Wee * so 
Mesozoic and Cenozoic Geology and Paleontology. 129 
the westward. Erosion has removed large masses of the strata, and 
exposed the Wasatch for a number of miles up stream. Eastward 
the Green River makes a sharp turn and passes north of Essex 
mountain. In this section the lower members of the Green River Group 
are composed of gray and bluish shales, more or less calcareous and 
arenaceous. Higher up the shales are yellow and light brown, mostly 
very sandy, but containing strata of impure argillaceous limestones, 
Above these follow concretionary sandstones and shales, of yellow and 
rusty brown color. The former contains one very prominent horizon 
of silicious material, appearing in the form of chalcedony and agate. 
Near the base a thin seam of oolite occurs. West of Packer’s creek, 
the total thickness of this group is from 1,700 to 1,800 feet. Of 
this the upper sandstones with their shales, occupy about 800 to 900 
feet, and the arenaceous beds near the base, about 150 to 200 feet, 
which leaves an average thickness for the shales of 700 to 800 feet. 
Both the shales and sandstones diminish in thickness in their north- 
ern extension. 
The Green River Group is succeeded by the Bridger Group in this 
section, wherever the bluffs rise high above the general level as on 
Steamboat buttes. A thickness is preserved of about 500 feet, but 
most of the group has been eroded. Toward the south and southwest 
it becomes thicker. On the northern edge of the Sweetwater plateau the 
Wasatch Group is succeeded by a local deposit, called the Sweetwater 
Group. It consists of brown, yellow and white arenaceous marls and 
clays, and near the top some sandstones without clearly defined strati- 
fication. Itis not conformable with the Wasatch. This group has 
suffered greatly by erosion, but retains a thickness in some places of 
1,200 to 1,400 feet. The hills south and southwest of Saint Mary’s 
ranch, the central butte in Elkhorn Gap, the Sweetwater hills and nu- 
merous bluffs are composed of the strata of this group. It is of Mio- 
cene age. 
The Sweetwater Group is succeeded by the Pliocene. Near the base 
it consists of a very loosely aggregated sandstone, of a light gray or 
yellowish color. Above this there is a succession of light marls and 
indurated clays. Usually these are either very light yellow or white, 
but pink and greenish beds are not wanting, Toward the eastern ter- 
mination of the group the strata become highly silicious. Thoroughly 
permeated by silica, the clays become very hard and brittle. The 
former occurs also in the shape of narrow veins, concretions and even 
strata. Innumerable moss agates are strewn over about six square 
