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<br />'I. <br /> <br />. <br /> <br />. <br /> <br />Co <br />:.0 <br />W <br />o <br /> <br />SPECIAL STUDIES (Continued) <br /> <br />Rocks of low permeability will, under equal head, transmit water <br />more slowly than those of high permeability. Water that flows through <br />rocks of low permeability (such as shale or siltstone) has more time, <br />before discharging to streams to dissolve mineral constituents than does <br />water that flows through rocks of high permeability (such as fractured <br />sandstone). Ground water runoff from rocks of low permeability is more <br />saline and adds more to the salinity of streams than does ground water <br />runoff from rocks of high permeability. <br /> <br />A large variety of geologic formations ranging in age from Pre- <br />cambrian to Holocene crops out in the Upper Colorado River Basin. They <br />include various types of igneous, metamorphic, and sedimentary rocks of <br />both continental and marine origin. Igneous and metamorphic rocks of <br />Precambrian and Cenozoic age mostly granite, quartzite, schist, gneiss, <br />basalt, and associated volcanic rocks are widely exposed in the higher <br />mountains of the basin, as are many marine sedimentary rocks of Pale- <br />ozoic age, which consist largely of limestone, dolomite, sandstone, and <br />quartzite. Marine and lacustrine sedimentary rocks of Mesozoic and <br />Cenozoic age, which are composed largely of shale, siltstone, mudstone, <br />and marlstone with some sandstone, conglomerate, and limestone, underlie <br />large parts of the Green River, Washakie, Uinta, Piceance, and San Juan <br />Basins; they also crop out in the Book Cliffs, San Rafael Swell, and <br />along the flanks of the high mountains and plateaus. Terrestrial sedi- <br />mentary rocks of Mesozoic age, consisting largely of windblown sand- <br />stone, are most Widely exposed in the Canyonlands. Unconsolidated <br />terrestrial deposits of Cenozoic age, which consist mostly of fluvial, <br />galciofluvial, colluvial, and windblown deposits, are widely scattered <br />throughout the basin. <br /> <br />Iorns, Hembree, and Oakland (1965, p. 4-80 grouped the rocks of the <br />Upper Colorado River Basin into eight hYdrolr~fc units on the basis of <br />their age and general hydrologic properties. These hydrologic units <br />are herein regrouped into five geohydrologic units (Figure 7) according <br />to their relative contribution to the natural salinity of runoff in the <br />basin. Criteria used in the regrouping included dominant lithology, <br />origin, and age of the rocks, and (where available) chemical quality of <br />ground and surface water known to be unaffected by irrigation return <br />fl ows . <br /> <br />The five geohydrologic units as used in this report are described <br />in Table F, with some additional explanatory material presented in the <br />following paragraphs. <br /> <br />Parts of geohydrologic units 3 and 4 underlie high plateaus and <br />mountainous areas where the average annual precipitation exceeds 15 <br />inches (381 mm). In these areas, some of the original highly soluble <br />mineral constituents have been leached. Therefore, the salinity of <br />runoff generated on those geohydrologic units at the higher altitudes <br /> <br />111 <br />