The Grossman Formation is named for exposures near Grossman Ranch in Mill Creek Valley about 1000 feet west of the Owyhee-Mountain City Quadrangle boundary, in Sec.4, T. 45 N., R. 53 E. (Coats, 1969).

The Grossman Formation rests unconformably on the Valmy Formation and is unconformably overlain by the Banner Formation, and was originally considered Late Devonian to Early Mississippian in age (Coats, 1969). Coats (1985) has tentatively assigned the Grossman an Early Mississippian age and suggests a correlation with the Copper Basin Formation of central Idaho.

The Grossman Formation is composed of coarse-grained, gray or green conglomerate, coarse-grained sandstone, and siltstone and minor phyllite. The conglomerate contains tectonically flattened clasts of gray quartzite, chert, phyllite, and magnetite-bearing siltstone, while the coarse-grained sandstone contains dark chert and light quartzite fragments (Coats, 1969). The entire unit is massive and generally fault bounded (Coats, 1985).

The top and bottom of the Grossman Formation are not exposed, however Coats (1969) estimated a thickness of about 2,000 feet in the Mountain City Quadrangle.

The Grossman Formation is exposed at Banner Hill, Mill Creek, and along the east side of the Owyhee River in the Mountain City and Owyhee Quadrangles (Coats, 1969, 1971).

The details of the depositional setting of the Grossman Formation are unknown. It certainly represents clastic deposition as a result of erosion of the "Antler Highland". These sediments may have been deposited as a shallow marine deltaic sequence.