THE LOESS TRANSPORTATION SURFACE IN WEST-CENTRAL WISCONSIN

For loess to be transported, silt grains must first be deflated, and deflation is usually initiated or assisted by saltating sand. As long as saltating sand is present, silt deflation may continue to be active, and final deposition of loess may be difficult to accomplish. This type of loess generation-transport system, first described by Joe Mason and colleagues in 1999 (Geomorphology 28:223-236), characterizes a type of “loess transportation surface” - where sand is widely available, winds are strong, and vegetative cover is minimal. In these types of situations, loess transport will continue until a topographic barrier such as a deep valley or a large ridge impedes the saltating sand, allowing sand-poor loess deposits to accumulate downwind. In this talk, I describe a classic loess transportation surface/system in western Wisconsin, across which I have sampled the eolian mantle at hundreds of sites. On the transportation surface, silty loess is absent, and any eolian sediment occurs as a thin, sandy-loamy-silty mantle. The transportation surface has two main sand sources: glacial outwash and isolated hills of friable sandstone. Most of these sand sources occur in the western (upwind) parts of the transportation surface. To the east, eolian deposits remain thin and loamy, with bimodal (sand and silt) grain size distributions. Thick loess deposits are eventually encountered at sites that occur downwind of: (1) high, isolated sandstone hills, and (2) the valley of the north-to-south flowing Black River. Maps of loess grain size and distribution at the many sample sites, relative to these two types of topographic barriers, comprise the bulk of this talk. These data clearly illustrate how loess texture, thickness, and distribution are related to topography, in areas where sand is available. This work may help others explain small- to medium-scale loess patterns on the modern-day landscape.