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<br />1 <br />_ ~s _ <br />' the Cretaceous Mesaverde group, which outcrops in the Grand Hogback, a sharp <br />monocline dipping locally approximately 40° to 45° into the subsurface towards the <br />t west. The Sunlight Mine, currently being mined, is at the site of the old coal mining <br />~' town of Sunlight on Fourmile Creek, about one mile northeast of the Sunlight Ski Area. <br />Four coal seams, called the A, B, C, and D seams, occur almost directly above the <br />' Rollins sandstone, a prominent marker bed in the area, and the stratigraphic equivalent <br />of the Trout Creek Sandstone of the Grand Hogback to the north (Figures G3, G.4, <br />'' G.S, and G.6). The coal beds generally vary from three to ten feet in thickness, and <br />may be overlain or underlain variably by sandstone, shale, or fossiliferous limestone, <br />all of deltaic origin (Collins, 1976). <br />' The geology of the area was descri bed specifically in reports by Collins, 1976; <br />Poole, 1954; Olander and others, 1974; and in general in a few reports of the USGS <br />' around the turn of the century. An excellent recent general geologic map was <br />provided by Tweto, etal (1978). The most detailed and recent map was done by <br />~, Donnell (1962). An environmental statement on the Thompson Creek mines of Anschutz <br />' Corporation, approximately 7 miles south of the Sunlight Mine, was done by the Bureau <br />of Land Management (updated). <br />Along the Grand Hogback the Mesaverde group has been subdivided into the Iles <br />and \V'illiams Fork formations (Figures G3, G.4, and G.5). The Iles formation overlies <br />~~' the hlancos shale, which is capped by the Cozzette sandstone, which forms the first <br />~' prominent hogback on the east side of the smal! valey just east of the Sunlight Mine. <br />The Iles formation contains interbedded sandstones, shales, carbonaeous shales, and a <br />' few thin limy beds, and the commonly ridge-forming Rollins sandstone at the top. The <br />i Fairfield coal group overlies the Rollins sandstone and comprises the lowermost units <br />~' <br />of both the Williams Fork formation and its lower subdivision, the Bowie shale (Figure <br />' G.5). <br />_1 <br />15 <br />