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W -..l r\) -..l The eas~-west-trending Uinta Mountains are a broad anticlinal arch of sedimentary rocks with the crest of the range composed of very old quartzites. This separates the Green River Basin in Wyoming from the Uinta Basin of Utah. The Wind River range in Wyoming forms the north- east b04ndary of the Green River Basin. Uplifting with accelerated erosion ,has exposed the oldest known crystalline granites as the crest and core of the mountains. The Green River area is underlain mostly by sedimentary rocks of Ter- tiary and Mesozoic age. These rocks contain vast deposits of coal, oil, and oil ,shale. During Tertiary times, the Green River Basin and Uinta Basin within the area were occupied by two large freshwater lakes. The Green River Formation, which contains the oil shale deposits and valuable trona deposits, was laid down in these lakebeds. Mesozoic rocks near Rock, Springs, Wyoming, contain both oil and gas in signifi- cant amounts. In many ,valleys of the Lower Basin, the basement rocks are overlain by a coarse material of generally low permeability, which has eroded from the nearby highlands. Concurrent with this sedimentation, faulting occurred and volcanic eruptions deposited lavas. The Colorado Plateau Province is underlain by sedimentary rocks ranging in thic~ness from 1,000 to 10,000 feet that overlie igneous and meta- morphic :basement rocks. The Colorado River became a through-flowing stream in late Cenozoic time. Downcutting by the river and its tributaries has caused deep entrencHment of the entire system, resulting in spectacular canyons. The occ4rrence of natural solute erosion of halite beds and solution phenomena associated with marine shales such as the Mancos Formation contributes an inordinate amount of salt to the river system. The geology of the Lower Colorado Basin includes a broad spectrum of sedimentary, metamorphic, and igneous rocks which produce a wide variety of soil~ locally and along stream courses. In short, the principal physical characteristics of the region are its great variety of land- forms, topography, and geology. 4. Soils The natural quality of the Colorado River is determined by the nature of the rocks and soils the river drains. The marine shales primarily degrade ,downstream water quality. Faults that cross streambeds also allow ground water to convey saline water to the river. Salt domes that lie directly in the path of some tributaries also make salt contri- butions. The shales of the plateaus were deposited in shallow seas that were often confined, containing high concentrations of calcium sulfate, sodium Chloride, and potassium salts. Significant shale formations are 11-3 'to ,'i ~> "~"... ...,," 2,1 &. ' ';;;-;L;"~"",{,~,-,,~U