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The Mancos Shale is the oldest formation exposed in the region. This unit is <br />composed of over 4,000 feet of gray marine shales and minor interbedded buff <br />sandstones. This unit is highly erodible and unstable. Erosion and <br />~; oversteepening of slopes in this formation produce the numerous rock falls and <br />landslides observed in the lower North Fork Drainage Basin (lunge, 1978). <br />The Mesaverde Formation conformably overlies the Mancos Shale. This formation <br />consists of approximately 2,300 feet of marine and terrestrial sedimentary <br />rocks. The Mesaverde Formation is the coal-bearing formation in the region <br />and is divided into four main members - the Rollins sandstone, the Lower Coal <br />Bearing (Bowie) member, the Upper Coal Bearing (Paonia) member, and the Barren <br />(Undifferentiated) member (Johnson, 1948). <br />The Rollins Sandstone Member is a 120- to 300-foot-thick, massive, <br />cross-bedded, medium- to fine-grained, buff to white sandstone. This <br />sandstone is regionally extensive and resistant in outcrop, and forms <br />prominent cliffs. This member is used regionally as a marker horizon to <br />define the top of the Mancos Shale and the bottom of the coal-bearing horizons. <br />The Lower Coal Bearing (Bowie) member consists of 260 to 350 feet of <br />interbedded gray shales, thin to thick lenticular beds of buff-colored, fine- <br />to medium-grained sandstones, and coals. The top of the member is usually <br />capped by a massive, buff-colored sandstone up to 90 feet in thickness. This <br />sandstone, however, appears not to be a single persistent bed, but is actually <br />several thick lenticular sandstones occurring at progressively lower <br />stratigraphic horizons from east to west. - <br />Three coal beds exist in the Lower Coal-Bearing member; the A (Old King> <br />horizon, the B (Somerset) horizon, and the C (Bear) horizon. The A horizon is <br />immediately above the Rollins sandstone and is not currently mined. The 6 <br />horizon contains two coal seams and occurs about 20 to 120 feet above the <br />Rollins sandstone. This horizon will be mined at the Sanborn Creek area of <br />the Somerset Mine and may be mined at the West Elk Mine. The C horizon <br />contains one coal seam that occurs 50 to 100 feet above the B horizon. This <br />horizon has been mined at the Somerset Mine, the Bear No. 1 and 2 Mines, will <br />be mined in the Sanborn Creek area of the Somerset Mine, and is currently <br />mined at the Bear No. 3 Mine. It may be mined at the West Elk Mine in the <br />future. <br />The Upper Coal-Bearing (Paonia) member consists of 200 to 500 feet of gray <br />shales; interbedded, buff-colored, lenticular sandstones; and coals. The top <br />of this member usually grades into a massive, cliff-forming sandstone. <br />However, like the similar sandstone at the top of the Lower Coal member, this <br />sandstone is not a single persistent bed. Drill hole data for both the West <br />Elk and the Somerset life-of-mine areas show this sandstone unit to occur as <br />several lenticular sandstones at slightly different stratigraphic horizons. <br />Imo. <br />Three coal horizons have been identified in the Upper Coal member - the D <br />(Oliver) Horizon, the E (Hawk's Nest> horizon, and the F horizon. The D <br />horizon occurs directly above the "massive" sandstone of the Lower <br />Coal-Bearing member and contains three seams. This horizon is currently being <br />mined in the Orchard Valley Mine. This seam may also be mined in the future <br />at the West Elk Mine, and by Bear Coal in the Bear No. 3 Mine. The E horizon <br />occurs about 130 feet above the D horizon and contains two coal seams. This <br />-5- <br />