. is ear Piedmont Station, in some arenaceous 

 clays which had been taken out of a cut, I found a few fragments of 

 turtle shells, which do not differ from those so common in the Green 

 River district. It seems that, throughout the West, rocks which are 

 characterized by this brick-red coloring matter are destitute of organic 

 remains. The red beds, or supposed triassic, which are so conspicuous 

 all along the flanks of the mountain ranges, are also singularly destitute 

 of fossils. It seems that wherever this ochreous color prevails in the 

 sediments, the physical conditions were not favorable for the existence 

 of animal life, for if life existed, 1 can see nothing in the composition of the 

 rocks why the remains should not have been preserved. At Bridger 

 Station, and from Bridger to Aspen, which is about twenty-four miles west 

 from Bridger Station, the ochreous beds of the Wasatch Group are well 

 exposed on both sides of the road, and will attract the attention of the 

 traveler. A few stunted cedars grow upon the hills and in the valleys 

 and plains. The alkali is as abundant as ever. There are also localities 

 where heavy deposits of drift occur, espesially in the valleys of the 

 streams. This is shown along the road wherever extensive excavations 

 have been made, and these deposits will be simply mentioned from point 

 to point, to be referred to hereafter in another connection. 



The valley through which the road passes from Piedmont to Aspen 

 is carved out of the beds of the Wasatch Group, and varies somewhat, 

 but seldom over a mile in width. The little stream that Hows through 

 the valley is not more than ten or fifteen feet wide. Prom Aspen to 

 Evanston the change- in the general appearance of the surface of the 

 country will be noticed at once. The hills are more rounded and more 

 thickly grassed over, presenting an older appearance. At Aspen the 

 cretaceous makes its appearance. The high hills on either side are 

 composed of cretaceous strata which seem to have been higher points 

 before the deposition of the sediments of the Wasatch Group, and also 



