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Coal Field. The rocks exposed in the Somerset Coal Field consist of <br />the Mancos Shale and the coal-bearing Mesaverde Formation of <br />Upper Cretaceous Age; and the Ohio Creek Conglomerate, the <br />Wasatch Formation and the Quartz monzonite porphyry of Early <br />Tertiary Age. Coal was to be produced from the Mesaverde <br />Formation, a 2,500- foot-thick sequence of sedimentary strata <br />overlain by the Ohio Creek Conglomerate and underlain by the <br />Mancos Shale. Beds exposed in the mine area dip 3 to 5 degrees <br />north-northeast. The E seam overburden ranges from less than 20 <br />feet near the western edge of the Blue Ribbon mine permit area to <br />over 1,400 feet in eastern portions of the permit area. <br />The late Cretaceous Mancos shale, the oldest unit exposed in the <br />region, is composed of over 4,000 feet of gray marine shales and <br />minor interbedded buffsandstones. This unit is highly erodible and <br />unstable. Erosion and oversteepening of slopes in this formation <br />produce the numerous rock falls and landslides observed in the <br />lower North Fork Drainage Basin. <br />The Mesaverde Formation is of Late Cretaceous age and <br />conformably overlies the Mancos Shale. This formation consists of <br />approximately 2,300 feet of marine and terrestrial sedimentary <br />rocks. The Mesaverde Formation is the coal-bearing formation in <br />the region and is divided into four main members; the Rollins <br />Sandstone, the Lower Coal Bearing (Bowie) member, the Upper <br />Coal Bearing (Paonia) member, and the Barren (Undifferentiated) <br />member. <br />The Rollins Sandstone member is a 120- to 200-foot-thick, massive, <br />cross-bedded, medium- to fine-grained, buff to white sandstone. <br />This sandstone is regionally extensive and resistant in outcrop and <br />forms prominent cliffs. This member is used regionally as a marker <br />horizon to define the top of the Mancos Shale and the bottom of the <br />coal-bearing horizons. <br />The Lower Coal Bearing (Bowie) members consist of 260 to 350 <br />feet of interbedded gray shales, thin to thick lenticular beds of <br />buff-colored, fine- to medium-grained sandstones, and coals. The <br />top of the member is usually capped by a massive buff-colored <br />sandstone up to 90 feet in thickness. This sandstone, however, <br />appears not to be a single persistent bed, but is actually several thick <br />lenticular sandstones occurring at progressively lower stratigraphic <br />horizons from east to west. Three coal horizons exist in the Lower <br />Coal-Bearing member; the A (Old King) horizon, the B (Somerset) <br />horizon, and the C (Bear) horizon. The A horizon is immediately <br />above the Rollins Sandstone and is not currently mined. 'The B <br />horizon contains two coal seams and occurs about 20 to 120 feet <br />above the Rollins sandstone. This horizon was mined at the <br />17 <br />