Named for its thickest exposed section which is 10 km west of Horse Valley in secs 20 and 21, T31S, R12E, Iron Co, UT in the Great Basin province. These exposures are designated the type section. Neither the base nor any complete section of the Horse Valley is exposed. Consists of 35+ m of volcanic mudflow breccia and at least 350 m of overlying intermediate lava flows at type. Is capped at type by a thin white air-fall tuff not included in Horse Valley. Rocks of the flows and clasts in the breccia are the same rock--that is a gray to pink rock with about 5 percent small phenocrysts of plagioclase and biotite in an aphanitic to glassy groundmass. Formerly mapped as two unnamed units. Is confined to the western Black Mountains, Iron Co. Postdates the Mount Dutton Formation. Is older than Mount Belknap Rhyolite. Three K-Ar dates--19.0 +/-0.6, 20.6 +/-0.4, and 21.9 +/-0.4 m.y.--confirming a Miocene age. Correlation chart; block diagram; measured sections.

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