UP IN THE REFRIGERATOR: GEOMORPHIC RESPONSE TO FULL- AND LATE-GLACIAL ENVIRONMENTS OF THE UPPER MISSISSIPPI VALLEY

Jim Knox was best-known for his work on Holocene and Historical changes in fluvial systems, but also had a long-standing interest in the effects of Late Pleistocene periglacial environments on landscape evolution in the Upper Mississippi Valley (UMV), when that region was just outside the ice margin, or as Jim said, “up in the refrigerator.” Jim argued strongly for accelerated hillslope erosion under periglacial conditions. This talk will summarize evidence on that issue, emphasizing new developments and unresolved problems. The sparse paleoecological data and features such as ice-wedge casts are consistent with a climate cold enough to allow extensive permafrost in the UMV around the last glacial maximum. In 1989 Jim used estimates of the post-depositional reworking of loess to document high rates of Late Pleistocene hillslope erosion in a Driftless Area watershed, results that can likely be extended to other high-relief, loess-mantled parts of the UMV. Accelerated erosion was probably a response to deposition of highly erodible loess on steep slopes as well as to periglacial conditions, however. Jim was also influenced by the work of Robert Ruhe, who inferred extensive Late Pleistocene hillslope erosion from the combination of anomalously thin or absent loess—indicating post-depositional removal—and the absence of well-developed soils in Middle or Early Pleistocene loess, glacial sediments, or bedrock residuum. Paradoxically, this combination is especially evident on low relief, very gently sloping landscapes of the UMV such as Ruhe’s Iowan Erosion Surface and parts of Wisconsin’s Central Sand Plain. I have argued that anomalously thin loess reflects thorough remobilization of dust from these landscapes under conditions of pervasive full- to late-glacial aeolian sand transport, because of both sparse vegetation and lack of topographic obstacles. New LiDAR data support this view by documenting extensive low-relief dunes (many with Late Pleistocene ages, as reported by Henry Loope in another abstract for this session). Wind erosion can at least partially explain the limited extent of well-developed pre-Late Pleistocene soils, and we may also need to reconsider the likely degree of development of those soils on poorly drained landscapes of dense till or sandstone residuum.