Roadside

Rock, from the geologic map

St. Kevin Granite (St Kevin Granite). Mesoproterozoic, about 1,600 to 1,000 million years old.

Medium- to coarse-grained, gray to light-pinkish-gray and pinkish-brown, euhedral equigranular to porphyritic, massive to locally weakly foliated biotite-muscovite monzogranite, with local gradational changes to granodiorite and other textural variations (Pearson and others, 1962; Tweto and Pearson, 1964; Tweto, 1974, 1987). The main batholith is characterized by a central region of coarse-grained monzogranite grading outward into medium- grained monzogranite and fine-grained foliated monzogranite and granodiorite with seriate-to-porphyritic texture commonly having Carlsbad twinning in aligned microcline phenocrysts. Within the map area, medium-grained, euhedral monzogranite is the common phase of the main plutonic body. Silicified, quartz-rich zones are mapped along the central-eastern portion of the map (Tweto, 1974) and indicated by dark blue, solid lines within unit Ygs. Rb-Sr isochron age is 1,390±60 Ma (Pearson and others, 1962), and a more recent U-Pb age from outside the quadrangle is 1,442±14 Ma (Moscati and others, 2017). Hydrothermally altered St. Kevin Granite, unit Ygs(a), bounds the north and south sides of the central region of Turquoise Lake along a generally north-trending fault zone. Farther north along the same north-trending fault zone, sheared and brecciated St. Kevin Granite, Ygs(s), is mapped between Porcupine and Longs Gulches and along the ridge between Homestake Reservoir and East Fork Homestake Creek

This is the map unit under this crag's map point, read from the 1:100,000 sheet (Ruleman and others, 2020, U.S. Geological Survey Scientific Investigations Map 3451). 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: St. Kevin Granite. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.