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I I I I I I I I I I I I I I I I I I I . ~ above the lower outlet works invert. Granite gneiss and migmatite constitute bedrock at the site. It is a high strength rock that is fresh to slightly weathered (some iron staining on joints), and is grey to green. Joints within the rock range from closely spaced (2 in. to 1 ft.) to widely spaced (3 ft. to 10 ft.) and appear tight in outcrop. Topography at the site is joint controlled. A joint survey was conducted at both abutments and the results of the survey were plotted stereographically and are shown on Figures 9 and 10. There are two significant joint sets at the site, both steeply dipping. The primary joint set, which strikes N 450E., controls the rec- tangular channel of Ouzel Creek. The secondary set strikes N 800 E. Soil deposits mantle only a small percentage of the terrain around the reservoir and types and location are shown on the Surficial Geologic Map (Figure 2). Shallow alluvium or stream deposited soils line Ouzel Creek. This material is composed primarily of sand, gravels, and cobbles. Beach deposits, generally sands, gravels, and cobbles have formed on the east shore of the lake due to per- vasive wave action. Lake deltas, or fan shaped soil deposits, have formed at the mouths of the poorly defined inlets. The materials within these deltas are a mixture of sand, clay, and organics with gravel and cobbles. A large talus slope, composed primarily of boulders, lies along the south shore of the lake. Slopewash, com- posed of sand and clay, mantles a small area south of the dam. Engineering Geology Thin arch dams 'work by transferring hydraulic loads into the abutment rock through arching action. Thus, given a coherent arch, it follows that the most important factor in arch dam stability is the abutment and foundation rock characteristics. The only historical failures of arch dams have been attributed -5-