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Show PART II NATURE, USE AND MANAGEMENT OF WATERSHED LAND Geologically, the exposed rocks range from very old to recent. Generally, the oldest rocks are hard and make up the mountian cores while the younger rocks are exposed in the intervening basins. Sedimen- tary rocks of widely diverse ages are represented. Volcanics generally are in the younger range. The oldest rocks are crystalline rocks of igneous and metamorphic origin such as granites, gneisses, schists, quartzites, and argillites, and are mainly in the mountains. Volcanic igneous rocks occur as lava flows, ash falls, and other volcanic deposits. The areas of hard rocks produce low sediment yields. Bordering the lower mountain slopes and extending over foothills, plateaus, and mesas are vast expanses where the underlying rocks are sedimentary. They are mainly alternating beds of resistant sandstone and soft shale. Valleys underlain by soft shales with sandstone cliffs or breaks along the margins are a characteristic feature in many land- scapes. High sediment yields are common where soft shales are dominant Erosion has produced many barren exposures of sandstone and shale that are associated with badlands. Some of these formations have relatively high concentrations of mineral salts. Younger unconsolidated deposits of alluvial, colluvial, glacial, and eolian origin mantle at least 75 percent of the region, but in most places they are only a few feet thick. Glaciers greatly modified the shape of many upper stream valleys and left glacial deposits in parts of the higher mountains. Sandy and gravelly alluvium is common on pediments, fans, and stream terraces. It is less erodible than the silty and clayey alluvium occupying the larger valleys. Colluvial and talus deposits are widespread on side slopes while eolian deposits mantle many of the mesas and plateaus. Slope gradient largely governs the velocity and erosive power of runoff. There is a high sediment yield hazard where erodible materials occupy steep slopes and are transported into well-defined water courses Landownership and Administration The Upper Colorado Region contains 72,234,000 acres of land and 405,000 acres of water area (water areas are defined as those with a minimum surface area of 40 acres and streams having a width of 1/8 statute mile or more). The landownership and vegetal cover figure shows acreage distribution within the region by ownership and major vegetal cover. State and federal ownership comprise 65 percent of the region. The remaining 35 percent is Indian trust, individual and corporate holdings. Additional information on landownership and administration including a generalized map is contained in Land Resources and Use, Appendix VI. 12 |