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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 thi...

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Bibliographic Details
Published in:Earth surface processes and landforms 2024-10, Vol.49 (13), p.4077-4087
Main Authors: Shepherd, Brianna, Portenga, Eric W., Peplinski, Alexys J.
Format: Article
Language:English
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Summary: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: Rmean (n = 30) versus ΔR (Rmax − Rmin of five consecutive R‐values on the same spot). In all assessments, Rmean is similar for erratic surfaces with no and light surface treatment, and both are significantly lower than Rmean 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 Rmean 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. Schmidt hammer R‐value metrics measured on glacial erratics in southeast Michigan are not related to the ages of the underlying glacial landforms. Light use of a carborundum puck to remove loose minerals and lichens does not significantly affect R‐value metrics; heavy use is not recommended for Schmidt hammer dating studies.
ISSN:0197-9337
1096-9837
DOI:10.1002/esp.5973