NEW INSIGHTS ON POSSIBLE ORIGINS OF THE “RIDGED-DRIFT” IN SOUTHWESTERN ILLINOIS

The “ridged-drift” in southwestern Illinois has been studied by many geologists for over a century; however, the exact origin of the landforms and associated deposits has been perplexing due to their high variability. Prior hypotheses for their origin range from crevasse fills to morainal to glaciofluvial to interlobate complexes. The “ridged-drift” refers to elongate ridges, along with associated hummocks, that are typically 0.5 km to 10 km in length and stand 10 to 30 m above the otherwise flat Illinoian till plain. These constructional ridges generally parallel the Kaskaskia River valley and the regional ice flow direction (SW) of the Illinoian glaciation, during which they formed.

New observations and data acquired during 1:24,000 mapping reveal deposits in ridges up to 30 m thick which vary from dominantly sand and fine gravel to dominantly fine-grained diamicton and lake sediments with minor sand lenses. Ice-thrust features, debris flow deposits and inclusions of pre-Illinoian deposits are common in the fine-grained facies. All ridged-drift deposits contain Sangamon Geosol development in the upper 2 to 3 m and are draped by 2 to 7 m of Wisconsinan loess. High resolution geophysics, including electrical resistivity and seismic reflection, have aided imaging the complex structure of subsurface deposits below the loess cover. GIS analyses has revealed that many ridge systems initiate at bedrock prominences or where bedrock topography ascending to the west or southwest has caused divergent flow. Thus, during the waning period of Illinoian glaciation (with thin downwasting ice), some ridges may have formed in local interlobate areas in the lee of bedrock highs, where possibly ice stagnation occurred and supraglacial and/or ice-marginal channels developed. In many areas, the SW terminus of a ridge system curves towards modern or buried valleys and merges into proglacial outwash deposits of similar age. Overall, a complex glacial origin can be envisioned for the ridge systems that includes supraglacial, subglacial and ice-marginal streams as well as crevasse fills and ice-thrust deposits, with such processes varying spatially and temporally. The deposits and landforms in the ridged-drift in some ways resemble the younger kettle-moraine area of southeastern Wisconsin (Carlson et al., 2005), but on a smaller scale.