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

Abstract

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 ΔR (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 ΔR, 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.