220 REPORT UNITED STATES GEOLOGICAL SURVEY. 
of the original boundaries of this great Tertiary basin. Within the area 
of this particular district observation seems almost to warrant the con- 
clusion, if it fails to establish the fact, that the waters of that time com- 
municated with the basin area of Green River, although denudation has © 
interrupted the continuity of the sediments then accumulated in the in- 
tervening straits between the Wind River and Gros Ventre Ranges. 
But in the latter quarter the vertical displacement or upheaval, in the 
region of the eastern extremity of the latter range, was not sufficient to 
erect an insurmountable barrier to the encroachments of the waters on 
either hand and the eventual union of the north and south expansions 
of the Cenozoic sea. From a geological point of view the evidence is 
somewhat conflicting, inasmuch as it is not yet known with that cer- 
tainty necessary to well-founded generalization to what extent the 
Cenozoic deposits themselves have been disturbed by forces acting 
within the present orographical boundaries, and, until the latter problem 
shall have reached solution, the original extent of surface occupied by 
these sediments must remain more or less a matter of conjecture. In the 
Wyoming Mountains they are known to reach high up on the summit in 
places, where they rest unconformably upon older geological formations ; 
at the same time their inclination, although moderate, shows that dis- 
turbing influences had not ceased subsequent to their deposition. Al- 
though in the Gros Ventre and Buffalo Fork uplifts these deposits at no 
point were observed in the more elevated portions of the mountains, yet 
they are more or less disturbed wherever they appear on the mountain 
flank. Whether their disturbed condition is due to elevatory move- 
ments within the mountain zones or to subsidence in the outlying basin 
areas is not so evident, although within the present topographically-de- 
fined basin area these beds exhibit marked evidence of disturbance, 
which resulted in their being uplifted into more or less well marked 
folds, whose parallelism with one or other of the bordering mountain up- 
lifts may be readily recognized. 
The northerly or northeast inclination of the Paleozoic and Mesozoic 
formations off the north flank of the Gros Ventre Mountains, as has been 
indicated in the preceding chapter, extends along the entire south border 
of the Gros Ventre Basin, so that it is difficult to draw the exact line 
- defining on the one side the basin limits from the mountain foot on the 
_ other. Of the above formations only the Mesozoics enter the basin area 
proper, where they occupy a rather wide belt in the southeastern border 
portion. In the westerly-rising declivity of the Gros-Ventre-Green di- 
vide, at a point perhaps 8 miles about north-northeast of Gros Ventre 
Peak, the Triassic “red beds” arch partially over a low fold, the eastern 
flank of which has been eroded and is at present covered with dense forests. 
East of the latter fold the channels of streams flowing down either side 
of the divide reveal the presence of the soft yellowish sandstones and 
light-drab clays of the Tertiary(?); but on the declivity rising up on the 
Wind River Mountains to the east, the surface is composed of drift ma- 
terials evidently derived from that range. West of the Triassic fold, in 
the gentle slopes descending the north flank of the Gros Ventre Range 
into the shallow intervening depression, the same deposits outcrop in 
low bluffs, the northerly inclination of the surface conforming to the dip 
of the strata. This low arch was not again recognized to the northwest, 
and it may be of merely local extent. 
In the southwestern half of the basin the strata are complicated by a 
series of flexures lying quite within the basin limits, although, of course, 
intimately related to the mountain upheaval culminating in the north 
fold of the Gros Ventre Range. This belt of flexed basin deposits extends 
