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The Iles Formation is the lower unit and is approximately 1,500 feet thick. It consists of interbedded light-brown to white, massive, fine-grained, ledge-forming sandstones, brown to black carbonaceous shale, sandy shale and coal beds. The coal beds are assigned to the lower coal group (Figure 4) of the Mesaverde, and are distributed throughout the middle and upper parts of the formation. Four persistent sandstone beds occur within the Iles Formation. They are: 11 the Tow Creek Sandstone member at the base, 21 a double ledge-forming sandstone sequence 400 feet or more above the base, 3) a light-gray sandstone sequence of variable composition associated with the upper fNo. 31 coals of the lower group situated about 900 to 1,000 feet above the base, and 41 the Trout Creek sandstone member which caps the formation (Figure 41. The Trout Creek Sandstone is a 50 to 100 foot thick, light-brown to light-gray, fine-grained, massive sandstone. The upper unit of the Mesaverde Group is the Williams Fork Formation which is approximately 1,000 to 2,000 feet thick and includes all beds between the top of the Trout Creek Sandstone Member and the base of the overlying Lewis Shale (Figure 41. The formation is conformable at its base and top, and it is divided into three units, the lower, middle and upper units. The lower unit is about 1,000 feet thick, consisting of shale, thin sandstone beds, sandy shale and several coal beds of the middle coal group. The middle coal group contains the coal seams of economic importance in this area. In ascending order they are the Wolf Creek, Wadge and Lennox seams, respectively. The extremely poor lateral continuity of the Wolf Creek and Lennox coal seams in the area of the Energy Mine No. 3 make them unmineable. The Lennox seam has been spoiled with the overburden at Energy Mine No. 3 in the past. The middle unit of the Williams Fork formation includes a massive, white, cross-bedded, cliff-forming sandstone about 100 to 200 feet thick, called the Twentymile Sandstone Member (Figure 41. The upper units of the Williams Fork Formation consist of interbedded sandstone, sandy shale, shale, sandstone and several thin coal beds of the upper coal group. Above the Mesaverde Group is the Lewis Shale (Figure 41, which is a 1,500 to 2,000 foot thick sequence of dark-gray to bluish, homogenous marine shale with several thin interbedded sandstones and calcareous concretions. Unconsolidated alluvial deposits of Quaternary age constitute the youngest geologic units in the area and are generally Less than 30 feet thick. The maximum thickness of these unconsolidated deposits occurs in the Yampa River alluvium which is estimated to be less than 100 feet thick (Brogden and Giles, 1981). These deposits are found most extensively along the Yampa River, Trout, Middle, Foidel and Fish Creeks in the basin and consist predominately of clay, sand and lenticular, discontinuous gravel layers. _lg_