INSIGHT FROM LOESS STRATIGRAPHY ON LANDSCAPE EVOLUTION IN THE CENTRAL LOWLANDS AND GREAT PLAINS

Although long-term landscape evolution is once again seen as a key problem of geomorphology, little recent work on this topic has focused on the Central Lowlands and Great Plains. Those regions seem likely to be far from dynamic equilibrium, with widely varying erosion rates across the landscape, since they are responding to glaciation and subsequent redevelopment of the drainage network, glacially driven relocation of major rivers, and other disturbances, at the slow rates expected in a tectonically stable setting. Such hypotheses have been tested elsewhere with cosmogenic nuclide analysis, but in the Midcontinent, loess stratigraphy can be used. I assume that well-defined Middle to Late Pleistocene loess units were deposited at rates that varied greatly at regional scales, but were locally uniform. If so, then hillslope-scale variation in their preservation should be related to variation in erosion rates averaged over the time represented by the loess sequence ( >100 kyr in some cases). For example, near Omaha, Nebraska, Middle to Late Pleistocene loess mantles fluvially dissected glacial deposits. The loess sequence, much of it deposited at high rates during the last two glaciations, is largely preserved on broad ridgetops and gentle slopes, indicating that loess accumulation since the Middle Pleistocene has exceeded long-term rates of erosion in those settings. The loess—especially older units—is truncated or absent on steep slopes, indicating much more rapid erosion. Thus, ridgetops are rising in elevation relative to streams, increasing local relief, but through loess accumulation rather than uplift. At the same time they are narrowed by slope retreat and eventually will cease to be stable locations of thick loess accumulation. In the Upper Mississippi Valley outside Late Pleistocene glacial limits, thick loess is preserved on ridgetops in dissected, relatively high-relief landscapes. Here, loess accumulation did not substantially increase local relief, but the pattern is otherwise similar to the Nebraska case. Loess stratigraphy rules out erosional lowering of the ridgetops since the Late Pleistocene (or the Middle Pleistocene in some cases), but is consistent with ongoing erosional retreat of steeper slopes. Both cases support the hypothesis that these landscapes are far from dynamic equilibrium.