This unit of the Wasatch Formation is that part of the formation which
produces the spectacular red-colored badlands in Fossil Butte National
Monument. Particularly typical exposures can be seen in the south-facing
scarp of Fossil Butte where the Main Body makes up the lower portion of
the butte.
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.