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<br />Natural Environmental Characteristics <br /> <br />Physiography and Geology <br /> <br />The Moapa Valley Subevaluation Unit is in the Basin and Range Province <br />and elevations vary from about 3,000 feet at the highest point to about <br />1,200 feet at Lake Mead. The relief and topography is characterized by <br />mountain ranges and ridges and intermontane valleys such as Moapa Valley, <br />Meadow Valley Wash and California Wash. <br /> <br />The four geomorphic units recognized in the area are the folded and <br />faulted mountains, the intermediate slopes below the mountains and slightly <br />above the valley floor, the valley floor, and low lying flood plains of <br />the drainage systems. <br /> <br />Thrust faulting and folding formed the mountainious terrain. As this <br />structural activity occurred, adjoining basins were subsequently filled <br />with deep alluvial and lacustrine sediments. <br /> <br />Rocks in the area are sedimentary and consist of limestone, dolomite, <br />shale, sandstone, conglomerate, and gypsum and salt (see Figures 4-A and <br />4-B). These rocks indicate the topographic and climatic conditions that <br />existed in the area during their time of deposition. <br /> <br />During much of geologic time the study area had an environment conducive <br />to the formation of calcium and magnesium carbonate rock (limestone and <br />dolomite), calcium sulfate (gypsum) and sodium chloride (salt). These <br />rock types have an effect on the salinity of water (surface and subsurface) <br />and soil s. <br /> <br />The following rock units mapped (see Geologic Maps Figure 4-A and <br />5) in the study area and interpretations as to environments of deposition <br />are from Longwell (l 928). . <br /> <br />Muddy Creek Formation: During time of sedimentation the region was <br />arid, with basin and range topography. Basins alternately held playas <br />and shallow lakes. Lake waters were strongly saline and when complete <br />evaporation occurred bodies of rock salt and gypsum were formed. <br /> <br />Horse Spring Formation: During time of sedimentation the area was <br />arid with low ridges and shallow basins in which playa deposits were formed. <br />The area had widespread discontinuous water bodies in which calicum and <br />magnesium carbonate, gypsum and associated saline materials plus sand and <br />silt were deposited. <br /> <br />Overton Fanglomerate: Characteristic of a deposit in an arid country <br />of high relief. Considered as a formation built on a relatively steep <br />grade adjacent to high scarps by swift intermittent desert streams. <br /> <br />Jurassic Cross Bedded Sandstone: Probably continental. <br /> <br />002688 <br /> <br />8 <br />