B-Lot Boulder

Rock, from the geologic map

Till of Pinedale age. Late Pleistocene, younger than 126,000 years.

Mostly nonsorted and nonstratified, subangular to subrounded boulders to granules in a silty sand matrix. Material less than 2 mm in diameter is estimated to be 20–40 percent of unit. This material consists chiefly of poorly sorted sand and ≤20 percent silt and ≤5 percent clay. Unit commonly forms large prominent, sharp-crested lateral and end moraines that are very bouldery and have distinct constructional morphology. Deposits in some areas have well-expressed knob-and-kettle topography. Surface soils have A/Cox profiles on moraines with narrow crests and A/Bw/Cox and A/Btj/Cox profiles on moraines with broad crests. Surface and near-surface O and E soil horizons are locally present. Cambic (Bw) and weak argillic (Btj) horizons are thin (10–40 cm) and commonly contain 1–5 percent more clay than the underlying till (<2 mm size fraction). Most of the biotite-rich granitic and gneissic clasts within the soil are unweathered, and disintegrated clasts are rare. Unit locally includes deposits of stratified drift, mass-movement and glacial deposits, undivided (Qmg), tills of the Satanta Peak and Triple Lakes advances of Benedict (1985), about 12–10 ka (Davis, 1988), till of Bull Lake age (Qtb), colluvium (Qc) and other mass-movement deposits, and valley-floor alluvium (Qva); locally may include till of pre-Bull Lake age. Radiocarbon (14C) and cosmogenic-exposure ages indicate that till of unit Qtp is about 30–12 ka (Nelson and others, 1979; Madole, 1986; Schildgen and Dethier, 2000; Benson and others, 2004, 2005). Subsurface deposits and locally some surface deposits of unit Qtp may be older than 30 ka, because uranium-series ages of travertine in the northern Yellowstone area suggest an early advance of Pinedale ice about 47–34 ka (Sturchio and others, 1994). Estimated thickness 1.5–30 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: Till of Pinedale age. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.