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<br />(, <br /> <br />( <br /> <br />l <br /> <br />t~ <br />OJ <br />['ow <br /> <br />9 <br /> <br />'. ,) <br /> <br />(:...-, <br />,::) <br /> <br />Based on the core drilling of the Mancos shale as described on <br />the borehole logs, the zone of weathering appears to extend <br />between 20 and 60 feet in depth. The shale exhibits gypsum in <br />the form of occasional crystals to veins in the rock fractures. <br />Other than the minor banding and occasional veins of gypsum, it <br />is very uniform in both color and composition. <br /> <br />(, <br /> <br />4.3 Geologic Hazards <br /> <br />The erosional potential of the site is moderate due to the highly <br />weathered nature of the Mancos shale and the scarcity of <br />vegetation, particularly on the slopes of the bluffs and in the <br />draws. This would be most severe during times of peak runoff and <br />severe thunderstorms. As an example-of this, a small stockpond <br />south of borehole C-18 exhibits a fairly narrow (3 to 4 feet <br />wide) and deep (4 to 6 feet) erosional channel adjacent to one <br />end of its embankment that presumably has formed during periods <br />of extreme runoff. <br /> <br />Landsliding and subsidence are not likely to be of major concern <br />at the site. The relatively flat topography and lack of <br />subsurface mining beneath the site preclude this from major <br />consideration. However, on some of the bluffs, boulders of <br />conglomerate that have migrated downhill can be observed and <br />toppling failure of this material can be expected in the future. <br /> <br />Of concern to previous investigators (CGS, 1982), is the <br />potential for a fault or folding within the McDonald Creek area <br />of the site caused by the Uncompahgre Uplift. No known faults <br />underlie the McDonald Creek area, however, several mapped faults <br />and suspicious lineaments occur northeast of the site and trend <br />towards it (Cashion, 1973; Kirkham and Rodgers, 1981; Schwochow, <br />1978). Cross sections (Plates 2 and 3) of the McDonald Creek <br />area derived from the borehole logs and surficial mapping <br />indicate that the pediment deposits are discontinuous across the <br />site and that the surface of the Mancos is curved. <br /> <br />( <br /> <br />As mentioned previously, the pediment deposits on the easternmost <br />bluff have been defined as Pre-Wisconsin and therefore continuity <br />would not be expected. However, it is not clear that the basinal <br />form of the creek and the irregular pediment contacts is a result <br />of erosion or recent faulting or folding. To determine the cause <br />of this basinal form would require a more extensive subsurface <br />investigation using closer spaced boreholes, test pits and <br />possibly geophysical exploration. <br /> <br />c <br />