*Arkansas Valley Bouldering North

Rock, from the geologic map

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

Unit commonly forms large, very bouldery, sharp-crested lateral and end moraines that have distinct hummocky surface morphology. Deposits in some areas have well expressed knob-and-kettle topography. Some of the surface boulders of granitic and gneissic composition are as large as ≥4.5×5.5×7.0 m. Most of the biotite-rich granitic and gneissic clasts within the soil are unweathered or slightly weathered. Youngest mapped till of Pinedale glaciation within cirques or near valley heads locally may include till similar in age to that of the Satanta Peak advance of Benedict (1973b, 1985), dated at about 10–12 ka (Davis 1988). Soils formed in the top of the unit Qtp commonly have thin (≤25 cm) cambic B horizons that are poorly developed and have little or no clay accumulation. Some soils lack B horizons and have A/Cox or A/Bk profiles (Nelson and Shroba, 1998; Shroba and others, 2014). Depth of oxidation (base of Cox soil horizon) ranges from about 50 to 170 cm. Unoxidized till is gray, whereas soil B horizons commonly are light brown. Locally, as much as 15 percent of the biotite-rich pebbles and small cobbles within soils are partly or completely disintegrated (Shroba, 1977). Locally, multiple recessional moraines of Pinedale age were formed in glaciated valleys, including Cottonwood Creek and North Cottonwood Creek. 10Be cosmogenic surface-exposure dating (aver- age ages) of exposed boulders and pebbles on moraine crests on the south side of Pine Creek suggests two episodes of glacial advance: one at 22.0±1.4 ka and a later advance at 15.2±0.9 ka (Briner, 2009). Recently reported 10Be cosmogenic surface-exposure ages of exposed boulders on crests of outer and inner moraines composed of till of Pinedale age (units Qtpo and Qtpy of Shroba and others, 2014, respectively) on the southeast side of the Twin Lakes Reservoir (Schweinsberg, and others, 2016) are similar to those of a possible early advance at Pine Creek (Briner, 2009). Surface-exposure ages of boulders on the outer moraines on the southeast side of the Twin Lakes Reservoir range from 21.1±0.3 ka to 22.2±0.3 ka and average 21.6±0.3 ka. Ages of boulders on nearby inner moraines range from 21.1±0.3 ka to 22.2±0.3 ka and average 21.7±0.3 ka (Schweinsberg, and others, 2016). The surface-exposure ages reported for Pine Creek and near the Twin Lakes Reservoir are in accord with (1) radiocarbon and cosmogenic surface-exposure ages for till of Pinedale age in the Front Range region of north- central Colorado of about 31–12 ka (Nelson and others, 1979; Madole, 1986; Benson and others, 2004, 2005; Ward and others, 2007; Madole, 2010); (2) retreat of Pinedale glaciers in the Front Range and San Juan Mountains, Colorado between about 19 and 12 ka (10Be surface-exposure ages of Guido and others, 2007; Ward and others, 2007); (3) last glacial maximum in the Taylor River drainage, about 18 km west of map area, of about 21–16 ka (Brugger, 2007), and a “late advance” in the Taylor River drainage of about 14.5 ka (10Be and 36Cl cosmogenic ages of Brugger, 2007); (4) 10Be and 36Cl surface-exposure ages of 23–16 ka for deposits of Pinedale age in type area for till of Pinedale age in Wyoming (Chadwick and others, 1997); and (5) 10Be and 3He surface- exposure ages of about 18.8–13.5 ka for till of Pinedale age in the Yellowstone regions and Teton Range (Licciardi and Pierce, 2008). 10Be surface-exposure ages of glacier- abraded bedrock within the cirque at the head of Rock Creek, just south of Turquoise Lake, suggest that deglaciation of the northern Sawatch Range occurred by about 15–13 ka (Mason and others, 2011; Ruleman and others, 2013). Estimated thickness is as much as 40 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: Till of Pinedale glaciation. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.