SEDIMENTOLOGY OF THE ILLINOIAN RIDGED DRIFT OF SOUTHWESTERN ILLINOIS: NEW IDEAS ABOUT CHANGING PROCESSES

The Ridged Drift of the Kaskaskia River Basin is a series of ridge belts in southwestern Illinois formed during the Illinoian glaciation. The ridges, elongated generally parallel to the Kaskaskia River valley, rise 40 m or so above the otherwise low relief till plain. Material composing the ridges (below loess cover) varies in sedimentology, with predominantly well sorted sand and gravel in some ridges and sandy, and in some cases stratified, diamicton in others. Although a number of researchers over the last century have proposed many different models for the formation of the ridge system, it is likely that multiple hypotheses will be necessary to address the issue in its entirety. New sedimentological observations suggest that depositional processes were not the same spatially across the whole ridge system and were also changing temporally.

Sand and gravel pits offer unparalleled views of the geometry and relationships of sediment bodies within the ridges. Two such pits near Highland and Keyesport in southwestern Illinois exemplify differences in deposits within the Ridged Drift. The Highland pit exhibits a core of rounded gravel, overlain by a fining upwards package of sand, and capped by discontinuous stratified diamicton. In some places rounded blobs of diamicton are seen embedded in the sorted sediments. This suggests a process change during ridge formation from primarily fluvial conditions with progressively decreasing energy to debris flows during the final stages of deposition. The Keyesport pit, on the other hand, displays a body of cross bedded sand of fairly uniform grain size nearly 30 m thick that, in some areas, is walled laterally by diamicton. The sedimentology here suggests fluvial conditions of fairly constant energy capable of incision.

Changes in sedimentology demonstrate examples of the complexity and variety of depositional processes within the Ridged Drift and underscore the importance of thorough documentation of the sedimentary relationships. Sedimentary evidence implies spatial and temporal differences in depositional environments from possible sub-glacial tunnels, in some examples, to open air ice walled channels in others. In some cases the cessation of fluvial conditions resulted in the deposition of the stratified diamicton as debris flows emanated from stagnant melting ice.