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<br />GEOLOGY <br /> <br />\) <br />" <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 Bountains reveal a structural <br />anomaly of anticlinal sedimentary rocks. <br /> <br />.... <br /> <br />-.J <br /> <br />The sedimentary rocks of the interior basins form the basis of most <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 />. <br /> <br />The sedimentary rocks are also the source of actual and potential <br />degradation of the water quality, ~any of the rocks contain soluble <br />minerals such as calcium sulfate, sodium chloride, and potassium salts, <br />which leach onto the surface hydrology and into the ground water. Salt <br />loading also occurs when surface water flows directly over salt domes. <br />Shales are the primary rock types containing soluble salts. Within this <br />rock group, the most significant contributor is the Late Cretaceous Hancos <br />Shale, which is common throughout the Region. The siltstones, claystones, <br />and mudstones add large volumes of sediment to the stream flows. Some <br />potentially large problems such as siltation and increased salinity can be <br />created in stream segments when these sedimentary rocks are disturbed. <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 1970-75, when the population increased from 345,000 to <br />an estimated 438,000, Dr 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 />Rural farm population has been declining steadily since the 1940s <br />as farms have been consolidated and as increased mechanization has <br />decreased labor demands, In 1970, the farm population in the region <br />was only 35,000, while two decades earlier it was 76,000, U.S. BUreau <br />of Census figures.show that farm ,population declined most sharply in the <br />San Juan-Colorado ASA from 1950 to 1970, <br /> <br />t <br /> <br />Only four cities in the region had populations of 10,000 or more <br />by 1970, Durango and Grand Junction were the largest Colorado cities <br />in the Region; Rock Springs, Hyom:i,ng, was the focal point in the Green <br /> <br />2-6 <br />