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Geology <br />Section 2.04.5 of the PSCM permit application describes the geology and hydrology in the permit <br />area and the surrounding area. The description includes information from the following two U.S. <br />Geological Survey Water-Resources Investigations Reports (WRIRs): Hydrology and Geochemistry <br />of a Surface coal Mine in northwestern Colorado, WRIR 92-4187, by R.S.Williams, Jr. and G.M. <br />Clark, and Geoh d?gic Evaluation of the upper part of the Mesaverde Group, Northwestern <br />Colorado, WRIR 90-4020, by S.G. Robson and Michael Stewart. <br />The PSCM permit area extends from the crest of the Tow Creek anticline westward to the axial area <br />of the Hayden syncline. This area is on the southern edge of the regional 'Sand Wash structural basin. <br />The local stratigraphic sequence that is exposed at the surface is of Cretaceous age. The sequence is, <br />in ascending stratigraphic order, the Iles Formation, Williams Fork Formation, and Lewis Shale. The <br />Iles and Williams Fork Formations are part of the regionally extensive Mesaverde Group. <br />Quaternary alluvial deposits, generally derived from the Mesaverde Group, are locally present in <br />stream valleys. Recent colluvial slides and slumps may also be present in canyons and gulches <br />throughout the area. <br />The Seneca II Mine extracted coal on approximately 1,800 acres of land that is proposed for transfer <br />to the PSCM. Coal seams were mined from the lower member of the Williams Fork Formation. <br />The lower member of the Williams Fork Formation is approximately 300 feet thick in the permit <br />area. Approximate thicknesses of the coal seams are, in ascending stratigraphic order, the Wolf <br />Creek seam, 15 feet ; the Wadge seam, 10 feet; and the Lennox seam, 4 feet. Shale, siltstone, and <br />sandstone separate the coal seams. The Wadge seam is approximately 150 above the Wolf Creek <br />seam, and is approximately 40 feet below the Lennox seam. <br />The PSCM will mine coal from the Wadge seam in the Hayden syncline. Underground mining <br />methods will be used because the significant depth (>100 feet) of the coal in this synclinal area <br />makes surface mining methods uneconomic. Surface mining methods were used at the adjacent <br />Seneca II Mine (on the Tow Creek anticline) and at the adjacent Yoast Mine (on the Fish Creek <br />anticline). <br />Ground Water <br />Ground water hydrology information can be found in Section 2.04.5 of the permit application. A <br />"hydrostratigraphic unit" is a group of strata which has hydraulic continuity. Hydrostratigraphic <br />units in and adjacent to the permit area that may serve as aquifers are (in ascending stratigraphic <br />order): the Trout Creek Sandstone member of the Iles Formation, coal and sandstone beds in the <br />lower member of the Williams Fork Formation, the Twentymile Sandstone member of the Williams <br />Fork Formation, alluvial bodies in local drainages, and coal spoil backfill in reclaimed surface mine <br />pits from the Seneca II Mine. <br />The sandstone and coal hydrostratigraphic units are referred to in this discussion as bedrock units. <br />Table 1 attached to the end of this findings document summarizes the hydrogeologic properties of <br />the bedrock units and the alluvium. Of the bedrock units, the Trout Creek and Twentymile <br />Sandstones have the greatest potential for serving as regional aquifers because individual sandstone <br />Peabody Sage Creek Mine 5 May 7, 2010