First published use in St. Francois Mountains, southeast MO, Midcontinent region. [Not clear if authors intended stratigraphic units within study area to be formal or informal.] No type locality designated; derivation of name not discussed. Mapped undivided with Iron Mountain Lake felsite (first used), Wolf Mountain ignimbrite (brief mention), and Ironton Hollow rhyolite (first used) in area between Iron Mountain, St. Francois Co, and Ironton, Iron, Co. Consists of ash flow breccia with intercalated volcaniclastic sedimentary rocks. Contains quartz and plagioclase phenocrysts in generally granular quartz-feldspar matrix; flattened pumice fragments; lithic fragments include rhyolitic ash flow tuff. Thickness is 200 m. In contact with Grassy Mountain ignimbrite. Younger than Iron Mountain Lake felsite; older than Wolf Mountain ignimbrite. Table of stratigraphic units; geologic map. Most volcanic rocks of St. Francois Mountains are 1485 m.y. old (Proterozoic).
[Authors' intentions are unclear as to which, if any, of the geologic units discussed should have formal status.]

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

Mapped (fig. 1) in parts of Iron Mountain Lake and Graniteville quads, central St. Francois Mountains, MO, Midcontinent region. Occurs west of regional angular unconformity. (Rocks east and north of unconformity are older than rocks west of it). Unit is a crystal-poor ignimbrite with abundant recrystallized volcanic xenoliths and matrix recrystallized to silica and feldspar with rare pumice fragments. Maximum thickness is 210 m. Petrologic data. Conformably overlies Iron Mountain Lake Ignimbrite; underlies Ironton Hollow Rhyolite. Proterozoic age (1440-1480 Ma).

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