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Cretaceous in age. Overlying the Lance Formation are the Tertiary Fort Union and Wasatch <br />Formations. Quaternary alluvial deposits, generally derived from the Mesaverde Group, are <br />locally present in stream valleys. Recent colluvial slides and slumps may also be present in <br />canyons and gulches throughout the area. <br />Surficial geology in the area consists largely of consolidated sedimentary rock of the upper <br />Cretaceous Mesaverde Group. A veneer of colluvial /alluvial material is present in drainages, <br />and becomes more significant along the major streams in the area. <br />The Seneca II Mine extracted coal from the Middle Coal Group of the Williams Fork <br />Formation, a member of the Mesaverde Group. The Williams Fork Formation, which <br />conformably overlies the 1,550 -foot thick Iles Formation, is a 1,100 to 2,000- foot -thick <br />sequence of interbedded 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 <br />Creek Sandstone, considered to be a regional aquifer, is separated from the Wolf Creek Coal <br />by about 70 feet of carbonaceous shale, sandstone, siltstone, and clean shale. <br />Overlying the Wolf Creek Coal is about 150 feet (stratigraphically) of lenticular sandstone, <br />siltstone, shale and thin coal seams. This interburden is overlain by the Wadge Coal, a nine <br />to ten foot thick coal seam. The Wadge overburden is a moderately thick sequence of <br />sandstone, siltstone and shale. About 50 feet above the Wadge Coal is the 2.5 to 5.5 foot <br />thick Lennox Coal. This poor quality coal was recovered in the southern portion of the permit <br />area. Overlying the Lennox Coal is a thin sandstone cap which is overlain by a thick shale <br />unit. <br />Three coal seams in the Williams Fork Formation have been mined at the site. In ascending <br />order, these seams are the Wolf Creek, Wadge and Lennox. The Lennox Coal is <br />discontinuous and was recovered 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 Probable <br />Hydrologic Consequences (PHC) discussion of this document. <br />Surface Water <br />The Seneca II Mine permit area is located within the Fish Creek drainage basin. The Fish <br />Creek drainage basin is a sub -basin of the upper Yampa River basin in northwestern <br />Colorado. A discussion of surface water conditions at the Seneca II Mine is provided in the <br />PHC discussion of this document. The former Wadge mining area lies in the Fish Creek <br />drainage, while the Wolf Creek mining area lies within the Grassy Creek drainage. Both of <br />4 <br />