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Quaternary alluvial deposits, generally derived from the Mesaverde Group, aze locally present in <br />stream valleys. Recent colluvial slides and slumps may also be present in canyons and gulches <br />throughout the azea. <br />Surficial geology in the azea consists largely of consolidated sedimentary rock of the upper <br />Cretaceous Mesaverde Group. A veneer of colluviaUalluvial material is present in drainages, <br />and becomes more significant along the major streams in the azea. <br />The Seneca II Mine extracts coal from the Middle Coal Group of the Williams Fork Formation, a <br />member of the Mesaverde Group. The Williams Fork Formation, which conformably overlies <br />the 1,550-foot thick Iles Formation, is a 1,100 to 2,000-foot-thick sequence of interbedded <br />shales, sandstones, sandy shales and coal. <br />The lower-most economically recoverable coal member of the Middle Coal Group is the Wolf <br />Creek Coal, which averages 15 feet thick at the Seneca II Mine site. The underlying Trout Creek <br />Sandstone, considered to be a regional aquifer, is sepazated from the Wolf Creek Coal by about <br />70 feet of carbonaceous shale, sandstone, siltstone, and clean shale. <br />Overlying the Wolf Creek Coal is about 150 feet (stratigraphically) of lenticulaz sandstone, <br />siltstone, shale and thin coal seams. This interburden is overlain by the Wadge Coal, anine- to <br />ten-foot thick coal seam. The Wadge overburden is a moderately thick sequence of sandstone, <br />siltstone and shale. About 50 feet above the Wadge Coal is the 2.5- to 5.5-foot thick Lennox <br />Coal. This poor quality coal was recovered in the southern portion of the permit azea. Overlying <br />the Lennox Coal is a thin sandstone cap which is overlain by a thick shale unit. <br />Three coal seams in the Williams Fork Formation have been mined at the site. In ascending <br />order, these seams aze the Wolf Creek, Wadge and Lennox. The Lennox Coal is discontinuous <br />and was_recov_ered where economically feasible. <br />Ground Water <br />Aquifers identified within and adjacent to the permit area include the Trout Creek Sandstone, <br />Wolf Creek Coal, Wadge Coal and overburden, resaturated spoils and the alluvial bodies <br />associated with local drainages. A discussion of groundwater is provided in the PHC discussion <br />of this document. <br />Surface Water <br />The Seneca II Mine is located within two adjacent drainage basins: Grassy Creek and Fish <br />Creek. Both of these aze sub-basins of the upper Yampa River basin in northwestern Colorado. <br />A discussion of surface water conditions at the Seneca II Mine is provided in the Probable <br />Hydrologic Consequences discussion of this document. The former Wadge mining azea lies in <br />the Fish Creek drainage, while the Wolf Creek mining azea lies within the Grassy Creek <br />drainage. Both of these drainages aze tributary to the upper Yampa River. <br />4 <br />