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Description of the Environment The Seneca II-W Mine is located in northwest Colorado approximately 7 miles south of Hayden in Routt County. The Seneca II-W Mine lies 7 miles west of the Seneca II Mine, Permit No. C-80-005 and 3.5 miles west of the Yoast Mine, Permit No. C-94-082. See Figure 1 for further detail, The mining area can be found on the Hayden, Hayden Gulch, Mount Hams and Dunkley Quadrangles 7.5 Minute Series Maps. The coal from Seneca II-W is hauled over county and private roadways by private hauler to the Hayden Station Power Plant operated by Public Service of Colorado. Haulage distance is approximately ten miles each way. Baseline environmental information collected for the Seneca II-W Mine can be located in Tabs 4 through 11 in the permit application. Geology and To raohy Elevation of the general mine site varies from approximately 6400 to 8350 feet. Slopes range from a gentle 7% to vertical ledges formed by the Twenty Mile and Trout Creek sandstones. Many of the drainages are deeply incised. The Seneca II-W Mine is situated in the southeastern synclinal portion of the Sand Wash Structural Basin of northwestern Colorado and south central Wyoming. Exposed strata near the mine site vary in age from Late Cretaceous to Quaternary. Most units in the azea are sedimentary. However, some Miocene extrusives aze locally present. Many faults and folds modify the regional synclinal structure, however, no faults have been identified at the site. The most significant local fold is the Sage Creek Anticline, located just east of the mine. Rock units dip 8 to 14 degrees to the west. The Seneca II-W Mine lies on the western flank of this anticline. The largest local structure is the Tow Creek Anticline, located about 10 miles northeast of the permit area. The surficial geology of the area is composed chiefly of sedimentary rocks, of marine and non-marine origin. These sedimentary rocks were formed through the transgression and regression of an epicontinental sea. Rock outcroppings within the permit boundary aze predominately of the Upper Cretaceous Mesaverde Group. The Mesaverde Group is divided into two Formations, the lies Formation and the Williams Fork Formation. Both Formations aze comprised of sequences of sandstones, siltstones, shales and coal. The older Iles Formation outcrops in the easternmost portion of the permit boundary while throughout the rest of the permit area, the surficial rock is from the Williams Fork Formation. There are some thin surf~cial deposits of Quaternary alluvium and colluvium scattered throughout the area. The local stratigraphic sequence, in ascending order, is the Mancos Shale; the Mesaverde Group, which is subdivided into the Iles and Williams Fork Formations; the Lewis Shale; and the Lance Formation. All are Upper Cretaceous in age. Overlying the Lance Formation aze the Tertiary Fort Union and Wasatch Formations. See Figure 2 for further detail. 4