The Georgetown Buttress

Rock, from the geologic map

Young stream-terrace alluvium. Late Pleistocene, younger than 126,000 years.

Glacial outwash composed chiefly of cobbly pebble gravel that underlies one terrace (locally two) along streams in major mountain valleys near and below lower limit of glaciation and fluvial deposits of nonglacial origin along Pole Creek near Tabernash. Amount and size of small boulders (commonly 30–50 cm in diameter) and cobbles in deposits decrease downstream from ice-contact deposits. Pebbly sand or silty overbank sediments less than about 50 cm thick locally overlie the gravel. Fluvial deposits of nonglacial origin north of Pole Creek, about 1 km southwest of Tabernash, consist of slightly cobbly pebble gravel overlain by about 35 cm of slightly pebbly, sandy, clayey silt overbank sediment. Deposits of unit Qg1 in glaciated valleys are composed of gravelly glacial outwash deposited during the Pinedale glaciation, about 30–12 ka (Nelson and others, 1979; Madole, 1986; Benson and others, 2004, 2005). Cosmogenic dating (based on measurement of isotopes 10Be and 26Al) of Pinedale outwash, 4–14 m above stream level in Boulder Canyon east of Nederland, yielded exposure ages of 32–10 ka (Schildgen and others, 2002). Unit underlies terrace treads commonly about 6 and 12 m above the Fraser River between Winter Park and Fraser, about 5 m above the Fraser River near Tabernash, about 5–9 m above streams tributary to the Fraser River, and commonly less 3 than 12 m above stream level elsewhere in map area. Unit locally may include a minor amount of valley-floor alluvium (Qva). Along Swan River, near southwest corner of map area, unit Qg1 includes extensive dredge tailings produced by placer mining during the late 1880s. Unit Qg1 is a source of coarse aggregate. Estimated thickness 3–20 m

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: Young stream-terrace alluvium. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.