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_25_ 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. This may represent the erosional remnants of a volcanic flow feeder. Sills have injected the Lower 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. These 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 by 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 Water 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 Mesaverde Formation; and the Rollins sandstone. The most significant occurrence of ground water in the region is associated with the alluvium of the North Fork of the Gunnison River (Figure 4). Significant thicknesses of alluvial sand and gravel between 30 to 80 feet exist along the North Fork. Numerous wells are developed in the alluvium with an average yield of 17.4 gpm. A pumping test conducted by Bear Coal Company on an alluvial well near the site of the proposed Bear No. 3 Mine yielded a value of 806.5 g/d/ft for transmissivity and 0.2 for storativity. Three water quality analyses were conducted for the Bear No. 3 Mine on three wells completed in the North Fork alluvium. Two wells exceeded drinking water standards for total dissolved solids, sodium and sulfate. The well located closest to the North Fork (the Bear No. 3 - office well) showed the lowest levels of these constituents. This is due to the dilution of alluvial ground water by North Fork River water at this well. Except for areas in the Minnesota Creek Drainage, the alluvium along the lower reaches of tributaries to the North Fork is predominantly thin and confined to discontinuous narrow bands along the stream courses. The deeply incised channels of these tributaries restrict the width of the alluvium, while the stream gradient and the presence of resistant sandstone in the stream channels limit the thickness and downstream extent of the alluvium.