Grand Lake

Rock, from the geologic map

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

Subangular to subrounded boulders, cobbles, and pebbles set in a sandy silt to silty sand matrix. Prominent, sharp-crested moraines are numerous; rugged knob-and-kettle topography is well developed in some areas. Gullying is negligible. Moraine-dammed lakes, swamps, and perennial kettle-hole ponds are abundant. Most soils (A and B horizons only) are 45-75 cm thick. Less than 5 percent of stones in B horizon are rotted. Description from Gable and Madole (1976). Patterned lines within unit indicate trend of lateral and terminal moraines. Pinedale glaciation probably began in the region more than 35,000 years ago, and occurred in several pulses. Last major advance of Pinedale glaciers approached its maximum about 22,000 years ago, and these glaciers began to recede from their terminal moraines between 13,000 and 15,000 years ago. Most Pinedale glaciers were probably gone by 10,000-11,000 years ago (age data from Madole and Shroba, 1979, pages 128-129). GRI Source Map ID 141 (I-1973).

This is the map unit under this crag's map point, read from the 1:100,000 sheet (National Park Service Geologic Resources Evaluation program, 2006, National Park Service). 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.