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. 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 the 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.

Credits and Sources:

“Fossil Butte: The Geologic History of Fossil Butte National Monument and Fossil Basin,” National Park Service, http://www.nps.gov/parkhistory/online_books/fobu/sec1.htm, Accessed on June 29, 2015.