| Abstract: | The enigmatic DeKalb mounds in north-central Illinois, United States, are oval, inactive hillocks of Wisconsinan age. They have heights up to 5 m, lengths up to 1 km, and are composed either of till and lacustrine sediment with a thin cap of loess, or entirely of outwash. They are underlain by one till member within a large depression that is flanked by higher, elevated, morainic till members. Different origins have been proposed for the DeKalb mound field but the only previous in-depth study identified them as pingo remnants, presumably due to the degradation of significant ground ice and permafrost. However, the investigators who reported this viewed their own hypothesis as problematical. These problems can be resolved by correlating the relict Illinois landforms to relict glacial landforms that show similar morphologies and field relationships in Saskatchewan, Canada, and North Dakota, United States. In this study, the DeKalb mounds are presumed to have developed from an intricate set of glacial dead-ice and mass movement processes. This reinterpretation reflects a range of either discontinuous permafrost or negative permafrost within the paleoenvironment. |
