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-19- GENERAL BACKGROUND The Mt. Gunnison No. 1 Mine lies on the southeast margin of the Piceance Basin. Sediments ranging from late Cretaceous to early Tertiary age, and dipping 3-5 degrees north to northeasterly, are exposed in the mine areas. Structural-stratigraphic cross sections are presented in Exhibits 2.7.2.D. - 2.7.2.F. The structure on top of the F-Seam is presented on Exhibit 2.7.S.A and geologic maps are presented in Exhibits 2.7.1.A.A and 2.7.2.D.1. Coal is pcoduced from the Mesa Verde Formation, a 2,500-foot-thick sequence of beds overlain by the Ohio Creek Conglomerate and underlain by the Mancos Shale. The Mesa Verde Formation is composed of four members, the Rollins Sandstone, the Uppec and Lower Coal members, and the "Barren" Member. See generalized stratigraphic column, Figure 2.7.2.A, and description on pages 2-219 through 2-227d of the original permit application. The lowermost member of the Mesa Verde Formation, the Rollins Sandstone, is a white to buff-colored, well-sorted, medium-grained sandstone, ranging from 150 feet to 300 feet thick in the permit area. The Lower Coal Member overlies the Rollins Sandstone and consists of interbedded sandstones, siltstones, coals, and shale. The unit averages 270 feet thick in the pecmit acea and bears three significant coal seams--the A, B, and C seams. A massive sandstone, 25 feet in thickness, lies at the top of the Lower Coal Member and separates it from the Upper Coal Member. The Upper Coal Member is approximately 220 feet thick, containing shales, siltstones, sandstones, and the D, E, and F coal seams. The Mt. Gunnison No. 1 Mine will be located in the upper most seam, the F seam. At the mine site, the coal is immediately overlain and underlain by shale. ~eclying the Upper Coal Member is the Barren Member of the Mesa Verde Group. This unit consists of interbedded sandstones, siltstones, shales, and coals. The unit is not thought to be of marine origin and, as a result, the sandstones and the coals are highly lenticular, discontinuous, and of limited lateral extent in outcrop. This unit ranges up to 1,500 feet thick and outcrops throughout most of the five year permit area. Ground water occurs in both the sandstones and the coal seams of the Mesa Verde Formation. Regional ground water movement is northeasterly, down the dip of continuous stratigraphic units. Local ground water movement is controlled by topography, faults, fractures, and discontinuous lenticular sandstones. The most continuous sandstone aquifers of any significance in the region are the Rollins Sandstone and the massive sandstone separating the Upper and Lower Coal Members of the Mesa Verde Formation. However,