Named after Horse Mountain, 2 mi north of Aravaipa, Graham Co, AZ, Basin-and-Range province. No type locality designated. Extends along southwest side Turnbull and Santa Teresa Mountains. Isolated outcrops in Klondyke Wash. About 9.7 percent of quad underlain by Horse Mountain. A heterogeneous assemblage of tuff and agglomerate ranging from andesite to rhyolite and small amounts of conglomerate and breccia. Rhyolitic and dacitic rocks predominate. Lenticular red conglomerate 100-180 ft thick at base in old Deer Creek and on Horse Ridge. Andesites include dark gray coarsely porphyritic, and gray and purple porphyritic lavas and tuffs. Lavas have phenocrysts of plagioclase and brown biotite. Groundmass minerals are sodic plagioclase, iron oxide, and apatite. The thickness is unknown but may be well in excess of 4,000 ft. Fossils possibly from the lower part of formation indicate a pre-Tertiary age. Unconformably overlies the Williamson Canyon Volcanics; unconformably overlain by the Hell Hole Conglomerate. May be correlative with Galiuro Volcanics. Assigned a Mesozoic and Cenozoic age. Geologic map.

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