Named for exposures in Summerhouse Canyon, Lemhi Range, ID. Type section measured on east wall of canyon in SE1/4 SE1/4 sec 35, T12N, R25E, Gilmore quad, Custer Co, ID in Idaho Mountains province. Is composed of brownish-red to reddish-gray, glauconitic, Scolithus-bearing, calcareous to dolomitic sandstone, locally includes red to gray shale and quartzite. Is 1003 ft thick at type. Generally up to 1,000 ft thick. Overlies Precambrian Y Swauger Formation with angular unconformity at type; overlies Precambrian Z Wilbert Formation (new) from Foss Mountain south to South Creek. Underlies Middle Ordovician Kinnikinic Quartzite with angular unconformity at type. Present in southern half Lemhi Range, locally in Lost River Range, and in Beaverhead Mountains. Has Lower Ordovician conodonts in lower part.

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