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