Named after a valley partly surrounded and largely underlain by lavas about 8 km southwest of Mammoth, Yellowstone National Park, WY in the Yellowstone province. Type area designated as Sheepeater Cliffs, on east side of Gardner River Canyon southeast of Bunsen Peak, northeast part of the Park. Is light gray to moderate gray. Contains sparse to abundant large phenocrysts of plagioclase, as much as 1 cm across, and locally some olivine phenocrysts. Is nearly 200 m thick east of Sheepeater Cliffs. Age suggested by stratigraphic relations. Is conformable on the middle Pleistocene Lava Creek Tuff. Is older than the canyon cutting that predated Osprey Basalt. Is part of the third volcanic cycle of the Yellowstone Plateau. Stratigraphic table; cross section.

Source: GNU records (USGS DDS-6; Denver GNULEX).