Named as lowermost of three formations of Thunderbird Group (rank raised). Derivation of name not stated. Type locality is about 3000 ft northeast of Smugglers Pass in Fusselman Canyon, latitude 31 deg 53' 06" and longitude 106 deg 29' 34", El Paso Co, TX. Outcrops between North and South Franklin Mountains in southern part of Franklin Mountains, Southwestern Basin-and-Range region. Conglomerate consists of well-rounded cobbles and pebbles of quartzite, quartz-sericite siltstone, shale, chert, jasper, ignimbrite, porphyritic trachyte and rare marble and granite in recrystallized matrix of sand-sized quartz, alkali feldspar, devitrified pumice and glass shards. Thickness ranges from 35 to 90 ft. Unconformably overlies Lanoria Quartzite; underlies Smugglers Pass Formation (new) of Thunderbird [contact relationship not specified]. Geologic map. Stratigraphic column. Late Precambrian age (950 m.y. old).

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