INFLUENCE OF ROCK STRENGTH PROPERTIES ON BEDROCK CHANNELS AND HILLSLOPE EROSION IN ILLINOIS

Bedrock exposures along river channels in the Midwest are not common, but they do create interesting regional landscapes to study the response of channel evolution following deglaciation. The strength of the rock is an important factor in how the channel and bedrock hillslopes and banks respond to changes in climate, base level, and or glacial isostatic adjustments. This work presents some preliminary studies of the evolution of bedrock channel and hillslopes in Ordovician sandstones and dolomites that are exposed at the surface in rivers and streams in Illinois. Fracture properties including spacing and width influence the rock strength. Tree roots growing within the fractures combined with heavy rainfall or freezing water forming in the fractures with or near the tree roots appear to contribute to slope failure incidents. This study looks at the variability of rock strength in areas that experienced recent rock falls. It also investigates the rate of granular erosion or rock surfaces that are weakly cemented. Rock thin sections help identify the role of lithology, cement, and pore space on the strength of the rock in materials derived from slope failures. Spatial distributions of hillslope deposits and channel width and slopes help assess potential contributions to hillslope failures including channel orientation, location or large scale joints and fractures, and the frequency of groundwater seeps and sapping.