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<br />2 <br /> <br />GENERAL GEOUOGY <br />Cretaceous Pierre Shale forms the bedrock for the entire area <br /> <br />of the dam, reservoir, and surrounding territory. Structurally it is <br /> <br />almost flat-lying but the upper surface is quite irregular due to long <br /> <br />continued stream erosion. The Pierre Shale is a sandy siltstone, quite <br /> <br />compacted, and measuring several thousand feet in total thickness. The <br /> <br />shale varies considerably in grain size and texture as might ~e expected <br /> <br />in.a sediment deposited under non-marine conditions. at least in the <br /> <br />upper part of the formation. <br /> <br />Unconformably overlying the Pierre Shale is a series of Pleisto- <br /> <br />cene and Recent alluvial deposits confined, at least in this area, to <br /> <br />the present and former courses of the South Platte River. These <br /> <br />deposits have been dated on the basis of continental fossil remains <br /> <br />and soil profiles and correlated with known glaciations in the Rocky <br /> <br />Mountains. <br /> <br />In the immediate area of the Narrows Dam site, the South Platte <br /> <br />River has been pushed northward out of its former channel by huge <br /> <br />alluvial fans from Kiowa and Bijou Creeks. The result is an alluvium- <br /> <br /> <br />filled buried channel a few miles south of the South Platte River. <br /> <br /> <br />This buried channel serves as an excellent aquifer and will permit <br /> <br /> <br />some water to by-pass the proposed Narrows Dam. The control of water <br /> <br />moving through this buried channel is perhaps the most critical item <br /> <br />in the design of the dam and cutoff wall. <br /> <br />As the climate changed during late Pleistocene and Recent <br />times, several blankets of windblown soil were deposited. These are <br /> <br />"-- <br />