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<br />GEOLOGY <br /> <br />f~~~ <br /> <br />,..., <br />-.J <br />~... <br />C <br /> <br />In general, the geology of the Region consists of igneous and meta- <br />morphic rock boundaries, with interior basins of sedimentary rock. The <br />igneous and metamorphic rock fringes in the mountains have been eroded <br />down to Precambrian rocks consisting of granite, gneiss, and schist. <br />Volcanic rocks of Tertiary age are scattered throughout .the Rockies and <br />have formed thick beds of volcanic ash and basaltic rock along the <br />western flank of the mountains. The Uintah Mountains reveal a structural <br />anomaly of anticlinal sedimentary rocks. <br /> <br />The sedimentary rocks of the interior basins form the basis of much <br />of the mineral wealth of the Region. These rocks were formed in the <br />Mesozoic era, a period of marine deposition, so they are richly supplied <br />with vegetative sources of carbons and hydrocarbons. <br /> <br />The sedimentary rocks are also the source of actual and potential <br />degradation of the water quality. Many of the rocks contain soluble <br />minerals such as calcium sulfate, sodium chloride, and potassium salts, <br />which leach into surface and ground waters. Salt loading also occurs <br />when surface water flows directly over salt domes. Shales are the pri- <br />mary rock types containing soluble salts. Within this rock group, the <br />most significant contributor is the Late Cretaceous Mancos Shale, which <br />is common throughout the Region. The siltstones, claystones, and mud- <br />stones add large volumes of sediment to the stream flows. Some poten- <br />tially large problems such as siltation and increased salinity can be <br />created in stream segments when these sedimentary rocks are disturbed. <br /> <br />,..~.' <br />tHD <br /> <br />DEMOGRAPHIC CHARACTERISTICS <br /> <br />The population of the Upper Colorado River Region grew from 271,000 <br />in 1940 to an estimated 438,000 in 1975. Much of that growth occurred <br />in the period from 1970-1975, when the population increased from 345,000 to <br />an estimated 438,000, or a 30.3 percent increase. In terms of its <br />distribution, the population is divided about evenly between the Green <br />River, Upper Colorado Main Stem, and San Juan-Colorado ASAs, with the <br />Upper Colorado Main Stem containing slightly more than one-third, or <br />about 170,000 persons, in 1975. <br /> <br />Farm population has been declining steadily since the 1940s as farms <br />have been consolidated and as increased mechanization has decreased labor <br />demands. In 1970, the farm population in the Region was only 35,000, while <br />two decades earlier it was 76,000. U.S. Bureau of Census figures show that <br />farm population declined most sharply in the San Juan-Colorado ASA from <br />1950 to 1970. Since 1970, however, farm .population has stabilized. <br /> <br />Only four cities in the Region had populations of 10,000 or more <br />in 1970. Durango and Grand Junction were the largest Colorado cities <br />in the Region; Rock Springs, Wyoming, was the focal point in the Green <br /> <br />2-6 <br />