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-23- Three coal horizons have been identified in the Upper Coal member; the "D" (Oliver) Horizon, the 'E" (Hawk's Nest) horizon, and the 'F" horizon. The "D" horizon occurs directly above the "massive° sandstone of the Lower Coal-Bearing member and contains three seams. Thts horizon is currently being mined in the Orchard Yalley Mine, and is planned to be mined at the Hawk's Nest Mine. This seam may also be mined in the Future at the Mt. Gunnison Mine, and by Bear Coal 1n the Bear No. 3 Mine, The 'E' horizon occurs about 130 feet above the "D" horizon and contains two coal seams. This horizon is currently being mined at the Hawk's Nest Mine and was mined at the Blue Ribbon Mine. It may be mined at the Mt. Gunnison Mine and at the Bear No. 3 Mine in the future. The "F° horizon contains two coal seams and presently is only mined at the Mt. Gunnison Mine. Coal seams of the 'F' horizon do not exist to the north of the North Fork in thicknesses sufficient for mining. The Barren (Undifferentiated) member of the Mesa Yerde Formation consists of up to 1,500 feet of terrestrial sedimentary rocks. This unit consists of fine-grained, buff-colored, lenticular sandstones; gray shales and thin lenticular coal beds. The sandstones predominate and are highly lenticular, discontinuous and of limited lateral extent in outcrop (Johnson, 1948). The Mesa Verde Formation is unconformably overlain by the Tertiary Age Rudy or Ylasatch Formation, This formation consists of red to buff-colored shales, red sandstones, and red to gray conglomerates, fie sediments of this formation • are weathered volcanic rocks. The Ohio Creek conglomerate is the basal unit within the fortnatlon and is 100 to 200 feet thick. Tertiary igneous intrusive rocks exist within the North Fork Drainage Basin. A diorite plug about 1,000 feet in diameter outcrops along Hubbard Creek in the SE 1/4 of Section 7, T13S, R91W of the 6th P.M, Th15 may represent the - erosional remnants of a volcanic flow feeder. Sills have injected the Lgwer Coal-Bearing member, particularly the 'B' and °C" seams. These sills consist of diorite and appear to have their source to the northwest of Terror Creek. The alluvium of the North Fork of the Gunnison River consists of Quaternary Age deposits of mixed coarse sand, cobbles and boulders, fiese coarse sediments are composed primarily of igneous and metamorphic rock types, and have their source area in the headwaters and upper reaches of the North Fork. This coarse alluvium is capped Dy finer sands and silts. The North Fork alluvium in the area of the Hawk's Nest. Bear, Mt. Gunnison and Somerset Mines is fairly narrow in width and between 50 to 70 feet thick. About a mile below the town of Somerset, Colorado, the width of alluvium increases while the thickness of alluvium decreases to about 35 feet. DESCRIPTION OF THE GROUND HATER REGIME Four categories of potential aquifers occur in the Somerset Coal Field. These are: the alluvial and terrace deposits associated with the North Fork of the Gunnison River; the localized, shallow alluvium along creeks tributary to the North Fork; the discontinuous, lenticular and laminar sandstones of the Mesa . Verde Formation; and the Rollins sandstone.