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<br />Geology
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<br />The Virgin River drainage includes parts of two great physiographic
<br />provinces, with approximately one-fifth of its total area falling in the
<br />Colorado Plateau Province and the remaining portion in the Basin and Range
<br />Province. The general dividing line is formed by the Hurricane fault in
<br />the Ash Creek-LaVerkin area. This line then swings southwestward to St.
<br />George, Utah, thence through Littlefield, Arizona, to Lake Mead.
<br />
<br />The Colorado Plateau portion is characterized by high plateaus
<br />5,000 to 10,000 feet above sea level, separated and cut by steep-walled
<br />canyons. The land forms of the area are determined by the nearly horizon-
<br />tal position of the rock layers and rather simple faults and flexures to
<br />which they have been subjected. The relief is characterized by wind-
<br />eroded buttes, pedestals, and related land forms and by deeply incised
<br />canyons cut by water. The rocks making up this portion of the watershed
<br />are mostly sedimentary in origin and range in age from Carboniferous to
<br />Tertiary, Limestones, sandstones, conglomerates, and shales are predomin-
<br />ating but are loc?lly capped by basaltic and rhyolitic lava flows. The
<br />outstanding structural features of this area are the Hurricane fault which
<br />crosses it in a north-south direction and the Grand Canyon of the Colorado.
<br />Tuffs and cinders are locally conspicuous.
<br />
<br />The Basin and range portion has land forms which contrast sharply with
<br />those of the adjacent plateaus. The relief for the most part, is character-
<br />ized by alternating mountain ranges and intermontane plains or valleys,
<br />roughly trending in a north-south direction.
<br />
<br />The altitudes of these plains vary between 500 and 3,000 feet in the
<br />extreme southern portion of the area to about 5,000 or 6,000 feet in the
<br />north. The mountain ranges lie generally 3,000 to 6,000 feet higher than
<br />the adjacent plains. The highest point in the Basin is Charleston Peak,
<br />west of Las Vegas, which rises to 11,912 feet. The rocks of the mountain
<br />ranges consist chiefly of Paleozoic limestones, shales, and quartzites,
<br />and Tertiary lavas and tuffs. Much of the outwash from the mountains has
<br />accumulated in the region forming the present desert plains. These desert
<br />plains consist of valley fills made up of interbedded clay and silt, sand,
<br />gravel, cobbles, and occasional boulders.
<br />
<br />Soils
<br />
<br />Variations in climate, biological forces, relief, parent materials and
<br />age of land surfaces within the Virgin River Basin has resulted in a complex
<br />pattern of soils and land types. Major Soil Orders represented in the Basin
<br />include Aridisols, Entisols, Mollisols and small areas of Alfisols and
<br />Inceptisols. Major land types include Rock Outcrop and Badlands.
<br />
<br />Aridisols
<br />
<br />Aridisols are extensive soils in Arizona, Nevada and Utah. They occupy
<br />the arid to subarid desert mountains, plateaus, alluvial fans, terraces, and
<br />
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