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Evaluation of Bear No. 3 Mine Landslide <br />• A review was made of CDMG photographs and files on January 13, 1998 by Mr. Rold. <br />• This report was prepared to sunttnarize the data obtained and present our conclusions. <br />GEOLOGY <br />' The Soterset area lies on the southwest limb of the Piceance Basin. Regionally, the Mesaverde <br />bedrock formations dip gently at 3 to 5 degrees to the northeast. The 2,500-foot-thick Mesaverde <br />formation consists of alternating sandstones, shales, and mudstones with several economic coal <br />beds occurring in its lower part. The competent 100- to 400-foot thick Rollins Sandstone marks <br />the base of the formation. Varying thicknesses of colluvium cover much of the bedrock, <br />particulazly on north-facing slopes. Alluvial sands and gravels occur along major streams such <br />as the North Fork of the Gunnison River (North Fork) as well as sporadically along smaller <br />tributaries. <br />Landslides, debris flows, and unstable slopes have occurred on the steep slopes of the North Fork <br />valley for many years and date back to the Ice Age (Pleistocene) 10,000 years ago. The <br />Colorado Geological Survey (CGS) noted the problems in the early 1970s. Rabe Junge of the <br />CGS addressed the stability problems in a regional sense and published geology and hazards <br />' maps of the area as OF-78-4 and OF-78-12 in 1978. His maps show most of the north-facing <br />slopes of the North Fork valley near Somerset to be covered by landslides, slope failure <br />complexes, and debris flows of various ages. <br />The instability of the area has plagued the Colorado Department of Transportation with major <br />slide problems east of the West Elk Mine, at Muddy Creek above Paonia Reservoir, and on <br />' McClure Pass. Slope stability has also caused many serious problems for mines and agricultural <br />ditch companies. The slide activity is correlated to precipitation, with greater activity in wet <br />years followed by near dormancy in the dry years. Likewise, slides usually initiate or reinitiate <br />movement in the spring, particulazly afrer a winter of heavy snowfall Movement usually slows <br />831-032.411 Wright Water Engineers, Inc. Page 2 <br />