The lasting legacy of glacial landscape dynamics: Capturing the transport of boulder armor and hillslope retreat with geochronology in the Flint Hills of Kansas

We present new geochronological data derived from hillslope boulder armor in the Flint Hills in northeastern KS, United States, that provides insights into the rates and timing of lateral retreat in this landscape. Our results show that the surfaces of these limestone boulders date back to the Pleistocene era, well within the last glacial period. We also found that there is a significant increase in the ages of hillslope armor with increasing distance downslope from the modern limestone bench, the source of the boulders. Based on the age-distance relationship of the boulders, we estimate the rate of lateral retreat in this landscape to be 0.02 mm/yr, which falls between the geometrically estimated retreat rates based on calculated denudation rates of the Flint Hills region. We propose that the cooler temperatures and higher effective moisture due to less efficient evapotranspiration during the late Pleistocene period resulted in more effective freeze-thaw and transport processes, such as creep due to soil expansion and contraction. The production and transport of new boulder armor would then have effectively ceased once the climate transitioned to warmer conditions during the Holocene. Our findings suggest that the boulder armor we observe on the soil mantled hillslopes today are relict features from before the LGM-Holocene transition. These results provide important insights into the long-term evolution of these ubiquitous layered sedimentary landscapes.