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i <br /> <br />1 Formation, containing the Fort Hays and Smokey Hill members, and <br />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 South <br />Platte formations. <br />' The Lytle Formation consists of nonmarine fluvial deposits. <br />The 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 finer <br />materials in a secondary silica cement. This bed is approximately <br />' 90 feet thick. The upper portion of the Lytle Formation consists <br />of a series of variegated red and yellow claystones that, on <br />' exposure, weather to a reddish surface soil. This deposit varies <br />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-marine <br />in origin. The Plainview Member is a platy, fine-grained, hematite <br />stained quartz and sandstone. It varies in thickness from 20 to 30 <br />' feet through differential incising of the underlying claystones. <br />The middle part of the South Platte Formation is a 125 to 175 foot <br />' thick gray to black carbonaceous shale interbedded with thin, buff- <br />colored siltstones and sandstones. The uppermost part of this <br />' formation is the Muddy member, a massive, ridge-forming tan <br />quartzose sandstone 20 to 30 feet thick that is slightly cross- <br />bedded and distinctly jointed. <br />' The Ft. Benton Formation is a 500 foot thick layer of fine <br />grained marine deposits. The lowest part is composed of dark gray <br />' to black fossiliferous sandy shales. The middle portion is a zone <br />of light to dark gray argillaceous limestone and dark gray to black <br />' calcareous shales. The upper part is calcerous dark gray sandy <br />' 46 <br />~l <br />