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<br />-,,-(1 <br />1J;j...~1 <br /> <br />y <br /> <br />2 <br /> <br />GENERAL GEOLOGY <br /> <br />Cretaceous Pierre Shale forms the bedrock tor the entire area <br /> <br />of the dam, reservoir, and surrounding territorf. Structurally it is <br /> <br />a~st flat-lying b~t the uppe~ surface is quite irregular due to long <br /> <br />continued stream erosion. ~e Pierre Shale is a sandy siltstone, quite <br /> <br />compacted, and measuring several thousa.'1.d feet in total thickness. The <br /> <br />shale varies consider3bly in qrain size and texture as might be expected <br /> <br />in. a sediment deposited under ncn-marL-le conditions, at least in the <br /> <br />upper part of the formation. <br /> <br />Unconformably overlying the Pierre Shale is a series of PIe is to- <br /> <br />c~~e ar.d 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 Sloil profiles and correlated with known glaciations in the ~cky <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 f!'om Kiowa and Bijou Creeks. The result is an alluvit1m- <br /> <br />filled buried channel a few miles south of the South Platte River. <br /> <br />This buried channel serves as an excellent aquifer and will permit <br /> <br />some water to by-pass the ,proposed Narrows DaI:l. '!'he control of water <br /> <br />moving through this buried channel is perhaps the most critical it~ <br /> <br />in the design of the dam and cutoff wall. <br /> <br />As the climate changed during late Pleistocene and Recent <br /> <br />times, several blankets of windblown soil were deposited. 'i:'hese are <br /> <br />"- <br /> <br />~ <br />