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<br />The Iles Formation is the lower unit and is approximately 1,500 ft. thick. It <br />consists of interbedded light-brown to white, massive, fine-grained, <br />ledge-forming sandstones, brown to black carbonaceous shale, sandy shale and <br />coal beds. The coal beds are assigned to the lower coal group of the <br />Mesaverde, and are distributed throughout the middle and upper parts of the <br />formation. Four persistent sandstone beds occur within the Iles Formation. <br />They are: 1) the Tow Creek Sandstone member at the base; <br />2) a double ledge-forming sandstone sequence 400 feet or more above the base; <br />3) a light-gray sandstone sequence of variable composition associated with the <br />upper (No. 3) coals of the lower group situated about 900 to 1,000 feet above <br />the base; and 4) the Trout Creek sandstone member which caps the formation. <br />The Trout Creek Sandstone is a 50 to 100 foot thick, light-brown to <br />light-gray, fine-grained, massive sandstone. <br />The upper unit of the Mesaverde Group is the Williams Fork Formation which is <br />approximately 1,000 to 2,000 feet thick and includes all beds between the top <br />of the Trout Creek Sandstone Member and the base of the overlying <br />Lewis Shale. The formation is conformable at its base and top, and it is <br />divided into three units, the lower, middle and upper units. <br />The tower unit is about 1,000 feet thick, consisting of sequences of shale, <br />thin sandstone beds, sandy shale and several coal beds. The middle coal group <br />of this unit contains the coal seams of economic importance for most of the <br />mining in this area. In ascending order they are the Wolf Creek, Wadge and <br />Lennox seams. All the mines considered in this CHIS surface water area mine <br />one or more of these coals except the Grassy Gap Mine and the Meadows No. 1 <br />Mine. The Seneca II-W Mine mines the Wadge coal seam. The Lennox seam has <br />been eroded throughout much of the mine site and the Wolf Creek seam lies too <br />deep for recovery by surface mining methods. The middle unit of the Williams <br />Fork Formation includes a massive, white, cross-bedded, cliff-forming <br />sandstone about 100 to 200 feet thick, called the Twentymile Sandstone <br />Member. The upper units of the Williams Fork Formation consist of interbedded <br />sandstone, sandy shale, shale, sandstone and several thin coal beds of the <br />upper coal group. <br />Above the Mesaverde Group is the Lewis Shale, which is a 1,500 to 2,000 foot <br />thick sequence of dark-gray to bluish, homogenous marine shale with several <br />thin interbedded sandstones and calcareous concretions. <br />Unconsolidated alluvial deposits of Quaternary age constitute the youngest <br />geologic units in the area and are generally less th an 30 feet thick. The <br />maximum thickness of these unconsolidated deposits occurs in the Yampa River <br />alluvium which is estimated to be less than 100 feet thick (Brogden and Giles, <br />1981). These deposits are found most extensively along the Yampa River, <br />Trout, Middle, Foidei, Fish, Sage and Dry Creeks and consist predominately of <br />clay, sand and lenticular, discontinuous gravel layers. <br />The alluvium is thin or absent in areas where streams cross the resistant <br />sandstones of the Mesaverde Group. The alluvial aquifers are wider where the <br />streams cross less resistant rock units of the Lewis and Mancos Shales. <br />- 26- <br />