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<br />o <br />~ <br />C:'..l <br />C' <br /> <br />CHAPTER II <br /> <br />UNIT SETTING <br /> <br />,-) <br /> <br />A zone of weathered and fractured shale exists at the top of the Mancos <br /> <br />Formation. This zone acts as an aquifer which allows water from convey- <br /> <br />ance system seepage and deep percolation to pass on to drains and <br /> <br />eventually the Colorado River. <br /> <br />Another aquifer, referred to as the cobble aquifer, overlies Mancos <br /> <br />Formation bedrock in ancient Colorado River channels in a strip 2 to 3 <br /> <br />miles wide from the town of Palisade to the community of Loma. It <br /> <br />consists of gravel and cobbles of mostly granite, basalt, and quartzite. <br /> <br />Alluvium derived from Mancos Shale and other formations fills stream and <br /> <br />wash channels and in some places overlies the cobble aquifer. Portions <br /> <br />of the cobble aquifer are overlain by a clay layer which decreases <br /> <br />permeability and allows perched water tables to develop above the aquifer. <br /> <br />Soils <br /> <br />Most of the soils in the valley are either alluvial deposits or <br /> <br />residual type soils formed on and from the Mancos Shale. The shallower, <br /> <br />residual-type soils normally occur on or near the visible shale hills <br /> <br />while the deeper alluvial soils are more evident on the mesas, alluvial <br /> <br />fans, and floodplains. <br /> <br />Textures normally range from fine sandy loam to <br /> <br />silty clay loam with silty-type soils predominating. <br /> <br />Soil depths vary <br /> <br />from a few inches to over 50 feet. The saline soils generally have low <br /> <br />permeabilit ies and high water-holding capacit ies. <br /> <br />Localized areas of <br /> <br />high saline-alkaline concentrations are scattered throughout the valley. <br /> <br />Climate <br /> <br />The climate of the unit area is characterized by hot summers, <br /> <br />mi ld winters, and a high percentage of sunny days. <br /> <br />The mean annual <br /> <br />temperature is 52' F with the hottest termperatures occurring in July. <br /> <br />7 <br />