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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Classification System. The amount of silt contained in the sands varies from <br /> <br /> <br />approximately 1 percent to 15 percent in different areas. Whereas the <br /> <br /> <br />topography of the South Platte River valley is relatively flat, the topography <br /> <br /> <br />of the Sand Hills Formation tends to have many undulating topographic highs <br /> <br /> <br />and lows. The topography is characteristic of sand dunes. <br /> <br />b. Subsurface Conditions <br /> <br /> <br />The subsurface conditions encountered in this investigation were <br /> <br /> <br />generally consistent with the geologic section and logs presented in the <br /> <br /> <br />Woodward-Clyde-Sherard report of 1964. The composite geologic cross section, <br /> <br /> <br />integrating the recently collected and interpreted data, is presented on <br /> <br /> <br />Figure VI-l (see Chapter VI). <br /> <br />The subsurface conditions consist of a deep, buried alluvial channel cut <br /> <br /> <br />into the Pierre Shale and Fox Hills bedrock approximately 160 ft from the <br /> <br /> <br />existing surface or to elevation 4360 ft. The width of this deep alluvium- <br /> <br /> <br />filled channel is approximately 1800 ft. The entire dam axis is underlain <br /> <br /> <br />by bedrock which consists of the Fox Hills Formation and the Pierre Shale <br /> <br /> <br />Formation. The Fox Hills and Pierre shale are in a transitional zone over <br /> <br /> <br />most of the length of the dam axis. This means that the Fox Hills and the <br /> <br /> <br />Pierre Shale Formations are intertongued which complicates identification of a <br /> <br /> <br />single contact between them. The Pierre Shale consists of silty claystones, <br /> <br /> <br />silty sandstones and sandy siltstones which are normally gray or gray-green in <br /> <br /> <br />color. The Unified Soil Classification description of the Fox Hills is brown <br /> <br /> <br />and gray mottled silty clay and clayey silt with occasional lenses of fine <br /> <br /> <br />sand. The Fox Hills also has occasional brown, very hard sandstone layers and <br /> <br /> <br />lenses in some areas. <br /> <br />The flood-plain alluvium consists of a well graded, silty, fine to medium <br /> <br /> <br />sand with some fine to medium gravel. These deposits are normally very dense <br /> <br /> <br />because they were deposited by water. The Sand Hills Formation, which over- <br /> <br /> <br />lies the flood-plain alluvium and, in some places, the Fox Hills Formation, <br /> <br /> <br />consists of a very fine to medium, poorly graded silty sand. These materials <br /> <br />III-17 <br />