206 Geological Survey of the Territories in 1872. 
the hills on either side close in to sharp cafions, 400 feet deep 
by 200 feet wide, with coarse gray sandstone walls. ese 
also are supposed to be of Tertiary age. At the head of the 
basin, a few miles farther on, there are high rugged walls of 
thin-bedded limestones and sandstones, probably of Quebec 
Group age, though possibly capped with Carboniferous. 
The valley of the Gros Ventre, which is the first stream 
south of Buffalo, is narrow, with precipitous slopes on either 
hand, walled near its mouth by Carboniferous limestones and 
sandstones, which are followed above by Triassic (?) red shales 
and sandstones, Jurassic (?) gray and buff magnesian limestones, 
and Tertiary (?) white friable sandstones. A large butte which 
stands on the bank of Snake River, opposite the mouth of the 
Gros Ventre cafion, shows Carboniferous fossiliferous limestones, 
followed below by limestones apparently destitute of fossils, 
which are referred to the Quebee Group, and quartzites which 
are referred to the Potsdam. The southern portion of the butte 
shows only the soft Pliocene whitish sandstones and mars, 
which cover the place where we should naturally look for meta- 
morphic rocks beneath the quartzites ; but, from the strike of the 
upper rocks, it is probable that a metamorphic axis runs across 
here from the Tetons toward, if not to, the metamorphic nucleus 
of the Wind River Mts. A short distance south of this point, 
the metamorphics of the Teton Mts. disappear beneath the 
limestones which come forward from the western slope and 
which now form the mass of the range for several miles. 
The South Gros Ventre Buttes, which stand on the bank of 
the Snake just below the junction of the Gros Ventre, contain 
the last outcrop of voleanic rocks seen in this basin. The up- 
per slopes were not examined, in this neighborhood ; and it 1s 
possible that, at a higher level, these rocks may continue south- 
ward: but the general appearance of the country gave the 1m 
pression that this was their southern limit,—that, before their 
eruption, the drainage of the region flowed northward an 
escaped westward through the broad and deep valley which then 
existed beyond the northern end of the Teton range; that that 
eruption dammed up the waters over Jackson’s Hole, so that 
the Pliocene sandstones and marls were deposited beneath the 
lake thus formed ; and that the southern outlet across the southern 
continuation of the Teton range was subsequently eroded to 1ts 
present level, during the progress of which erosion the terraces 
were formed which are here exhibited on a grand scale. 
At the South Gros Ventre Buttes, the party again divided, 
the main train crossing the Teton Pass to Pierre’s Hole, and 
striking the Snake again where it emerges from the mountains 
about twenty miles from Taylor’s Bridge, while a survey!08 
party followed the river through the so-called Grand Caion- 
