Entrance Slab

Rock, from the geologic map

Colluvium — colluvium. Quaternary, younger than 781,000 years.

Unconsolidated to weakly consolidated, mostly non-stratified and non-sorted, dark-brown to light-gray-brown deposits that mantle surfaces that commonly underlie slopes less than about 40°. Clasts and finer- grained sediment are transported and deposited mainly by downslope movement. Unit commonly contains angular and subangular granule- to boulder-sized clast with lithologic compositions that reflect that of bedrock and clasts in surficial deposits farther upslope. Unit includes material transported by creep, sliding, debris flow, hyperconcentrated flow, rockfall and rockslide processes. Unit locally includes minor unmapped deposits of stream-channel and sheetwash alluvium. In the southern part of the map area, unit Qc is locally overlain by thin, unmapped, loess deposits as much as 50 cm thick that are composed chiefly of very fine sand and silt. Unit locally includes extensive mass-movement deposits formed by periglacial processes mainly during glacial episodes (chiefly solifluction and ice wedging) in non-glaciated, alpine areas above an elevation of about 3,500 m. The upper part of these latter deposits consists of material derived chiefly from mechanical weathering of blocks and smaller rock fragments and, in part, from the deposition of loess. Surface of periglacial deposits commonly exhibit patterned ground features produced by particle sorting chiefly by near-surface frost sorting and ice wedging. Kellogg and others (2013) mapped extensive periglacial deposits in high country surrounding Buffalo Peaks in the Harvard Lakes quadrangle. Many deposits of unit Qc mapped above treeline throughout the map area probably are composed of or locally include periglacial deposits. At lower elevations, deposits of unit Qc are locally common in open meadows, sagebrush-covered areas, and aspen groves. Estimated thickness is less than 10 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. Every crag on this unit: Colluvium. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.