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from south to north, and is joined by its two ephemeral tributaries, Little Grassy Creek and Scotchman's <br /> Gulch, near the northern boundary of the permit area (Little Grassy Creek has perennial flow in its lower <br /> part as a result of coal spoil spring inflow from Pond 002 on lands transferred from the Seneca II Mine <br /> to the PSCM.) Grassy Creek flows into the Yampa River two miles north of the permit area. The <br /> Yampa River is one of the largest tributaries of the upper Colorado River system. <br /> Geology <br /> Section 2.04.5 of the PSCM permit application describes the geology and hydrology in the permit and <br /> surrounding areas, which includes information from the following two U.S. Geological Survey Water- <br /> Resources Investigations Reports (WRIRs): Hydrology and Geochemistry of a Surface coal Mine in <br /> northwestern Colorado, WRIR 92-4187, by R.S. Williams, Jr. and G.M. Clark and Geohydrologic <br /> Evaluation of the upper part of the Mesaverde Group,Northwestern Colorado WRIR 90-420, by S.G. <br /> Robson and Michael Stewart. <br /> The PSCM permit area extends from the crest of the Tow Creek anticline westward to the axial area of <br /> the Hayden syncline. This area is on the southern edge of the regional Sand Wash structural basin. The <br /> local stratigraphic sequence that is exposed at the surface is of Cretaceous age. The sequence is, in <br /> ascending stratigraphic order,the Iles Formation, Williams Fork Formation, and Lewis Shale. The Iles <br /> and Williams Fork Formations are part of the regionally extensive Mesaverde Group. Quaternary <br /> alluvial deposits, generally derived from the Mesaverde Group, are locally present in stream valleys. <br /> Recent colluvial slides and slumps may also be present in canyons and gulches throughout the area. <br /> The Seneca II Mine extracted coal on approximately 1,800 acres of land that was transfered to the <br /> PSCM. Coal seams were mined from the lower member of the Williams Fork Formation. The lower <br /> member of the Williams Fork Formation is approximately 300 feet thick in the permit area. <br /> Approximate thicknesses of the coal seams, in ascending stratigraphic order are: the Wolf Creek seam, <br /> 15 ft; the Wadge seam, 10 ft; and the Lennox seam, 4 ft. Shale, siltstone, and sandstone separate the <br /> coal seams. The Wadge seam is approximately 150 above the Wolf Creek seam and approximately 40 <br /> feet below the Lennox seam. The PSCM will mine coal from the Wadge seam in the Hayden syncline. <br /> Groundwater <br /> Groundwater hydrology information can be found in section 2.04.5 of the permit application. A <br /> "hydrostatigraphic unit" is a group of stata which has hydraulic continuity. Hydrostatigraphic units in <br /> and adjacent to the permit area that may serve as aquifers are (in ascending stratigraphic order): the <br /> Trout Creek Sandstone member of the Iles Formation, coal and sandstone beds in the lower member of <br /> the William Fork Formation, the Twentymile Sandstone member of the Williams Fork Formation, <br /> alluvial bodies in local drainages, and coal spoil backfill in reclaimed surface mine pits from the Seneca <br /> II Mine. <br /> CO10 <br /> e <br /> 1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 I `�' O <br /> �`1876� <br />