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_22_ 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 acsending order they are the Wolf Creek, Wadge and Lennox coals. The extremely poor lateral continuity of the Wolf Creek and Lennox coals in the area of the proposed Foidel Creek Mine make them unmineable. 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 4). 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 4), which is a 1,500 to 2,000 foot thick sequence of dark-y ray 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 ft. thick. The maximum thickness of these unconsolidated deposits occurs in the Yampa River alluvium which is estimated to be less than 100 feet thick (8rogden and Giles, 1981). These deposits are found most extensively along the Yampa River, Trout, Middle, Foidel and Fish Creeks in the basin and consist predominanatly of clay, sand and lenticular, discontinuous gravel layers. The alluvium is thin or absent in areas where streams cross the resistant sandstones of the Mesaverde Group. The alluvial aquifers are wide where the streams cross less-resistant rock units of the Lewis and Mancos Shales. B. Ground Water Although ground water occurs in all the sedimentary rocks within the Twentymile Park Basin, the only identifed strata capable of regionally storing and transmitting water are the Tow Creek, the Trout Creek and the Twentymile sandstones, along with the lenticular and interbedded sandstones of the three coal grroups. The aquifer characteristics, the ground use for each aquifer and the relationship of geologic structure to ground water flow are discussed in the following paragraphs. Wells tapping the Mancos Shale generally yield small quantities of water, less than 5 yal/min. The water is predominant]y a ca]cium-bicarbonate type (8royden and Giles, 1977). Dissolved solids concentration ranges from 338 to 2,590 mg/1, and the water may contain concentrations of chloride, flouride, iron and manganese in excess of U.S. Public Health Service (1962) drinking water standards (8rogden and Giles, 1977). Although the Mancos Shale contains limited ground water that may be suitable for domestic and stock use, the extensive shales in the upper part of the formation act as relatively impermeable barriers to the subsurface movement of water and the Mancos Shale can be considered as an impermeable, confining layer underlying the Mesaverde Group.