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Geoteehnical Evaluation of Mlne-Induced Seismicity on Monument Dam Dorsey 8 Whitney, LLP Section 3 -Geology 3.1 Regional Geology Monument Dam and Reservoir is located on the eastern edge of the Colorado Plateau physiographic province, adjacent to the Southern Rocky Mountain province. The site is between the Colorado Grand Mesa and the West Elk Mountains. Structurally the site is within the Piceance Basin. Bedrock units generally dip to the north-northeast and east at less than 5 degrees. Stratigraphically, the area is contained near the contact between the Late Cretaceous Mesaverde Group and the Mancos Shale. Most of the present day topography at the site was established by the late Quaternary time (10,000 years ago). The site topography is influenced by glaciation and subsequent surface erosion. At least three ages of glaciation influenced the present topography and provided substantial valley-fill material. 3.2 Site Geology • The following paragraphs describe the area surrounding the dam and reservoir and are based on documents provided as part of this evaluation. A geologic map of the site is provided on Figure 3.1. Most of the surficial soil deposits were derived from the bedrock formations either as residual weathered deposits or slope-wash and landslide deposits. The slope-wash material is generally fine grained material and rock fragments that has been transported and deposited by flowing water. Landslide deposits may contain a wider variety of particles due to the dovmward movement and mixing of the material. The stratigraphy of the valley where the dam is located primarily includes poorly consolidated siltstone representative of the gradational contact between the Mancos Shale and the overlying Mesaverde Formation, as shown on Figure 3.2. The Mancos Shale varies in color from dark brown to gray and is derived predominantly from the deposition of marine silts that are easily eroded and prone to landslides and mass wasting. The Mesaverde Formation transitions from mostly dark brown to gray siltstone near the contact with the Mancos Shale, to light gray to white sandstone, near the top of the formation. Commercially viable coal seams occur in the lower portion of the Mesaverde Formation above the Rollins Sandstone Member. The sandstone layers occurring near the project site are more resistant to erosion than the shales and create over-steepened slopes that are undermined by the softer shales. GEI~ 10 0603500fiA1-l6Monumm[OamGmmdwcdEvil March 2006