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I :. . . . . . . . . I I . . I I I I I A large part of the present geomorphology is the result of glacial activity. During the last Pleistocene ice age. Beaver Creek drainage was occupied by a valley glacier which advanced eastward down the drainage. During the advance. the glacier eroded and/or modnied the overburden depos~s and scoured the bedrock surface. eroding the more weathered and weaker portions of the rock, The overburden depos~s which were overridden by the glacial lee were compacted by the weight of the ice. As the glacier retreated, debris carried by the glacier was depos~ed in the valley e~her directly by the glacier or by melt waters emitted from the glacier, The materials depos~ed directly by the glacier lack the sorting and bedding characteristic of the stream deposits. The previously mentioned terminal moraine marks a location where the glacier stagnated during retreat, resulting in a thicker accumulation of glacial deposits. Subsequently, the glacier continued ~s retreat up the drainage, During the retreat and wasting of the glacier, blocks of ice became detached from the main body of ice. becoming isolated in the outwash plain. Sediments were deposited around these Ice blocks, Eventually, the ice meted, forming depressions in the plain which are referred to as kettles, Small kettle features are present in the area west of Hourglass Reservoir, It Is likely that a natural ancestral lake at the location of Hourglass Reservoir was formed in a kettle feature. With the exception of the entrenchment of Beaver Creek and the construction of Comanche and Hourglass Reservoirs, the valley geomorphology has probably changed little since the retreat of the glacier. 5.3.1.2 Bedrock Bedrock Is exposed at the surface or is interpreted to be at a shallow depth on the steeper slopes of the valley walls. The bedrock is identified on the Preliminary Geologic Map of the Greeley 1. x 2. Quadrangle (Open File Report 78-532) as Precambrian metamorphic rock, felsic and homblendic gneiss including metabasalt, metatuff, and interbedded metagraywacke with IocaJ interlayered biotite gneiss. Bedrock is exposed near the southern side of Hourglass Reservoir. The U,S,G,S. map shows a fault in the bedrock at the bottom of the valley to the southwest of the reservoir. This fault is shown as a splay from a relatively long (about 50 miles) fault that passes near Comanche Peak and has a nearly east-west strike. The fault splay is inferred to extend beneath the glacial deposits on the southeast side of the reservoir, 5-7