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<br />terrain consists of high, rugged mesas and irregularly surfaced <br />plateaus separated by deep canyons. A web of streambeds and water- <br />ways covers the entire region. Elevations vary from approximately <br />4,500 feet near Frui ta in the northwest, to approximately 9,000 <br />feet in the high mesas in the south, to over 11,000 feet in the <br />mountains along the southeastern border. <br /> <br />Mesa County is part of the Canyon Lands subdivision of a larger <br />physiographic region known as the Colorado Plateau. Following <br />periods of intense volcanic activity, glaciation, and massive <br />crustal upheaval s, during which the basic structure of the Rocky <br />Mountains was formed, evolution of the present land forms of <br />western Colorado began at the close of the Mesozoic era <br />(approximately 60 mi 11 ion years ago). Another broad upl ift of the <br />region occurred at that time and, in a following cycle of erosion, <br />vast quantities of rock material were carried away. Following this <br />erosion and surface wasting, layers of sands, clays, and gravels, <br />which are known as the Eocene beds, were deposited over much of the <br />area. Then, a renewal of erosion and the development of a drainage <br />pattern on the Eocene beds occurred. It was at this time that the <br />existing main drainage patterns were fixed, and it is since that <br />time that the major topographic characteristics of the county were <br />carved by water and wind. As valleys deepened, the streamways were <br />superimposed on the older, underlying rock. Erosion continued and <br />an old-age erosion surface evolved. <br /> <br />The hundreds of plateaus in the region are more or less sharply <br />separated: some ri se, table-l ike, above thei r neighbors on all <br />sides; others adjoin their neighbors as steps or terraces; and some <br />are basin-like, with surrounding infacing cliffs. They also differ <br />in their degree of dissection by streams; as a result of differing <br />elevations and stages in the erosion cycle, the various plateau <br />formations may differ greatly in temperatures, rainfall, and <br />vegetation. <br /> <br />All streams in Mesa County are either direct or indirect <br />tributaries of the Colorado River, which traverses the north- <br />central and northwes tern sectors. From the northern county line, <br />the river flows southwesterly for 41 miles to its confluence with <br />the Gunnison River, thence northwesterly 21 miles, and again <br />southwesterly for 15 miles in its remaining course in the county. <br />In general, the Dolores River, Gunnison River, and West Creek <br />systems drain the western, southwestern, and south-central portions <br />of the county. The Plateau Creek system drains the eastern sector, <br />except for the easternmost portion, which is drained by the Divide <br />Creek system, which flows northerly to the Colorado River in <br />Garfield County. A group of minor creeks and washes flowing <br />southerly from the Roan and Book Cliffs regions drain the <br />northwestern portion of the county, and a group of similar <br />streamways convey drainage to the river from the north-central <br />portion. <br /> <br />7 <br />