Named as a formation (1 of 5) in Phantom Lake Metamorphic Suite in the Medicine Bow Mountains for exposures (its type locality) on south and west sides of Rock Mountain, secs 27 and 33, T19N, R79W, Carbon Co, WY in the Northern Rocky Mountain region. Crops out on Rock Mountain and near the northwest limit of Precambrian outcrop in secs 27 and 33, T19N, R79W. Is absent in Stud Creek area although similar conglomerates occur as lenses in Stud Creek Metavolcaniclastics (named). Petrographic data. Stratigraphic charts. Geologic map. Is predominantly a granular to pebbly muscovitic quartzite and quartzose schist with paraconglomerate beds one meter to several hundred meters thick. Paraconglomerate contains stretched clasts of quartz, quartzite, amphibolite, and schist in a strongly foliated quartz-muscovite matrix, has striking green fuchsitic schist clasts, and is locally garnetiferous. Contact with older Stud Creek Metavolcaniclastics not exposed; intertonguing suggested. Estimated as 400 m thick. Gradational contact with overlying Bow Quartzite (named). Thought to have been a prograding alluvial fan deposit. Coarsens to east and upwards in formation. Of Late Archean age.

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