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• 2.04.6 Geo[o_y Descri~on <br />The coal mined at the Colowyo operation is located in the northern extent of the Danforth <br />Hills coal field of the Uinta Region. The Danforth Hills field comprises the coal deposits on <br />the northeast flank of the Piceance Creek basin and is defined to the northeast by the Axial <br />Basin Anticline. <br />Stratioraphv <br />The area is underlain by as much as 13,500 feet of sedimentary rock consisting of <br />approximately 4,500 feet of Paleozoic (~70 million to 225 million years old) quartzite, <br />limestone, shale, sandstone. and gypsum and 9,000 feet of Iviesozoic (225 million to 70 <br />million years old) limestone, shale, mudstone, sandstone, and conglomerate. The coal seams <br />mined at the Colowyo operation are of upper Cretaceous Age. <br />The generalized geological structural setting is outlined in Figure 1 Geologic Structural <br />Setting. The stratigraphic position of the coals mined by Colowyo are graphically illustrated <br />in Figure 2, Generalized stratigraphic Column. <br />The two most important geologic formations within the permit area are the upper Cretaceous <br />Iles and Williams Fork Formations of the Mesa Verde Group. Details of each formation are <br />set forth below. <br />Iles Formation - In general, the Iles Formation consists of light brown- to white, fine- to <br />medium-grained, poorly-sorted calcareous sandstone interbedded with red and dark maroon <br />sandy and silty carbonaceous shale. Thin lenticular coal beds are found in this formation. The <br />uppermost sandstone of the Iles Formation, the Trout Creek Sandstone Member, appears to <br />be reliable marker horizon at the base of the principal coal beds within the Williams Fork <br />Formation. The 75 foot thick Trout Creek member is a white, fine-grained, well-sorted, <br />massive sandstone that is fairly uniform <br /> <br />2.04.6-1 <br />