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