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<br />002307 <br /> <br />yellow pine or aspen at the highest.elevationsand piilon-juniper at the lower alti- <br /> <br />tudes. Sagebrush, bunchgrasses, and desert shrubs are found in the lower,' <br /> <br />drier sections. <br /> <br />In the central section, high mountain ranges exten4 in, a general northwest- <br /> <br />southeast direction. The mountain tops generally rise 4,000 to 6, 000 feet above <br /> <br />. <br /> <br />, " <br />the vaUey floors and several peaks exceed 11,000 feet above sea level. The <br /> <br />vegetative cover in the central section is similar to that of the northern plateau <br /> <br />area, but '<Vith a greater proportion of desert types as a result of the generally <br /> <br />.lower average elevation of the region. <br /> <br />The southern section is sometimes referred to as the desert, or plains <br /> <br />region. It consists largely of vast stretches of desert plains broken by short <br /> <br />m.ountain chains from 1, 000 to 3, 000 feet, in elevation. The mountains are <br /> <br />seldom forested and the plains or broad level valley support typical desert <br /> <br />. flora, or are devoid of vegetation. Portions of the area are extensively <br /> <br />irrigated, particularly in the Gila and Salt River Valleys. <br /> <br />B, Geology <br /> <br />. That portion of the study area within the Colorado Plateau Province is dis-' <br /> <br />tinguished primarily by the horizontality of its rock formations in contrast to <br /> <br />the folded formations in adjoining provinces. Stratigraphically, the Colorado <br /> <br />Plateau Province is characterized by massive sandstone formations with <br /> <br />some shale interbeds. As the formations have been subjected to extensive <br /> <br />erosion in an arid climate, mesas, cuestas, escarpments, canyons, and dry <br /> <br />7 <br /> <br />