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<br />Natural EnVironmental Characteristics <br /> <br />Physiography and Geology <br /> <br />Virgin Valley Subevaluation Unit is chiefly in the Basin and Range <br />ProVince, with a very small portion in the Colorado Plateau Province. <br />The general dividing line between these provinces runs throQgh St. George, <br />Utah, then swings sout.hwestward near Littlefield, Arizona to Lake Mead <br />at Grand Wash. <br /> <br />The Basin and Range ProVince within the Virgin River Sub evaluation <br />Unit varies in elevations from about 8,075 feet at Virgin Peak the <br />highest point to about 1,200 feet at Lake Mead. This province generally <br />has sharply contrasting land forms. <br /> <br />The four major geomorphic units recognized in the area are the <br />folded and faulted mountains, the intermediate slopes below the mountains <br />and slightly above the valley floor, the valley floor, and the low- <br />lying flood plains of the drainage systems. <br /> <br />Thrust faulting and folding formed the mountainous terrain. Subsequent <br />to the structural activity adjoining basins were filled with alluvial <br />and lacustrine sediments. <br /> <br />The rocks of the surrounding mountain ranges consist chiefly of <br />Paleozoic limestones, shales and quartzites, and Tertiary lavas and <br />tuffs and some granites and schists. The alluvium from the mountains <br />has accumulated in the region forming the present desert plains. These <br />desert plains consist of valley fills made up of interbedded clay <br />and silt, sand, gravel, cobbles, and occasional boulders. These rocks <br />indicate the topographic and climatic conditions that existed in the <br />area during their time of deposition. <br /> <br />, , <br />, <br />I <br /> <br />During geologic time, the study area had an environment conducive <br />to the formation of calcium and magnesium carbonate rock (limestone <br />and dolomite), calcium sulfate (gypsum) and sodium chloride (halite). <br />These rock types have an effect on the salinity of water (surface and <br />subsurface) and soils. <br /> <br />The following rock units (see Geologic Ma~ Figure 4) are in the <br />study area. Interpretations as to environments of deposition are from <br />Longwell (1928). <br /> <br />Muddy Creek Formation: During time of sedimentation the region <br />was arid, with basin and range topography. Basins alternately held <br />playa and shallow lakes. Lake waters were strongly saline and when <br />complete evaporation occurred bodies or rock salt and gypsum were formed. <br /> <br />Chinle Formation: Characteristic of arid climate deposition in <br />water with baSin or lagoon conditions indicated by gypsum layers. <br /> <br />O'JC~6(1 <br /> <br />7 <br />