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• 2.04.9 SOILS RESOURCE 1NFORMAT[ON <br />a. Overall Setting <br />The soils of the northeastern plains of Colorado in general have developed under a temperate, <br />semi-arid climate (average annual precipitation from 11.12 to 16.32 inches per year). The <br />native vegetation consists primarily of short grasses interspersed occasionally with low- <br />growing shrubs. These conditions have resulted in soils that are somewhat immature and have <br />only moderate amounts of organic matter. <br />The productive agricultural lands of northeastern Colorado have soils which are developing on <br />parent materials from three main sources: 1) local outwash over broad, gently sloping <br />pediments along the Front Range of the Rocky Mountains, 2) alluvial materials deposited <br />along bottom lands and terraces of rivers and streams, and 3) relatively deep deposits of <br />windblown silts and fine sands (loess) occurring in upland positions between stream channels. <br />In addition to the above, relatively minor amounts of glacial outwash materials, water <br />reworked loessial materials, and deeply decomposed residual geologic materials serve as <br />parent materials for highly productive agricultural soils. <br />The soils of the pediments and the soils of the river valleys vary widely in texture, depth, and <br />degree of development, as well as in their chemical status. Generally, however, they are <br />medium to moderately coarse textured, deep, and free of harmful accumulations of salt. <br />• The soils of the uplands are predominantly deep, medium textured, and also free of harmful <br />accumulations of salts. The soils on the pediments and uplands which are most productive for <br />agriculture are generally moderately well to well developed, since they occupy the level to <br />gently sloping portions of these land forms. Soils of the river valleys range from almost no <br />development in the more recent alluvium to very well developed on the older high terraces. <br />Soils of the project lands show great variation in source, depth, and material. They were <br />formed largely from wind and waterborne materials which have been resorted and reworked <br />after deposition. The South Platte River Valley is underlain with alluvial sands and gravels <br />which form the aquifer for irrigation well development. The bottom lands are recent alluvium, <br />ranging from a few inches to several hundred feet in depth. They consist of a heterogeneous <br />mixture of clay, sand, and gravel. Terraces occur in contact with the bottom lands and were <br />formed from calcareous alluvial deposits on old stream terraces. <br />Most of the proposed mining area is covered with windblown fine sand. The valley areas, <br />located outside and adjacent to the site, are residual or eolian in origin. They have been <br />deposited and reworked over periods of time. Due to their origin, they may range in texture <br />from coarse sandy loam to clay. Most are covered by a thin layer of loess (windblown silts or <br />fine sandy materials). <br /> <br />72 <br />