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• sandstone and conglomerate layers and lenses and basalt flow= <br />transmit most of the ground water, and shale and coal layers usually <br />retard flow. However., highly fractured layers and near-surface <br />weathered zones in shale, limestone layers, and a few thick coal <br />. seams also transmit water. Formations comprised largely of shale, <br />including the Graneros Shale, lower Carlisle Shale, Smoky Hill <br />Marl, Pierre Shale, and Huerfano Formation retard the downward <br />movement of ground water and confine ground-water flow within <br />the aquifers. Precambrian and Tertiary intrusive and metamorphic <br />rocks, unless highly fractured, act as barriers to ground-water <br />flow. <br />The principal sources of ground-water information in Area 61 <br />are Griggs (1948), Powell (1952), McLaughlin and others (1961), <br />McLaughlin (1965), Water, Waste, and Land, Ltd. (1980),)and U.S. <br />Geological Survey computer files. <br />• <br />52 <br />