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Aeolian sheet deposits exhibit a regular four part sequence in the survey area. Two <br />thirds or more of the total deposit is middle Holocene in age, possibly with some lower <br />elements aging to the latest Pleistocene or early Holocene. The remaining depth represents <br />late Holocene accumulations after about 3000 years ago. Typically, the late Holocene <br />deposits are discontinuous in the part of the Piceance Basin surrounding the disparate survey <br />areas, but are best preserved in coppice mounds in open areas. On relatively flat terrain or <br />on protected (north- and east-sloping) slopes the upper deposits are more or less complete. <br />Loess deposition is relatively insignificant in the windward edges of the forests, but deeper <br />in and especially with a favorable aspect, such as on gentle north or east slopes, the loess <br />deposits are well preserved and somewhat thicker, although bioturbated. <br />The aeolian loess sequence on steep slopes is incomplete. On the north-facing <br />slopes bordering the south flanks of the main trunk streams, mixed loess and sheet wash <br />alluvial deposits are relatively young, probably a maximum of about 3000 years old, but <br />probably younger, and perhaps as young as the last 500 years in some areas. Some of the <br />deposits, however, may contain the remnants of older loess deposits that have been mostly <br />stripped away. <br />Maximum depth of loess deposits are on the order of a meter or a meter and a half in <br />thickness. The best preserved deposits and the most complete aeolian sequences are on sage <br />flats in the highlands, and these exhibit a four part sequence, with each individual sheet <br />separated from older and/or younger components by unconformities. Unconformities are <br />represented by serir deposits formed by deflation and in cross section appear as thin (1-2 cm <br />thick) zones of coarsening grain sizes. Serir deposits mark lacunas, periods of erosion, and <br />some are further enhanced by frost heaving. <br />The upper three loess deposits were emplaced in the last 3000 years, with the <br />lowermost of the three probably aging between 2800-1000 years ago, and the upper two to <br />between 500-350 years ago and into the 20th Century. The oldest sheet deposits probably <br />date to between 6500-3500 years ago, but could include remnants of older loess deposits <br />which were weathered during the same period in the middle Holocene. The separate <br />deposits are frequently distinguishable by secondary mineral formation prospered by in- <br />place or syndiagenetic weathering, with older deposits displaying correspondingly greater <br />degrees of weathering and secondary mineral accumulations. The important secondary <br />minerals are calcite, smectite, oxy-hydroxides, and sulfides. <br />Vegetation <br />Elevations of the study area fall within the Upper Sonoran vegetation zone. Two <br />main plant communities are present: sage shrubland, and pinyon-juniper woodland. It is <br />assumed that the plant species present today were there in the past; however, the <br />proportionate representations of the vegetation communities has undoubtedly been altered in <br />modern times by grazing of domestic animals or displaced deer populations. <br />Sagebrush communities occur in the drainage bottoms areas and in open parks on the <br />6