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potential of local soils is not good. The land is mainly useful for grazing and wildlife <br />habitat. Along the major drainages of the region, the warm alluvial soils can support grass- <br />hay and other crops if they are irrigated (Fox 1973:111-19). <br />The alluvial and aeolian depositional systems have a regular stratigraphy that is <br />expressed, at least in part, over much of the project area. Colluvial deposits have no unique <br />stratigraphic sequence, but interbed with or overlie other types of deposits. However, <br />frequency of significant colluvial events – slides and slumps – are somewhat regulated by <br />overall climatic conditions, especially during times of climatic transition from cooler, <br />concomitantly moister conditions, when slopes are stabilized by vibrant vegetal growth, to <br />warmer, dryer conditions, when slopes are destabilized and sediment stored on slopes is <br />more heavily affected by the force of gravity. Periglacial deposits are integral to loess <br />deposits, but are generally absent in alluvial deposits that existed at the time of frost heaving <br />and survived to the present day. <br />Alluvial processes are the most visible process affecting the landscape, expressed by <br />the deep alluvial fill and well developed fan complexes. Colluvial or mass wasting <br />processes are almost as important, with striking evidence in many places. The evidence for <br />other processes is more subtle. Aeolian processes are represented by discontinuous sheet <br />and shadow deposits on leeward and north-facing slopes or blanketing the flatter, highland <br />terrain expressing gentle slopes with a northern or eastern aspect. Periglacial processes (i.e. <br />frost heaving) are even more subtle, with virtually no surface evidence except for infrequent <br />exposures in older aeolian deposits. <br />Alluvial deposits span the latest Pleistocene and Holocene and are displayed in a <br />regular sequence of vertical accretion alluvium. Since the beginning of the Holocene <br />climatic envelope, drainages in the area have accumulated sediment, in braided stream <br />deposits early on, but as overbank, crevasse splay and backwater deposits later. The fact <br />that so much sediment is available to the systems has resulted in deep alluvium in the major <br />trunk drainages with at least three periods of deposition and one cut terrace apparent inside <br />the arroyos. <br />The important aeolian deposits in the survey area are loess (wind-blown silt) sheets <br />or blankets and shadows. The term “sheet” or “blanket” defines thin, albeit widespread <br />deposits that drape the surface (they are also termed goze, e.g., Reineck and Singh 1975). <br />Shadows are accumulations, generally leeward of effective obstructions, and are best <br />developed on east-facing slopes; however, obstructed shadows form on the windward side. <br />Deposition of either sheets or shadows is a result of wind turbulence caused by expanding or <br />compressing wind flow; in which, the turbulent flow allows suspended sediment to fall out <br />to form the deposits. Preservation and long-term stability of these deposits on the landscape <br />is in large part the result of vegetal growth and vitality, so the deposits are phytogenic in <br />nature, meaning they depend on vegetation for accumulation and stability. Vegetation <br />benefits through the accumulation of sediment which enhances water storage and provides <br />nutrients. Climate change to warmer, concomitantly drier conditions decreases vegetal <br />vitality and this leads to degradation of the deposits. <br />5