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<br />(} <br /> <br />( <br /> <br />l. <br /> <br />..... <br />('- <br />" <br />r-- <br /> <br />7 <br /> <br />c.. <br />t::._ 4. <br /> <br />SITE GEOLOGY <br /> <br />(\ <br /> <br />Of primary interest to development at the site are the recent <br />Quaternary deposits and the near surface Cretaceous age Mancos <br />shale (Km). Quaternary deposits at the site range in thickness <br />from 2 to 31 feet. These in turn are underlain by Mancos shale <br />which ranges in thickness from 500 feet or less in the southern <br />part of the site to 1,400 feet thick in the northern part of the <br />site. The Mancos shale in the area is reported to be dipping <br />north to northwest at 20 to 50 (CGS, 1982). <br /> <br />Exposures of Mancos shale on-site are quite weathered and <br />fissile. Therefore, mapping of dip magnitudes and direction was <br />quite infrequent and suspect. As shown on Plate 1, the Mancos <br />locally dips north to northwest from 50 to 200. The Uncompahgre <br />Uplift, just south of the site, is thought to be responsible for <br />the regional and localized changes in dip direction of the Mancos <br />as well as the large regional anticline whose axis runs through <br />the site (Figure 3). <br /> <br />The following sections describe in more detail the features of <br />each stratigraphic unit and their significance for development of <br />the site. <br /> <br />4.1 Surficial Geology <br /> <br />There are four primary_surficial materials encountered at the <br />site. The location of these materials is shown on an airphoto <br />covering most of the site presented as Figure 5. Each of these <br />materials are described below: <br /> <br />( <br /> <br />o Quaternary Pediments (Qp)--consist of light brown to <br />yellow brown silty and clayey sands, gravels, cobbles, <br />and boulders. As shown on Plate 1, these deposits <br />comprise most of the surficial soils on-site. On the <br />west side of the site they form a north-south ridge, <br />while on the east side they form a resistant capping for <br />two major bluffs (Figure 6). As shown by Plates 2 and 3, <br />the pediments are discontinuous across McDonald Creek. <br />The Colorado Geological Survey (1982) indicates the <br />pediments on the easternmost bluff are Pre-Wisconsin, <br />while the pediments to the west are younger yet <br />undifferentiated. <br /> <br />In all areas the sands and gravels are typically capped <br />with a thin veneer (0 to 6 feet) of light brown to red <br />brown silt and clay thought to be wind blown (CGS, 1982). <br />Their thickness ranges from 5 to 31 feet across the site <br />generally being thin near the drainages on the west side <br />of the site, somewhat irregular in the middle eastern <br />part of the site, with the Pre-Wisconsin pediments being <br />fairly uniform in thickness on the eastern border. <br /> <br />c <br />