Quantifying the effects of carborundum rock surface smoothing for Schmidt hammer R-value metrics measured on glacial erratics in the North American Great Lakes region

Schmidt hammer R-values, which reflect the degree of rock surface weathering, can be paired with independent rock surface exposure ages to produce Schmidt hammer exposure-age dating (SHD) calibration curves that are used to provide rapid estimations of exposure ages for undated rock surfaces. In this Letter, we lay the groundwork for later establishing a SHD calibration curve based on glacial erratics deposited by the Laurentide Ice Sheet in the North American Great Lakes region. First, we establish a reference R-value for a calibration boulder, which may be used to facilitate direct comparison of R-values collected by different individuals using mechanical N-type Schmidt hammers, specifically. We then assess how no, light or heavy use of a carborundum puck to smooth rock surfaces affects resulting R-value data. Lastly, we compare two different R-value metrics on the same rock surface: R-mean (n = 30) versus Delta R (R-max - R-min of five consecutive R-values on the same spot). In all assessments, R-mean is similar for erratic surfaces with no and light surface treatment, and both are significantly lower than R-mean of surfaces with heavy treatment. Thus, we advise against the use of heavy surface treatment in SHD applications. We observe no relationship between inferred erratic age and either R-mean or Delta R, which could arise from inferred ages of erratics being inaccurate, from inferred ages of erratics being too similar or from measuring too few erratics from each landform.