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• shades of gray and blue gray (Every, 1998). <br />The Granite Cobbles form the basal overburden unit. Although Figure H3 shows the Granite Cobbles to <br />be a fairly thick and uniform bed, boring logs, exposures, and verbal communication (Every, 1998) indicate <br />that the unit is variable in thickness, distribution, and composition. Much of the cobble zone may, in fact, <br />be quite thin or missing and/or mixed with a clay or other fine-grained matrix. The cobbles are an alluvial <br />deposit of the ancestral North Fork River. <br />Bedrock under the entire area is the Cretaceous Mesaverde Formation. The rock bordering and underlying <br />the slide is predominately sandstone with interbeds of claystone, siltstone, and thick beds of coal. The rock <br />is variably jointed and fractured, particularly in the coal beds. The bedding in the vicinity of the slide dips <br />less than 5 degrees to the northeast. A buried mound of badly fractured rock separates the deepest scour <br />into the buried bedrock terrace from the North Fork River valley. <br />Colluvium. Colluvium is defined by Bates and Jackson (1984) as: "A general term applied to loose and <br />• incoherent deposits, usually at the foot of a slope or cliff and brought there chiefly by gravity." Other <br />references (I-Iunt, 1984 and Junge, 1978) expand Colluvium to include everything from small rock falls to <br />massive landslides and debris flows. The term Colluvium therefore can be used very broadly and the use of <br />the classification for material below the slide plane and above the granite cobbles seems generally <br />acceptable. However, this classification is too simple to explain the possible differences between the <br />Upper and Lower Colluvium. The main shear zone for the lower slide appears to coincide with the contact <br />between the Upper (brown) and Lower (gray) Colluvium. Color contrasts, due to oxidation in a thick <br />geologic unit, are common. Because oxidation generally takes place above the water table, the contact <br />between an upper oxidized (brown) zone and a lower unoxidized (gray) zone normally represents the low <br />groundwater level during the period of oxidation. In addition, a zone of water-table fluctuation is typically <br />mottled brown and gray. However, if the water table rises after a zone is oxidized, the color stays brown <br />and does no[ return to gray. <br />Frequently, the oxidized material keeps all of the physical characteristics of the unoxidized material. <br />However, the oxidized zone may develop cracks due to desiccation which allows the infiltration of water, <br />and may result in a gain or loss of strength due to drying, cementation or leaching. Such types of changes <br />Barr Enginxring Company July 31, 1999 <br />PCDOCS1208220/CAK 3 Appendix H <br />1999 Addendum to <br />1997/98 Landslide Corrective Measures Report <br />