The most remarkable feature of the Main Body is its color. On the lower slopes of Fossil Butte are bands of bright to dull red, pink, purple, yellow, and gray color arranged in various patterns. This unit is best observed at a distance, especially after a rain. The bright hues of the Wasatch contrast markedly with the whites and tans of the overlying Green River Formation.
Individual bands of color range from 1 to 10 ft thick (Oriel and Tracey 1970:78). The colors are brightest in the upper part of the member and drabber in the coarser-grained lower part.
The predominant rock types in the Main Body are banded, variegated mudstone with interlayered sandstone, conglomerate, marlstone, siltstone, and claystone.
The upper part of the Main Body is mainly mudstone composed of fine silt and very fine, bedded sand with a clay binder. Conglomerates occur as channel fills and contain calcium carbonate as cement as do a number of sandstone and siltstone layers in the upper Main Body.
Conglomerates and sandstone are more common in the lower part of the Main Body. Some are part of Veatch’s old Almy Formation. They are best developed along the edge of Fossil Basin. The Main Body overlaps the Evanston Formation in some places and may rest directly on Mesozoic or Paleozoic rocks.
SANDSTONE TONGUE.
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.
MUDSTONE TONGUE.
This tongue of the Wasatch Formation can be seen in the northwestern part of Fossil Butte National Monument and over most of the northern part of Fossil Basin. In the area of its distribution, the Mudstone Tongue separates the underlying Fossil Butte Member of the Green River Formation from the overlying Angelo Member of the Green River Formation. The tongue thins and disappears to the south, that is, basinward. As with the Sandstone Tongue, the edges of the Mudstone Tongue where it pinches out represent an ancient shoreline of Fossil Lake. To the north and west the Mudstone Tongue merges with the Tunp Member of the Wasatch Formation.