BOULDER HARDNESS AS AN INDICATION OF RELATIVE AGE OF MORAINES IN EASTERN CALIFORNIA

Determining the ages of glacial deposits provides important information in paleoclimatological studies, but current methods such as cosmogenic 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. Weakening of rocks due to weathering should produce lower and more variable SH rebound (R) values, so R values should decrease as moraines age. Our results show that SH measurements can be an efficient and economical method of determining relative ages of moraines.

We examined boulders from three glacial time periods: Tahoe (140,000-80,000 years old), Tioga (26,000-18,000 years old), and Little Ice Age (700-200 years old). 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 that a large number of measurements be taken to obtain an accurate representation of the average hardness of the rock population. 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. Although the standard deviations of these measurements are high (e.g., 12.9 for boulders from Tioga moraines), the mean R values of each glacial period decrease with increasing age: Little Ice Age = 59.9, Tioga = 53.0, Tahoe = 49.2, and 95% confidence intervals of the mean R values do not overlap: Little Ice Age = ± 2.2, Tioga = ± 1.1, Tahoe = ± 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.

These data support the use of the SH for indicating relative ages of moraines. Bootstrap analysis supports the hypothesis that each glacial period has a discrete average and that these averages become discrete at a sample size of at least 100 R values. These results support the use of the SH as an inexpensive method for determining the relative ages of glacial deposits.