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The Iles Formation directly overlies the Mancos Shale and is approximately 1,400 feet thick. It consists of interbedded light brown to white, massive; medium to fine grained, ledge-forming sandstones; brown to black carbonacecous shale; sandy shale; and coals. The coals are generally thin and discontinuous, and are not mined in the immediate area. The Tow Creek Sandstone is the basal member of the I1 es Formation; the Trout Creek Sandstone is the uppermost member, These two sandstones crop out in prominent cliffs and are used in the general area as stratigraphic markers. The Trout Creek sandstone averages 78 feet thick in the permit area. The Williams Fork Formation conformably overlies the Trout Creek Sandstone (See Figure 4). The Williams Fork is over 1,700 feet thick in this area and has been divided into three parts. The lowest unit is called the lower coal group and contains sandstones, shales, and coal deposited in transitional, lower delta plain environments. Economically important coal seams found in the lower coal group are the "C", "E", and "F" seams. The "F" seam is being mined by the Eagle No. 5 operation, The Eagle No. 6 Mine will be developed in the "E" seam. The Middle Sandstone, some interbedded sandy shale, shale, and coal; and the overlying massive Twentymile sandstone make up the second unit of the Williams Fork Formation (See Figure 4). The Middle sandstone contains sedimentary structures typical of barrier island and/or delta depositional environments, The Middle sandstone crops out in the valley walls of the Yampa and Williams Fork Rivers. Two mineable coals, the Hart and "H" seams, are present in the section between the Middle and Twentymile sandstones, The Twentymile sandstone crops out in the same general areas as does the Middle sandstone. It is a regionally more prominent unit, however, and averages 120 feet in thickness in the general area. The upper Williams Fork Formation is the third subdivision. This unit overlies the Twentymile sandstone and consists of approximately 680' of interbedded sandstone, siltstones, sandy shale, shales and coals. The Eagle No. 9 Mine, abandoned in 1985, was located in the "A" seam in the upper Williams Fork (See Figure 4), The Trapper Mine is developed in coals of the upper Williams fork but employs a different identification system for the coal seams that it mines. Within the strata of the upper Wiitiams Fork Fomation is a series of three massive sandstones, cumulatively called the White sandstone. The combined thicknesses of these sandstones is about 350 to 450 feet. In the permit area of the Eagle Mines, these sandstones are numbered in ascending order: one, two, and three. A reverse numbering order is used at the adjacent Trapper Mine. Thin sequences of shales and coals separate the individual White sandstones. Above the Mesa Verde Group lies the Lewis Shale, 1,500 to 2,000 feet of dark gray to bluish, homogeneous, marine shale with several thin interbedded sandstones. The shales are bentonitic and swell when hydrated. The Lewis Shale occupies topographically and structurally low areas such as the center of the Round Bottom and Big Bottom synclines. -22-