PHOTOGRAMMETRIC AND SCHMIDT HAMMER ANALYSES TO UNDERSTAND EROSION OF THE ST. PETER SANDSTONE IN LA SALLE COUNTY, ILLINOIS

Sandstone exposures in North-central Illinois are investigated using Structure-from-Motion (SfM) and Schmidt hammer exposure age dating techniques to understand post-glacial canyon incision. The retreat of Wisconsinan glaciers ~15,000 years ago is thought to have caused outwash floods, carving ~50-200m deep canyons into the St. Peter Sandstone, a weakly cemented quartz arenite. Because the St. Peter is very friable, the canyons are susceptible to human disturbance and rapid natural erosion. Erosion rates may also be affected by cement and porosity of the rock. Case hardening occurs when minerals precipitate within pore spaces where rocks are exposed at or near the surface. Increasing cement can strengthen the rock and increase resistance to erosion. The focus of this study is to explore how quickly short-term erosion occurs, and how erosion relates to rock strength, case hardening, and porosity. Porosity and cement concentrations are assessed through thin section analysis. Repeat SfM photographic data are collected from study sites where canyon walls exhibit carvings from human interaction. Densely collected photos are used to create 3D point clouds. Point clouds are converted to rasters to calculate differences between site visits and compare changes in surface topography following erosion events. Additionally, Schmidt hammer techniques are implemented to determine rock strength at the study sites. Two sampling methods are used to compare accuracy. In the first method, ten measurements are taken from across an outcrop surface with an N-type Schmidt hammer and averaged. The second is a double-strike method, where the hammer strike is measured at the exact same location on the outcrop surface. In the double-strike method, the difference between two hammer strikes indicates a degree of weathering or hardening on the surface. Greater rock strength values are likely associated with higher cement concentrations in bedrock thin sections. Where rock strength values are high, photographic analyses are expected to show little change on the surface over time. Alternatively, where rock strength values are low, quantifiable changes are expected. Changes observed in the SfM analyses will allow for short-term calculations of erosion rates influenced by natural erosion and anthropogenic interactions with the surface.