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Above the Mesaverde Group is the Lewis Shale (Figure 4), 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, 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-resistent 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 identified 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 groups. The geologic formations within the study area are described with respect to their general water bearing characteristics, their water quality parameters and their use in the following paragraphs. The formations will be described from oldest to youngest. Wells tapping the Mancos Shale generally yield small quantities of water, less than 5 gpm. The water is predominantly a calcium-bicarbonate type (Brogden and Giles, 1977). Dissolved solids concentration ranges from 338 to 2,590 mg/1, and the water may contain concentrations of chloride, fluoride, iron and manganese in excess of U.S. Public Health Service (1962) drinking water standards (Brogden 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. The major bedrock aquifers in the Mesaverde Group occur within the massive, regional sandstone units of the Iles and Williams Fork Formations. Wells tapping the Tow Creek sandstone and Trout Creek sandstone of the Iles Formation and the Twentymile sandstone of the Williams Fork Formation are used primarily for domestic and stock purposes. Well yields from these sandstone aquifers are generally less than 10 gpm (Brogden and Giles, 1977). Higher yields generally occur at greater depths where water in the sandstone is under artesian pressure, and where the sandstones have been fractured extensively. Hydraulic conductivity values for fractured sandstones of the Iles and Williams Fork Formations range from 3.7 to 26 feet/day (Brogden and Giles, 1977). -34-