This is a tongue of cross-bedded sandstone. It is brown in color and is
composed mostly of quartz with some black chert grains. This unit is
limited in distribution to the south of Fossil Butte National Monument.
The Sandstone Tongue thins and pinches out to the north and is not
present within the monument. The area of pinchout represents the
shoreline at one stage of Fossil Lake. The Sandstone Tongue wedges into
the Fossil Butte Member of the Green River Formation. The sediment
sources were probably the Uinta Mountains at the south edge of Fossil
Basin. Tectonic events caused an uplift and erosion of Mesozoic and
Paleozoic rocks in the Uintas and debris was deposited in Fossil Lake as
an encroaching delta of sand. When deposition of the sand halted, the
lake expanded and covered the sand, encasing it within the shales of the
Green River Formation.
The Sandstone Tongue can be traced into the Main Body of the Wasatch,
hence its assignment to the Wasatch Formation.