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<br />:::> <br /> <br />~ <br />..-1 <br />...~ <br /> <br />BACKGROUND AND SETTING <br /> <br />C~~' <br /> <br />This section describes the socio-economic and physical conditions in the <br />Grand Val lev. Included are discussions of soils properties and agricul- <br />tural practices that have an effect on salt loading. <br /> <br />DESCRIPTION OF THE GRAND VALLEY <br /> <br />The Grand Valley of Colorado is near the western edge of Mesa County. <br />The valley covers an area of about 126,000 acres and is within the <br />Canyon lands section of the Colorado Plateau physiographic province. <br />The valley was carved in the Mancos Shale formation (a high salt bearing <br />marine shale) by the Colorado River and its tributaries and for the most <br />part is surrounded by steep, rough terrain. Deep canyons flank the <br />valley to the southwest; a sharp escarpment known as. the Book Cliffs <br />rises above it to the north and northeast; foot slopes of the Grand Mesa <br />lie to the east; and rough, broken and steep, hilly land that borders <br />high terraces of mesas lies to the south. Within the valley, the irri- <br />gated lands have developed on recent alluvial plains consisting of broad <br />coalescing alluvial fans and on older and higher alluvial fans, terraces, <br />and mesas. <br /> <br />In past time, the Colorado River has eroded a deep wide channel through <br />the Mancos Shale formation which has since been refilled with alluvial <br />materials to depths as great as 100 feet. The present river channel <br />is incised in alluvial materials which are much more permeable than <br />the shale and serve as a general aquifer for the floodplain area. <br /> <br /> <br />Capilarity and EvaporatIon causes accumulation <br />of salts on the ground surface. <br /> <br />10 <br />