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<br />Physiography and Geology <br />The Lower Gunnison Unit contains two million acres. About 42 percent <br /> <br />of this area is privately owned and consists of 182,500 acres of irrigated <br /> <br /> <br />cropland including the 171,000 acres within the study area, and 663,500 <br /> <br /> <br />acres of non-irrigated rangeland. The irrigated land, mostly within the 0-2 <br /> <br /> <br />and 2-5 percent slope groups, developed from alluvium, glacial outwash and <br /> <br />shale. Forest land managed by the U.S. Forest Service makes up 36 percent <br /> <br />of the area and is considerd a low producer of salts and sediment. Range- <br /> <br />land, highly susceptible to erosion, managed by the Bureau of Land Manage- <br /> <br />ment comprises 22 percent of the area. <br /> <br />Mancos shale underlies most of the valley surrounded by older Dakota <br /> <br />sandstone and younger volcanic rock as shown in Figure 111-2. The Mancos <br /> <br />formation, a marine shale with high salt content, consists mainly of gray <br /> <br />silty clay. It is the principal source of salt in the Lower Gunnison Unit. <br /> <br />Salts are concentrated in certain areas within the shale and the younger <br /> <br />alluvial deposits. Lenses of crystalline salt often are exposed during <br /> <br />excavation into shale. <br /> <br />Elevation in the area ranges from 4550 feet above mean sea level where <br /> <br />the Gunnison River joins the Colorado River, to 14,300 feet on Uncompahgre <br /> <br />Peak in the southern end of the basin. Because of the arid climate below <br /> <br />about 7,000 feet elevation, salts have not been leached naturally from the <br /> <br />soil. The leaching process is greatly accelerated by applying irrigation <br /> <br />water to these salt laden soils. <br /> <br />Notable sites of scenic and geologic interest in the study area include <br /> <br />Box Canyon, the San Juan Mountains, Young's Peak - a chimney rock near <br /> <br />Crawford, and the Black Canyon of the Gunnison River. <br /> <br />History and Archaeology <br /> <br />An archaeological survey done under contract to the Colorado Division <br /> <br />001220 <br /> <br />of Highways near the area indicates the possibility of many sites of <br /> <br />III-2 <br />