The Roost

Rock, from the geologic map

Granite of Log Cabin batholith, undivided — granite. Mesoproterozoic, about 1,600 to 1,000 million years old.

Granite of Log Cabin batholith—Light-gray, grayish-orange, pale pink, or tan monzogranite to syenogranite that contains 1‒8 percent biotite; texture ranges from equigranular to seriate porphyritic to distinctly porphyritic, and grainsize ranges from fine to coarse. Coarse-grained, porphyritic phase is prominent along the western and southern batholith margins and in the Red Feather Lakes area and displays a flow alignment of microcline phenocrysts that dips steeply outward. Mylonitic foliation is locally present in the vicinity of the Skin Gulch shear zone. Granite of Log Cabin batholith typically weathers tan and light orange-brown and forms a landscape of rounded tors surrounded by grus aprons. Age of intrusion is similar to granite of Longs Peak (YgLP) and other Silver Plume-type granites of the Front Range; U-Pb SHRIMP age on zircon grains from the coarse, porphyritic phase is 1,407±13 Ma, and from the medium-grained, equigranular phase is 1,408±15 Ma (DeWitt and others, 2010; Premo and others, 2012). Along southern margin of batholith, porphyritic and equigranular phases have been mapped separately

This is the map unit under this crag's map point, read from the 1:100,000 sheet (Workman and others, 2018, U.S. Geological Survey Scientific Investigations Map 3399). 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: Granite of Log Cabin batholith, undivided. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.