Abstract

Kelso Dunes lie in a mountain-rimmed basin, 35 miles east of their principal sand source, where the predominant, sand transporting, westerly wind is locally counterbalanced by strong, orographically controlled winds from other directions. In the sand-mantled source area most grains larger than 1 mm travel principally by creep under saltation impact. The size distribution and sorting characteristics of eolian sand become well established after 10–12 miles of saltation transport. Further transport produces greater rounding and more mineralogical fractionation but does not greatly alter other characteristics.

Fifteen years of measurements along established lines across individual transverse dunes record a high degree of activity but only a hesitating advance, involving three steps forward and two and a half steps back. Opposing winds shift the sand back and forth from one dune flank to the other, with frequent reversals in crestal asymmetry.

Crestal position is not a reliable index of bulk movement. Dune crests shifted back and forth within a 30–40-foot zone, moving several hundred feet in 10–12 years but ending up only a few feet from first-observed positions. Greatest accumulation and removal changes lie just to either side of the crest and are associated with reversals in dune form. Although tens to hundreds of feet of sand may be moved, net surface change in level after 10–12 years can usually be measured in inches in the crestal zone.

Lee-slope beds dipping 10–25 degrees predominate within these dunes and are overlain by a thin, well-laminated, windward-slope layer inclined as much as 16 degrees in the opposite direction. Slip-slope deposits as steep as 30 degrees are only scantily preserved.

The orientation of several hundred lee slopes reflects only modestly the influence of prevailing western winds. Storm winds from other directions, earlier winds with a more southerly component, and local orographic controls complicate the picture. This suggests that reliable interpretation of paleo-wind directions from cross-bedding in ancient eolian sandstones requires a knowledge of the type of dunes represented and their response to a possibly varied wind pattern.

Observations with smoke pots during high winds lead to the conclusion that no strong, fixed eddy lies to the lee of transverse dunes in the Kelso complex. When strong transverse winds blow, currents on the slip face are for the most part capricious and too gentle to produce significant sand movement. Occasional gusts and traveling eddies traverse the slip face, usually in a longitudinal or oblique direction. However, no powerful, fixed, lee-side eddy was found which significantly influences dune behavior and morphology in the manner and to the degree postulated by Vaughan Cornish.

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