CHAPTER III 



DESCRIPTIVE GEOLOGY— GEEEN RIYEK DRAINAGE AREA. 



GREEN RIVER BASIN — WIND RIVER MOUNTAINS — OYSTER RIDGE — HAM'S 

 FORK BASIN — HAM'S FORK PLATEAU — ABSAROKA RIDGE — 3IERIDIAN 

 RIDGE — FONTENELLE HOGBACKS — IVIERIDIONAL VALLEY — WYOMING 

 RANGE. 



The present chapter will be devoted to the description of the Green 

 River Basin, or rather to that portion of it which falls within the limits 

 of our district, i. e., the northern and western portion, and the western 

 boundaries. So uniform, however, are the conditions of the geological 

 strata and the structiu-e of the basin, that the description of one part 

 naturally gives a good idea of the whole. 



GREEN RI^^ER BASIN. 



The Green River Basin is an elevated plain, or plateau, extending 

 northward from the Uintah Mountains to the Wind River and Gros Ventre 

 Ranges, and reaching eastward from the others of the Wahsatch system 

 until it becomes merged in the Laramie i^lains, which extend westward 

 from the uphfts of the Rocky Mountain system. Thus we see that the 

 northern, southern, and western boundaries are well defined, while on 

 the east an arbitrary line must be assumed. That portion of the basin 

 lying within our district extends southward to about the latitude ot the 

 mouth of the Big Sandy and north to that of the mouth of Lead Creek. 

 The eastern line is apijroximately the Big Sandy River and on the west 

 the Absaroka Ridges and the Wyoming Mountains. With the Ham's 

 Fork Basin it includes an area of more than 5,200 square miles. 



The elevation of the basin is from 6,000 feet to 7,000 feet. The general 

 level between the Big Sandy and the Green is about G,500 feet, and this 

 is very uniform, although the surface is considerably eroded in places. 

 The streams are generally in canons or caQon-valleys a couple of hundred 

 feet below the general level. On the west the country rises somewliat 

 as the mountains are approached, and the same is true as we approach 

 the Wind River Mountains. Geologically considered, the basin is Ter- 

 tiary, the Green River Group prevailing. In the centre of the Ijasin 

 the strata appear to be horizontal, but as the edges are approached the 

 beds are seen inclining towards the central part of the basin, showing 

 that an elevation took place after the deposition of the Tertiary l)eds 

 • that now form an undistui-bed succession in the centre of this ancient 

 Lake Basin. 



The accompanying illustration gives a good general idea of the ap- 

 pearance of the basin, showing especially the eliects of erosion, which 

 are more marked towards the south. Between the Big Sand^' and Green 

 Ri^er the suiface is much more level, as we have already said. Early 

 in the summer, when the surrounding mountains are still snow-covered, 

 the scenery possesses great beauty. Dr. Hayden, speaking of the Wind 

 River Moimtains as seen from the Big Saudj^, in 1870, says:* ''As the 



'Report U. S. Geol. Surv. Terr., 1870, 1871, p. 46. 



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