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Gunnison River Valley, Delta and Gunnison Counties, <br />Colorado." <br />The Blue Ribbon Mine extracted coal from the E seam of the <br />Upper Coal Member of the Mesaverde Formation. The Upper <br />Coal Member ranges from 400 to 600 feet thick in the <br />Somerset Coal Field, also known as the North <br />Fork-Minnesota Creek or Paonia Coal Field. The rocks <br />exposed in the Somerset Coal Field consist of the Mancos <br />Shale and the coal-beazing Mesaverde Formation of Upper <br />Cretaceous Age; and the Ohio Creek Conglomerate, the <br />Wasatch Formation and the Quartz monzonite porphyry of <br />Eazly Tertiary Age. Coal was to be produced from the <br />Mesaverde Formation, a 2,500- foot-thick sequence of <br />sedimentary strata overlain by the Ohio Creek Conglomerate <br />and underlain by the Mancos Shale. Beds exposed in the mine <br />area dip 3 to 5 degrees north-northeast. The E seam <br />overburden ranges from less than 20 feet near the western <br />edge of the Blue Ribbon mine permit azea to over 1,400 feet <br />in eastern portions of the permit azea. <br />The late Cretaceous Mancos shale, the oldest unit exposed in <br />the region, is composed of over 4,000 feet of gray mazine <br />shales and minor interbedded buff sandstones. This unit is <br />highly erodible and unstable. Erosion and oversteepening of <br />slopes in this formation produce the numerous rock falls and <br />landslides observed in the lower North Fork Drainage Basin. <br />The Mesaverde Formation is of Late Cretaceous age and <br />conformably overlies the Mancos Shale. This formation <br />consists of approximately 2,300 feet of marine and terrestrial <br />sedimentary rocks. The Mesaverde Formation is the <br />coal-beazing formation in the region and is divided into four <br />main members; the Rollins Sandstone, the Lower Coal <br />Bearing (Bowie) member, the Upper Coal Bearing (Paonia) <br />member, and the Barren (Undifferentiated) member. <br />The Rollins Sandstone member is a 120- to 200-foot-thick, <br />massive, cross-bedded, medium- to fine-grained, buff to white <br />sandstone. This sandstone is regionally extensive and resistant <br />in outcrop and forms prominent cliffs. This member is used <br />regionally as a marker horizon to define the top of the Mancos <br />Shale and the bottom of the coal-beazing horizons. <br />18 <br />