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<br />w <br />00 <br />tV <br />-..1 <br /> <br />. <br /> <br />rainfall does occur, thus imparting a distinct chemical quality to the <br /> <br />water, <br /> <br />. <br /> <br />Extensive deposits of alluvium exist in interior valleys of the Upper <br /> <br />Basin and interchange between water and alluvium can be expected to re- <br /> <br />sult in a close relation between the chemical quality of water in streams <br /> <br />. <br /> <br />and alluvium. <br /> <br />Basin rocks range in age from Precambrium to Recent. The Pre- <br /> <br />. <br /> <br />cambrium rocks include plutonic and metamorphic rocks which form the <br /> <br />foundation upon which the more recent sedimentary rocks rest. These <br /> <br />older. rocks have been exposed in the mountains through uplifting, folding, <br /> <br />. <br /> <br />faulting and erosion. Sedimentary rocks, the material of which was de- <br /> <br />posited when the area was covered by ancient seas, are abundant in the <br /> <br />. <br /> <br />basin and over 200 formations have been named. <br /> <br />The oldest and most extensive formation in the Upper Basin includes <br /> <br />the Wasatch, Green River, Uintah, and other related formations of the <br /> <br />. <br /> <br />Tertiary Age. The older rocks are lacustrine and fluviatile,containing <br /> <br />marl, siltstone, sandstone, limestone, . evaporite , frona, and related <br /> <br />minerals. About 30 percent of too basin (34,000 square miles) is under- <br /> <br />. <br /> <br />lain by this material. <br /> <br />Continual rocks which outcrop in25 percent of the basin form hog- <br /> <br />. <br /> <br />backs and ridges in southern Utah and southeast Colorado and are found <br /> <br />in the foothills of the San Juan.and Rocky Mountains. They include the <br /> <br />Glen Canyon group, the San Rafael group, the Dakota and Morrison <br /> <br />. <br /> <br />8 <br /> <br />. <br />