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-z4- <br />The Mancos Shale is the oldest strata exposed in the region, and is of <br />Late Cretaceous Pge. This unit is composed of over 4,000 feet of gray <br />marine shales and minor interbedded buff sandstones. This unit is <br />highly erodible and unstable. Erosion and oversteepening of slopes in <br />this formation produce the numerous rock falls and landslides observed <br />in the lower North Fork Drainage Basin (Junge, 1978). <br />The Mesa Verde Formation is of Late Cretaceous Age and conformably <br />overlies the Mancos Shale. This formation consists of approximately <br />2,300 feet of marine and terrestrial sedimentary rocks. The Mesa Verde <br />Formation is the coal-bearing formation in the region and is divided <br />into four main members; the Rollins sandstone, the Lower Coal Bearing <br />(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 200 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 <br />to define the top of the Mancos Shale and the bottom of the <br />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, <br />fine- to medium-grained sandstones, and coals. The top of the member <br />is usually capped by a massive buff-colored sandstone up to 90 feet in <br />thickness. This sandstone, however, appears not to be a single <br />persistent bed, but is actually several thick lenticular sandstones <br />occurring at progressively lower stratigraphic horizons from east to <br />west. <br />Three coal horizons exist in the Lower Coal-Bearing member, the "A" <br />(Old Ring) horizon, the "B" (Somerset) horizon, and the "C" (Bear) <br />horizon. The "A" horizon is immediately above the Rollins sandstone <br />and is not currently mined. The "B" horizon contains two coal seams <br />and occurs about 20 to 120 feet above the Rollins sandstone. This <br />horizon has been mined by U.S. Steel at the Somerset Mine. The "C" <br />horizon contains one coal seam that occurs 50 to 100 feet above the "B" <br />horizon. This horizon has been mined at U.S. Steel's Elk Creek Mine <br />and the Bear No. 1 and 2 Mines, and is currently mined at the Bear No. <br />3 Mine. <br />The Upper Coal-Bearing (Paonia) member consists of 200 to 500 feet of <br />gray shales, interbedded, buff-colored, lenticular sandstones, and <br />coals. The top of this member is generally considered to be capped by <br />a massive, cliff-forming sandstone. However, like the similar <br />sandstone at the top of the Lower Coal member, this sandstone is not a <br />single persistent bed. Drill hole data for both the Mt. Gunnison and <br />the Somerset life-of-mine areas show this sandstone unit to occur as <br />several lenticular sandstones at slightly different stratigraphic <br />horizons. <br />