Topaz rhyolite flow of Nathrop Volcanics, including basal vitrophyre
Basalt & volcanic. Rupelian, about 34 to 28 million years old. 1 crag on this site has its map point on it.
What the map says
Light-gray, light pinkish-gray, and purplish-gray, strongly flow-layered alkaline rhyolite flow forming prominent outcrops, about 8 km south-southeast of Buena Vista at Ruby Mountain, Sugarloaf Mountain, and upper part of Bald Mountain (Keller and others, 2004). Layering is moderately to strongly folded. The rhyolite consists of small, sparse phenocrysts of smoky quartz, sanidine, plagioclase, and rare biotite in a dense microcrystalline groundmass; accessory minerals are apatite, zircon, magnetite, garnet, and topaz (Keller and others, 2004). Locally contains spherules as much as 10 cm in diameter, and lithophysae as much as 6 cm in diameter, particularly on Bald Mountain. Lithophysal cavities locally contain euhedral crystals of spessar- tine garnet and topaz, some of gem quality (Voynick, 1994). The basal vitrophyre is glassy, composed mostly of light-gray, brownish-gray, or black perlite with subordinate pitchstone. The perlite typically shows an “onion-skin” structure. One 40Ar/39Ar age on plagioclase on Bald Mountain is 29.25±0.12 Ma, and three 40Ar/39Ar ages on sanidine at Ruby Mountain are 30.47±0.08 29.25±0.21Ma, and 30.74±0.08 Ma (McIntosh and Chapin, 2004; ages corrected for revised standards of Kuiper and others, 2008), indicating early Oligocene age (McIntosh and Chapin, 2004). Similarity of age and composition (for example, presence of accessory topaz) with evolved Mount Antero leucogranites suite of Sawatch Range suggests genetic connection (Shannon, 1988). On Bald Mountain, the rhyolite is estimated to be about 150 m thick and was depos- ited within a paleovalley that also contains remnants of older Wall Mountain Tuff and Tallahassee Creek Conglomerate (Keller and others, 2004)
1:100,000 sheet: Kellogg and others, 2017, U.S. Geological Survey Scientific Investigations Map 3382 (1 crag). A contact line at that scale is about 100 m wide on the ground.
Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.