Space Boulders

Rock, from the geologic map

Biotite gneiss. Paleoproterozoic, about 2,500 to 1,600 million years old.

Gray, medium-grained, equigranular, well-foliated gneiss typically containing approximately 25–50 percent quartz, 20–30 percent plagioclase (approximately An30), 0–30 percent microcline, 10–15 percent biotite, 0–15 percent muscovite, 0–10 percent sillimanite, 0–5 percent hornblende, 1–2 percent opaque minerals, and a trace zircon. Most outcrops contain 5–20 percent granitic layers (leucosomes). Sillimanite, where abundant, occurs in fibrous, elongate, light-colored aggregates or clots as wide as about 1 cm. Migmatitic biotite gneiss in Clear Creek gave preliminary U-Pb zircon date on nonmelt phase of 1,773±18 Ma; near Pine, just south of map area, similar rock gave U-Pb zircon date of 1,780±9 Ma (W.R. Premo, unpub. data, 2005)

This is the map unit under this crag's map point, read from the 1:100,000 sheet (Kellogg and others, 2008, USGS Geologic Investigations Series 3000). The point can be a few hundred metres from the rock, a cliff often stands on the line between two units, and at that scale a line on the map is about 100 m wide on the ground, so the wall itself may be the neighbouring unit. Every crag on this unit: Biotite gneiss. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.