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Geology and Physiographic Description' <br />Holbrook Lake Ditch watershed is located within the Colorado Piedmont section of the <br />Great Plains Physiographic Province (Fenneman, 1931). The Colorado Piedmont <br />represents an old erosion surface. It is a mature to old, broadly rolling, and elevated <br />plain with local scarps. <br />Bedrock consists primarily of Cretaceous marine shales and limestones. These <br />formations dip slightly to the northwest, toward the Denver structural basin. The <br />oldest formation that crops out in the watershed is the Upper Cretaceous Carlile <br />shale, which is found south of the Arkansas River at La Junta. Overlying the Carlile <br />Shale (from oldest to youngest) is the Fort Hays limestone and Sm.okey Hill shale <br />members of the Niobrara Formation, and the Pierre Shale. <br />Shales and limestones have higher concentration of some minerals than other rock <br />types have. This is particularly true of minerals such as sulfur and trace minerals <br />such as arsenic, boron, and selenium (Turekina and Wedepohl, 1961). <br />Studies by Schultz and others (1980) also showed elevated sulfur and trace mineral <br />concentrations in studies done of the Upper Cretaceous Pierre shale and equivalent <br />formations. The sediment source areas for these formations were to the west. The <br />watershed area is far from the source area, so sediments are almost exclusively fine- <br />grained marine shale and muddy limestone. As the amount of clays increase with' <br />'Check in reference section for geology reports for the area. <br />99 <br />