Named as a formation of Sunlight Group (new) of Absaroka Volcanic Supergroup (new) for Mount Wallace (p. C13), Montana folded belt province, MT. Occurs chiefly north of Yellowstone Park, but extends into northwest part of Park, Yellowstone province, WY. Is composed of andesite, trachybasalt, and latite lava flows, flow breccias, rhyodacite-welded ash-flow tuff, and interlayered volcanic sediments. Was called basic andesite breccia and flows in earlier reports. Occurs chiefly north of Yellowstone National Park. Thins to southwest, south, and southeast of type. Includes Slough Creek Tuff Member in middle of formation. Is 2,000 ft thick in type. Is as much as 1,000 ft thick between Slough and Buffalo Creeks where it fills in local depressions in the prevolcanic surface. Is 0-400 ft thick southeast of Slough Creek. Does not occur southeast of Soda Butte Creek. Outliers cap ridges in northwest part of Park and at south end of Gallatin Ridge northeast of Madison Valley. Rests on Washburn Group (new). Is overlain by and partly interlayered with Wapiti Formation. Considered to be of early Bridgerian, middle Eocene age.

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