A new model of topographic effects on the distribution of loess

A model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed, based on the critical role played by saltating eolian sand in the entrainment and long-distance transport of finer-grained suspended dust. In this model, long-term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand, and associated re-entrainment of dust. Regional and local patterns of distribution of late Wisconsinan loess in parts of the Upper Mississippi River basin are closely related to the distribution of topographic obstacles to sand transport. These cases of topographic influence occur in a region where evidence exists for sparse tundra-like vegetation during the deposition of loess in the late Wisconsinan. Similar topographic effects may not be evident further south in the Missouri and Mississippi River basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys. Thus, topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions.