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Geotechnical Data Report Brvice Park Dam <br />Bowie Resources, Limited <br />April 2001 <br />Section 6 - Geol <br />6.1 Regional Geology <br />Bruce Park Dam and Reservoir is ]ocaied on the eastern edge of the Colorado Plateau <br />physiographic province, adjacent to the Southern Rocky Mountain province. The site is <br />between the Colorado Grand Mesa and the West Elk Mountains. Structurally the site is <br />within the Piceance Basin. Bedrock units generally dip to the north-northeast and east at less <br />than 5 degrees. Stratigraphically, the area is contained within the Late Cretaceous Mesa <br />Verde Group just below contact with the Tertiary (Paleocene) Wasatch Formation. An <br />unconformity exists at the top of the Mesa Verde Group. <br />Most of the present day topography at the site was established by the late Quaternary time <br />(10,000 years ago). The site topography is influenced by glaciation of the high flat <br />• topography of the adjacent Grand Mesa, but not the typical alpine glaciation indicated by <br />cirques and valleys. Instead, icecaps formed on the mesa and spilled off in all directions into <br />tributaries. The glaciation extended down to elevations of 5,400 feet. <br />At least three probable glaciation periods, that are Pre-Bull Lake in age (greater than 70,000 <br />years ago), mark [he beginning of glaciation on Grand Mesa. The Pre-Bull Lake glacial tills <br />have been completely reworked and incorporated by later glaciation. The time periods of <br />these older tills are recorded by high pediments neaz Debeque and Rifle. <br />A Bull Lake-aged glacial till (28,000 to 70,000 yeazs ago), mapped as the Lands End <br />Formation, at one time covered the entire Grand Mesa and surrounding areas. The maximum <br />extent of ice coverage during the Lands End glaciation has been obscured by later glaciation, <br />but it apparently extended over the reservoir area by inference from the mapping and report <br />by Yeend (1969). The Lands End Formation is often found underlying the younger deposits <br />of the Grand Mesa Formation. <br />A Pinedale-aged till (10,000 to 12,000 years ago), mapped as the Grand Mesa Formation, <br />also covered the Grand Mesa and surrounding aeeas. The area azound the reservoir is <br />mapped as younger till by Junge, (1978), and although he doesn't use a specific formation <br />name, it probably represents the Grand Mesa Formation. <br />LJ <br />GEI Consultants, II1C. t$ 55^.9'<<~'-.4t5 Gectecnmrai ~aN Report <br />