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northeast. The geology is further modified in the northeast comer of Lease C-37210 where <br />coked coal gives evidence of igneous activity associated with the Miocene Iron Point pluton. <br />The Somerset Coal Field lies on the southeast margin of the Piceance Basin and just south of <br />Grand Mesa. The sedimentary strata exposed in the Somerset Coal Field dip at 3N to SN to the <br />north and northeast, and range in age from late Cretaceous to early Tertiary. <br />Coal is produced from the Mesaverde Formation, a 2500-foot-thick sequence of sandstone, shale <br />and coals overlain by the Ohio Creek conglomerate and underlain by the Mancos Shale (Figure <br />2). The Mesaverde Formation is composed of four members, which aze, in order of decreasing <br />age, the Rollins Sandstone, the Lower and Upper Coal members and the Barren member. <br />Minor faulting of limited vertical displacement has been observed in other nearby mines. <br />However, in the Bowie No. ] Mine, a fault with a displacement of fifty (50) feet was encountered <br />during mining, and drill-hole data indicates the presence of other faults in the life-of-mine area <br />with similar displacements. The faults which have been encountered in these mines tend to be <br />high-angle, notmal faults. <br />The steep slopes of the stream valleys and the instability of the rock strata in the North Fork <br />drainage basin have contributed to numerous landslides, mud flaws and rock falls. These mass <br />wasting features have been mapped by W.R. Junge of the Colorado Geological Survey and <br />published as an open file report entitled "Geologic Hazazds, North Fork Gunnison River Valley, <br />Delta and Gunnison Counties, Colorado." <br />Geologic units exposed in the North Fork Drainage Basin consist of Late Cretaceous to Eazly <br />Tertiary Age sedimentary strata, Tertiary Age igneous intrusives, and Quaternary Age alluvial <br />and colluvial deposits. The units of the Late Cretaceous in the general azea aze described below <br />in ascending order. A stratigraphic column representing the geology of the coal member of the <br />Mesaverde Formation in the permit area can be found on Map 2-10, Volume 2, and is found in <br />this document as Figure 3. <br />The Mancos Shale is the oldest formation exposed in the region. This unit is composed of over <br />4,000 feet of gray marine shales and minor interbedded buff sandstones. This unit is highly <br />erodible and unstable. Erosion and over- steepening of slopes in this formation produce the <br />numerous rock falls and landslides observed in the lower North Fork drainage basin. <br />The Mesaverde Formation conformably overlies the Mancos Shale. This formation consists of <br />approximately 2,300 feet of mazine and terrestrial sedimentary rocks. The Mesaverde Formation <br />is the coal-beazing formation in the region and is divided into four main members -the Rollins <br />Sandstone, the Lower Coal Bearing (Bowie) member, the Upper Coal Bearing (Paonia) member, <br />and the Barren (Undifferentiated) member. <br />The Rollins Sandstone member is a 120- to 300-font-[hick, massive, cross-bedded, medium- to <br />fine-grained, buff to white sandstone. This sandstone is regionally extensive and resistant in <br />outcrop and forms prominent cliffs. This member is used regionally as a mazker horizon to <br />define the top of the Mancos Shale and the bottom of the coal-beazing horizons. <br />12 <br />