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<br />o <br />a. <br />...:J <br />w <br /> <br />PART II. DESCRIPrION OF BASIN <br /> <br />A. Geology <br /> <br />The upper or northern portion of the Colorado River Basin in Wyoming <br />and Colorado is a mountainous plateau 5,000 to 8,000 feet in elevation <br />marked by broad, rolling valleys, deep canyons, and intersectin5 mountain <br />ranges. Hundreds of peaks in these mo~ntain chains rise to more than <br />13,000 feet above sea level and many exceed 14,000 feet in elevation. <br />Mountain lakes exist in considerable numbers. The so~thern po~tion of <br />the Upper Basin is studded with rugged mo:mtain peaks interspersed with <br />broad, alluvial valleys and rolling plateaus. The min stream and its <br />trib~taries in Colorado generally flow in deep mountain ca.nyons. The <br />Green River, primary tributary of the Colorado River, flows in similar <br />canyons in Wyo;nin5, Colorado, and Utah after rising in the Wind River <br />Ma~ntains. The San Juan River, a large trib~tary, emerges from the m::Jun- <br />tains of southwestern Colorado, floNs through northwestern New Mexico, <br />and then traverses the deep canyons of the San Ju~n in ~taQ before join- <br />ing the Colorado River in Glen Canyon. The Glen Canyon section of the <br />main stream ani trib'-ltaries lies almost entirely in deep canyons. <br /> <br />Rocks of all ages from those of the Archean age (the oldest known <br />geological period) to the recent alluvial deposits, in~luiing igneous, <br />sedimentary, and meta'llorphic types, a.!'e fOlL'1d in the Colorado River Ba- <br />sin. The high Rocky Mo~ntains which dominate the topography of the <br />upper regions are compased of granites, schists, gneisses, lava, and <br />sharply folded sedimentary rocks of limestone, sandstone, and shale. <br />Many periods of deposition, erosion, and upheaval have played a part in <br />the p~esent structure of these mountains. <br /> <br />In contrast to the folded rocks of the mountains which frin5e the <br />basin, the plateau country of southwestern WyomiIl5, eastern Utah, and <br />northern Arizona is co;npased p~incipally of horizontal strata of sedi- <br />mentary rocks. SloN but constant elevation of the land area has allowed <br />the Colorado River and its tributaries to cut narrow, deep canyons into <br />the flat-topped mesas. Tnis type of erosion reaches its culmination in <br />the Grand Canyo'~ where the Colorado River has cut through all ,Jf -ohe sed- <br />imentary rocks down to the oldest Archean 5rahites. <br /> <br />The Lawer <br />by low ranges. <br />vial deposits. <br /> <br />Basin is characterized by broad, flat valleys sepa!'ated <br />These valleys are filled by la:rge accum'llations of allu- <br /> <br />Sediment remo7ed by constant erosion of the upper areas was depos- <br />ited in _Arizona, California, and Mexico and n::J. forms the great delta of <br />the Colorado River. <br /> <br />7 <br /> <br />