206 REPORT UNITED STATES GEOLOGICAL SURVEY. 
bowlders. The nature of the erratic materials is determined to a dis- 
cernible extent by the prevalent rocks composing the nearest mountain 
wall. Paleozoic quartzite and limestone masses occurring at one place, 
and Archean bowlders predominating at other localities. 
Still farther east, and reaching over to. the angle in the mountains | 
at Gros Ventre Peak, the Tertiary deposits are much eroded by the 
streams that issue from the range, from whose foot they are here sepa- 
rated by a narrow intervening belt of Mesozoic formations, which prop- 
erly pertain to the mountain area. A few miles to the eastward of Gros 
Ventre Peak these deposits reach high up in an outlying shoulder, about 
10,000 feet altitude, where they rest unconformably upon deep red are- 
naceous beds holding the position of the Trias, and thence southward 
they compose the bulk of the water-divide between the Green and Ho- 
back. At the latter locality, about south of Gros Ventre Peak, the 
Tertiary is chiefly made up of light buff or yellow and drab sandstones 
and arenaceous clays, dipping gently southward or southwestward into 
the basin where they merge into the long lines of terraces or benches 
that form a characteristic topographic feature of this area. The occa- 
sional exposures in the bluff escarpments along the water courses and 
ravines reveal the bedding with tolerable distinctness and often the beau- 
tiful effects of weather action peculiar to these soft deposits. No indica- 
tions of coal were observed in connection with these deposits, although 
the observations made during a hasty visit-hardly warrant the asser tion 
that it does not exist. 
In the region east of the water- divide, the middle, and perhaps later, 
Mesozoic formations have been denuded over a considerable area of the 
rolling high lands at the eastern terminus of the Gros Ventre range, 
where the Tertiary has been swept away. But farther south the latter 
deposits are again encountered, and thence they constitute the superficial 
rocks, filling the Green River portion of the basin, extending over to 
the great morainal accumulations that bury the western foot of the Wind 
River Mountains and reach well out into the more level basin uplands, 
where the erratic matérials conceal the underlying deposits over exten- 
Sive areas. However, exposures sufficient to show the identity and dis- 
tribution of the Tertiary formation are not wanting, though to the action 
of fluvial, and to a less degree glacial, agencies, they have been generally 
leveled and covered with soil. South of Lac d’Auralia, the ight drab, 
soft Tertiary deposits, nearly horizontal, fill a rather wide bay-like re- 
cess, extending in towards the mountain foot from. the lower portions of 
the valley at a point on the south line of the district. This recess is 
defined on the north and south by great morainal ridges, so that it is 
impossible to ascertain the condition of the Tertiary deposits near the 
mountain border. These deposits probably extend not above 8 or 10 
miles north of the 43d parallel along the river, where they give way to the 
Mesozoies which cross the valley in a low fold extending over from the 
Gros Ventre to the Wind River Mountains, south of the great bend of 
the Green, and which will receive fuller notice under the head of the 
formerrange. Although the valley extends 15 to 20 miles farther north, 
the geological limits of the common basin area occupied by the Tertiary 
sediments, may be defined by a line projected southeasterly from Gros 
Ventre Peak across Green River Valley. Tertiary beds may occur north 
of this line, but they are to be regarded as remnants which have es- 
caped demolition in the process of erosion of the valley. Indeed these 
deposits may once have extended over the above mentioned geological 
fold, uniting and forming an uninterrupted belt with the Tertiary for- 
mations occurring in the otherwise well-defined basin area of the Gros 
