The Roost

Rock, from the geologic map

Elephant Rock Granite. Mesoproterozoic, about 1,600 to 1,000 million years old.

Gray to grayish-pink, coarse-grained, massive to moderately foliated, porphyritic monzogranite and subordinate granodiorite. In the Harvard Lakes quadrangle, unit contains about 30–35 percent quartz, 25–30 percent plagioclase (oligoclase), 30–32 percent microcline, 6–12 percent biotite, 1–2 percent sphene, and trace amounts of apatite, zircon, and opaque minerals (Kellogg and others, 2013). Microcline phenocrysts are as long as 3 cm. Commonly weathers into distinctive rounded, reddish-brown tors with intervening areas of reddish-brown grus. Forms large batholith east of Arkansas River that extends from the northern Harvard Lakes quadrangle (Kellogg and others, 2013) as far south as the northern Salida East quadrangle (Wallace and others, 1997), a distance of about 42 km. Referred to as “granodiorite” several quadrangles south of Harvard Lakes quadrangle. Unit includes numerous outcrops of similar coarse-grained granite interpreted as blocks incorporated into Langhoff Gulch Granite (Ygl) in South Peak quadrangle. Formerly suspected to be equivalent to Paleoproterozoic Henry Mountain Granite near Gunnison (DeWitt and others, 2010), but a U-Pb zircon age from about 4 km north of Buena Vista in an abandoned train tunnel just south of a pinnacle named Elephant Rock, located atNE1⁄4 sec. 31, T. 13 S., R. 78 W. and composed of Elephant Rock Granite is 1,436±8 Ma (Moscati and others, 2017). A U-Pb date from a roadcut on Highway 285 east of Buena Vista is 1,434±8 Ma (Moscati and others, 2017). The name Elephant Rock Granite is herein formalized and location X (38°52 38"N., 106°08 44"W. on the USGS Harvard Lakes quadrangle) is hereby designated as the type locality

This is the map unit under this crag's map point, read from the 1:100,000 sheet (Kellogg and others, 2017, U.S. Geological Survey Scientific Investigations Map 3382). 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: Elephant Rock Granite. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.