*Arkansas Valley Bouldering West

Rock, from the geologic map

Fan deposits. Quaternary, younger than 126,000 years.

Mostly unsorted and unstratified, matrix- supported bouldery debris-flow deposits that consist of granule- to boulder-size clasts in a sandy, silty sand, or finer-grained matrix. Unit locally includes poorly to moderately well sorted, crudely stratified, clast-supported, stream-deposited sand and gravel. These mass-movement and alluvial deposits commonly form fan-shaped landforms at the mouths of small tributaries streams. Locally these deposits coalesce to form a broad, relatively gently sloping apron. Clasts are mostly subangular to subrounded; some are as long as about 2 m, but most are much smaller. Unit Qf is commonly graded to levels at or slightly above present stream level. Depositional processes were periodically active during the Holocene, but probably were most active during Pinedale deglaciation. Surface is locally veneered by unmapped deposits of sheetwash alluvium and probably by hyperconcentrated-flow deposits. Fan deposits on the east side of the Arkansas River, in the southern part of the map area, are locally mantled by unmapped deposits of eolian sediments and alluvium undivided (Qes). Estimated thickness is as much as 15 m

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. The rock named at the top of this page comes from the public geology of this area; this card is the map's answer. Every crag on this unit: Fan deposits. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.