Loess distribution by variable winds

Abstract

Systematic decreases in loess thickness with distance from the source, usually considered to reflect a prevailing wind direction, may relate to variable wind directions. If winds blowing transverse to a linear source carry their load to a farthest distance designated X m , winds blowing at an angle α to the source will distribute their load over a reduced distance, X m sin α, leading to a proportionate increase in thickness by a factor of 1/sin α. Thus where α is small, for winds nearly parallel to the source, deposition should be limited to a narrow corridor adjacent to the source, leading to greater accumulations in this area. Representation of random wind directions by integration of the thickness equation gives a linear relationship between thickness and the logarithm of distance, as observed empirically along loess traverses and in sampling near dusty roads. Addition of a moderate prevailing wind to the variable wind model preserves the semilogarithmic thickness relation on the downwind side of the source but reduces thicknesses on the upwind side and substitutes a nearly linear relationship between thickness and distance from the source. Loess thickness contours should be approximately parallel to the source regardless of the prevailing wind direction. The contours bulge outward adjacent to wider source areas, probably because of increased dust concentrations in the air. The variable wind hypothesis does not fully account for extraordinary thicknesses of loess within a few miles of the source. These extraordinary thicknesses appear to relate in part to a still-rising dust cloud and hence increased low-level dust concentrations in air close to the source.