APPLICATIONS OF SCHMIDT HAMMER EXPOSURE-AGE DATING IN THE NORTH AMERICAN GREAT LAKES REGION: AN ELUSIVE ENDEAVOR

Schmidt Hammer exposure-age dating (SHED) provides a quick and widespread means of estimating ages of glacial erratics on undated glacial landscapes, after construction of a calibration curve between Schmidt Hammer rebound values (R-values) and independent exposure-age constraints on rock surfaces, which are often ¹⁰Be exposure ages. Wide spatial gaps exist between dated glacial landforms in the North American Great Lakes Region; SHED techniques could help develop or refine the glacial history of the Laurentide Ice Sheet following the last glacial maximum. In this study, we present initial findings from a 2022 attempt to construct a SHED calibration curve for the Great Lakes Region by measuring R-values of dated glacial erratics across Wisconsin and a 2023 field-based experiment in southeast Michigan testing the efficacy of different proposed R-value measurement or data reporting strategies. For the former, we visited glacial erratics (n = 34; all granite) across Wisconsin that were previously dated using ¹⁰Be exposure-age dating that ranged in age from 12.1 ka to 82 ka. No clear relationship between R-value and ¹⁰Be exposure age was found, nor did we find any clear relationship between R-value and lithological, topographical, or climatic variables between individual erratics. As such, we are unable to establish a SHED calibration curve for the North American Great Lakes Region at present. In 2023, we employed three different R-value measurement techniques: (1) taking 30 R-value measurements on untreated rock surfaces, (2) using a carborundum puck to pretreat rock surfaces before taking 30 R-value measurements, and (3) taking the difference between five consecutive R-values at the same location. We used each technique on one granite erratic from five glacial landforms in southeast Michigan, each consecutively younger than the previous. Initial results suggest no clear trend in R-value data from the oldest granite erratic to the youngest, suggesting that the lack of relationship between R-value data and ¹⁰Be exposure ages from Wisconsin erratics may not be due to the measurement technique we employed in 2022. We continue to explore reasons why a clear relationship between R-values and ¹⁰Be exposure ages did not emerge from our field data, but empirical explanations are thus far elusive.