POST-GLACIAL EROSION OF SANDSTONE CANYONS IN CENTRAL ILLINOIS

Glaciers defined many landscape features throughout Central Illinois. Meltwater floods after Wisconsin glacial events helped create the Illinois and Fox Rivers. The tributaries to these river systems carved vertical cliffs rising 30-40 meters high into Ordovician St. Peter Sandstone. The timing of torrents is defined, however post-glacial evolution of tributaries to these systems is less well understood. This study begins to look at the timing and rate of tributary incision after glacial retreat and drainage of meltwater lakes by measuring bedrock and catchment averaged erosion rates. Bedrock samples were collected from canyon walls in a tributary to the Fox River, and stream sediments were collected from four tributaries to the Illinois River for ¹⁰Be cosmogenic nuclide analysis. Bedrock samples were collected along the channel length to evaluate how quickly the channel formed. Two pairs of samples were collected at upstream and downstream locations approximately 1-3 meters above the channel floor, and one mid-stream sample was collected approximately 0.25 meter above the channel floor. Results from the Fox River tributary produced upstream samples that were 6-7 ka, the mid-stream sample was ~3 ka, and the downstream samples were 7-8 ka. The erosion rate for the youngest, mid-stream sample was 2.3 mm/yr. The erosion rate for the remaining bedrock samples was ~1.3 mm/yr. Catchment averaged erosion rates from the Illinois River tributaries averaged ~0.012 mm/yr. Bedrock samples in the Fox River tributary were collected only from weakly cemented St. Peter Sandstone. Streams in the Illinois River tributaries flow across the same St. Peter Sandstone, but also across Pennsylvanian bedrock units and glacial tills. The difference in erosion rates may be related to the timing of flood torrents that created the larger river systems, the size of the catchments, or the variations in rock units across the length of the catchments. It is also possible that quartz in the stream samples are derived from glacial till containing sediments with a more complex exposure history resulting in inherited cosmogenic nuclides and slower apparent erosion rates.