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zs <br />stratigraphic descriptions in this section are summarized from <br />three graduate theses that contained detailed lithologic <br />descriptions compiled during field mapping and section <br />measurement activities (Hunter; 1997, Masters, 1957). These <br />documents provide the best available specific lithologic <br />information on Uowe Flats. The most important stratigraphic unit <br />in Dowe Flats are, from oldest to youngest, the Dakota Group, <br />containing the Lytle and South Platte formations; the Ft. Benton <br />Formation, the Niobrara Formation, containing the Fort Hays and <br />Smokey Hill members, and the Pierre Shale. <br />The Dakota Group contains the upper beds on the cuestas <br />surrounding Dowe Flats. It has an average thickness in this area <br />of 330 feet, and is subdivided into the lower Lytle and upper <br />South Platte formations. <br />The Lytle Formation consists of nonmarine fluvial deposits. The <br />lowest part is a fine to coarse-grained massive brown sandstone <br />intercalated with a basal conglomerate containing quarter to half <br />inch diameter chert pebbles and granite fragments mixed with <br />finer materials in a secondary silica cement. This bed is <br />approximately 90 feet thick. The upper portion of the Lytle <br />Formation consists of a series of variegated red and yellow <br />claystones that, on exposure, weather to a reddish surface soil. <br />This deposit varies in thickness from 30 to 60 feet. <br />The South Platte Formation constitutes the upper part of the <br />Dakota Group. Deposits in this interval are marine and near- <br />marine in origin. The Plainview Member is a platy, fine-grained, <br />hematite stained quartz and sandstone. It varies in thickness <br />from 20 to 30 feet through differential incising of the <br />underlying claystones. The middle part of the South Platte <br />Formation is a 125 to 175 foot thick gray to black carbonaceous <br />shale interbedded with thin, buff-colored siltstones and <br />sandstones. The uppermost part of this formation is the Muddy <br />member, a massive, ridge-forming tan quartzose sandstone 20 to 30 <br />feet thick that is slightly cross-bedded and distinctly jointed. <br />