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1 <br /> <br />' 9.0 Geologic Context <br />Introduction <br />' A maximum of approximately 12,700 feet of sediments is found <br />in the vicinity of Dowe Flats. Only those formations beneath the <br />' lower Pierre Shale are present. In general, the geologic section <br />is composed of alternating sandstone and shale layers of some <br />' limestones. Beginning at the bottom of the section and working <br />upward in both depth and time, the individual units display a <br />variety of depositional origins. <br />Precambrian basement rock, including both plutonic and <br />metamorphic rocks, is exposed in the mountains West of Lyons. The <br />' rocks are generally massive, hard and tough when fresh, but coarser <br />grained materials can weather to grus. The basal Fountain <br />' Formation was deposited during erosion of the ancestral Rocky <br />Mountains. The Lyons Formation above it is a beach deposit. The <br />' overlying Lykins Formation is a marine shale and the Entrada is an <br />eolian deposit. The Morrison Formation and the Lytle Formation of <br />' the Dakota Group are both continental deposits, while all units <br />between the South Platte Formation of the Dakota Group and the <br />' Pierre Shale were deposited in various marine environments as the <br />seas transgressed and receded through the area. The overlying Fox <br />' Hills Sandstone is a transitional beach type sandstone formed as <br />the seas regressed from the area for the last time, and the <br />' uppermost Laramie Formation is a series of interbedded nonmarine <br />claystones and shales with an occasional coal seam. During the <br />Tertiary period, Precambrian, Paleozoic and Mesozoic rocks were <br />' invaded by igneous intrusives. One such large intrusive body is <br />present about 2 miles Southwest of Lyons and is currently being <br />' developed for commercial aggregate and riprap. <br />Mineral utilization near Dowe Flats includes production of <br />' cement form the Fort Hays limestone in a pit south of Dowe Flats, <br />~' 9 3 <br /> <br />