Named for extensive exposures in the Casto quad in Lemhi, Valley, and Custer Cos, ID in the Idaho Mountains province. These volcanics extend eastward and northeastward from the Casto quad. The exposures cover an area of about 200 sq mi. No type locality designated. Consists of altered lava flows, largely brecciated, and tuffs. The lava ranges from sodic andesite to dacite. Some beds are more calcic, and others more silicic. Conglomerate of pebbles, mostly derived from Paleozoic sedimentary rocks in a tuffaceous matrix, is present at base locally. Thickness unknown, probably exceeds 3,000 ft. Contact relationships not discussed. May be equivalent to volcanics in southwest Nez Perce, and western Adams and Washington Cos, ID that are of Permian age. Assigned to Permian on basis of tentative correlation.

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

Crop out over about 231 sq mi in quad. Have not been traced far beyond southern and western boundaries of quad, Custer Co., ID in Idaho Mountains province. Are composed of water-laid tuff, andesitic and rhyolitic rocks. The rhyolitic rocks are fine grained and chert-like. The andesite rocks are sodic andesite and dacite. Some coarse conglomerate of subangular to rounded pebbles 1 to 4 inches diameter of metamorphosed sedimentary and volcanic rocks in a tuffaceous matrix are present locally. One white siliceous limestone bed 200 ft long and 20-40 ft wide occurs near summit of Cache Creek. Is about 4,000 ft thick in quad. Is younger than an unnamed Ordovician limestone. Is older than Challis volcanics. Considered to be of Phosphoria or Permian age. Shown as Permian? on geologic map.

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

The Casto Volcanics as defined by Ross (thought to be older than Challis because they were altered) are assigned to the Challis Volcanics in this report on Valley and Custer Cos, ID in Idaho Mountains province. The "Casto" rocks were altered by the Tertiary granite that intrudes them. Only broad gradations exist between altered and unaltered rocks, mappable boundaries are not present. Name "Casto Volcanics" abandoned.

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