Fork Boulder

Rock, from the geologic map

Biotite schist — metamorphic. Paleoproterozoic, about 2,500 to 1,600 million years old.

Conspicuously banded rock marked by alternating layers of contrasting composition; dark-gray to black, medium- to coarse-grained layers are rich in biotite, sillimanite, and magnetite; light- to medium-gray or bluish-gray, fine-grained layers contain more quartz and feldspar than biotite; white, coarse-grained layers consist of quartz and feldspar and minor biotite. Weathers to mottled reddish or yellowish brown. Layering is typically folded into moderate warps and tight flexures and isoclines. Protolith consisted of alternating beds of shale, siltstone, and sandstone. Regional metamorphism at about 1,700 Ma (Peterman and others, 1968, p. 2284) produced characteristic mineral assemblages that vary across the region depending on maximum local temperature and pressure. Northeast of the North Fork Big Thompson River, maximum temperature of metamorphism was as low as 525 degrees C (J.W. Drexler, oral commun., 1986), partial melting was not initiated (migmatite is absent), and the typical prograde assemblage consists of quartz, plagioclase, lepidoblastic muscovite, biotite, staurolite, andalusite, and iron oxides. Throughout most of map area, peak temperatures probably exceeded 700 degrees C (Nesse, 1977, 1984), anatectic melting was widespread (migmatite is ubiquitous), and typical mineral assemblage contains quartz, plagioclase, microcline, sillimanite, biotite, +/- garnet, +/- cordierite, hercynite, and iron oxides. Presence of garnet (rare) and (or) cordierite depends on local bulk composition; prograde cordierite appears to have formed by reactions of the type: biotite + sillimanite = cordierite + anatectic melt. Mineral changes between the lower and higher temperature regions are shown by the red dashed-line boundaries: sillimanite is present west of the S boundary, whereas staurolite and andalusite disappear near this boundary; microcline appears and lepidoblastic muscovite disappears at the K boundary; migmatite appears south and west of the M boundary; cordierite is present south and east of an unmapped, broad zone that extends from Eagle Rock (north of Estes Park) through Beaver Meadows, Mount Wuh, Ptarmigan Mountain, and southwestward to the east end of Granby Reservoir. GRI Source Map ID 141 (I-1973). ** Areas where numerous sills, dikes, or irregular bodies of Silver Plume Granite (Ysp) intrude biotite schist (Xs) [indicated by stippled regions on source paper map]. Areas that contain numerous conformable layers of pegmatite (YXp) [indicated by crossed regions on source paper map].

This is the map unit under this crag's map point, read from the 1:100,000 sheet (National Park Service Geologic Resources Evaluation program, 2006, National Park Service). 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 schist. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.