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OSGS Quadrangle maps of Milner and Rattlesnake Butte cover the majority of the permit area. The proposed permit area lies on the south end of the regionally significant Sand Wash structural basin. Locally significant are the rack units of Twentymile Park Basin rhich dip 5° - 30° toward the central portion of the basin. Normal faulting has been identified throughout the permit area, trending northwest to southeast, with displacement of zero to BS feet. A reverse fault running in an arcuate line northward through the west central part of the basin shows displacement of 0-SO feet. The near-surface bedrock units are composed of sedimentary rocks of marine and non-marine origin. These sedimentary rocks sere formed through the transgression and regression of an epicontinental sea and are comprised of sequences of sandstones, eiltetones, shales and coal. Four geologic formations exist in the vicinity of the proposed Foidel Creek underground mine. They are, in ascending order, the Mancoe Shale, the Iles and Williams Fork of the Mesaverde Group, and the Lewis Shale (see Figures 3 and 4 of this document). These strata were all deposited during the late Cretaceous Period. Generally, the strata dip to the center of the basin at about 7°. Three economically important coal seams, the Lennox, Wedge, and Wolf Creek coals, are located within the locally important, Middle Coal Group of the Williams Fork Formation. The Wadge coal seam will be mined in the Foidel Creek underground mine. The Wadge Coal seam was strip mined in CYCC's adjacent Energy No. 1 and Eckman Park Mines. The Wadge coal seam, in the area of the Foidel Creek Mine, is separated from the underlying Trout Creek sandstone by about 250 feet of interbedded sandstones, siltstones, shales and coals. The Wadge coal seam ie separated from the overlying Twentymile sandstone by 700 feet of massive shale and interbedded sandstones, siltstones, shales and coals. Overburden depths in the proposed permit area range from 0 to 1,700 feet (see Map 6 of the permit application). The stracigraphy and geologic structure within the Twentymile Park Synclinal Basin control the ground water flow within the permit and adjacent areas. There are three regional bedrock aquifers within the permit and adjacent areas of the Foidel Creek underground mine. These aquifers are the Trout Creek sandstone, the Twentymile sandstone, and the Wadge coal-overburden zone. All three aquifers exhibit artesian conditions in central portions of Twentymile Park Basin. The quality in all aquifers is moderately poor and is tolerable for~use as a drinking .supply. The folding and faulting (geologic structure) within the Twentymile Park Synclinal Basin affects the direction of flow within the rock aquifers. In the permit and adjacent areas, the ground water flow in the bedrock aquifers is down the dip to the northeast into the Trentymile Park Basin. Faults in the area produce localized fracture zones rithin the rock strata. These zones of increased permeability, in turn, increase the potential for vertical and horizontal movement of ground rater rithin the rock strata. However, to-date experience in the Foidel Creek mine shows little to no inflow along the fault zones at the level of the Wadge coal seam. The Twentymile sandstone is separated from the Wadge coal-overburden aquifer by up to 700 feet of impermeable shale within the permit and adjacent areas. The underlying 700-foot shale and the overlying shales and eiltetones of the / appe~,Williame Fork Formation and the Lewis Shale Formations produce the confined artesian conditions within the Trentymile sandstone aquifer. To the ,:'~ northeast, the Twentymile sandstone recharges the alluvial aquifer and provides base flow to the tributaries of Trout Creek down gradient of the '~' .:mine. Salta in the Twentymile sandstone waters are predominantly sodium ~; .. _. 4