Boulder hardness as an indication of relative age of moraines in eastern California (2012)

Undergraduates: Tyler Kress, Camille Morgan, Rachel Medlin, Annie Jin

Faculty Advisor: Allen Glazner
Department: Mathematics

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Determining the ages of glacial deposits provides information in paleoclimatological studies, but current methods (e.g. radionuclide dating) are expensive and time-intensive. In this study, we used a Schmidt Hammer (SH) to measure the hardness of boulders in moraines to evaluate a relationship between hardness and relative moraine age. Weathered rocks should produce lower and more variable SH rebound (R) values, so R values should decrease as moraines age. Our results show that the SH can be an efficient and economical method of determining relative ages of moraines.
We examined boulders from three glacial time periods: Tahoe, Tioga, and Little Ice Age. We took samples from three moraines in eastern California: in McGee Canyon, along Convict Creek, and in Glacier Canyon below Mt. Dana. Variability of SH readings requires taking a large number of measurements. The sample sizes for Little Ice Age, Tahoe, and Tioga were n=199, n=529, and n=270 respectively, where n is the number of R values taken. Each boulder was measured an average of ten times. The mean R values (calculated with 95% confidence intervals) of each glacial period do not overlap and decrease with increasing age: Little Ice Age=59.9±2.2, Tioga=53.0±1.1, Tahoe=49.2±1.3. Bootstrap analysis shows that approximately 100 measurements are needed to obtain a mean R value that is representative of the moraine. This allows for extrapolation of the relative age of a moraine from an average of R values taken in the field.