FACTORS INFLUENCING THE DISTRIBUTION OF THE ILLINOIAN RIDGED DRIFT OF SOUTHWESTERN ILLINOIS

The Ridged Drift of the Kaskaskia River Basin is a series of major ridge belts generally parallel to the Kaskaskia River valley in southwestern Illinois. Over the past century, a number of researchers have proposed several hypotheses for the formation of the ridge system. While many have described the complexity of the Ridged Drift, no hypothesis yet has adequately explained the causes for the variations in the ridge forming processes throughout the region.

An early hypothesis was that the ridges were part of a morainic system bordering the western edge of a glacial lobe that persisted in southeastern Illinois after the ice margin in western Illinois had retreated. It was later found that some ridges contain large amounts of sand and gravel therefore a glaciofluvial origin was proposed. Another model was suggested when it was observed that the ridge system has the characteristics of a crevasse fill system with related moulin kames. Over a century after the ridges were first studied, a widely supported set of models for the formation of the ridge system is still sought.

A new study has been conducted to identify the glacial processes that led to the formation of the ridges. New detailed bedrock topography and lithology maps, created in a geographic information system, show relationships between ridges and trends in the glacial substrate. Many drift ridges follow the trend of bedrock ridges and valleys. In several cases, ridge trends align themselves with bedrock valleys that drain in the down ice direction. The bedrock valleys are themselves controlled by bedrock lithology and structure in the region and trend (northwest-southeast) along weaker units. Till fabric measurements at various locations on either side of the ridge system imply ice flow patterns from Illinoian glaciation in the region are more complex than previously thought. These ice flow patterns may be suggesting influence from the underlying bedrock topography and imply local variations from the previously assumed northeast to southwest pattern in southwestern Illinois.

Through this approach of integrating many different types of observations, a better explanation of the underlying processes that resulted in the formation of the Ridged Drift can be achieved. It is now becoming apparent that bedrock topography may have been a controlling factor in the occurrence of the Ridged Drift.