DEGRADATION OF MORAINE SLOPES WITH AGE IN QUATERNARY MORAINES OF EASTERN CALIFORNIA

Moraines serve as geologic fingerprints of glacial episodes, and as with other landforms, the steepness of a glacial moraine decreases with time and with exposure to the environment. Thus, observing moraines to determine past instances of climate change requires an understanding of moraine shape. The slope angles (here defined as the angle from the horizontal) of a moraine should decrease with time owing to weathering and downslope diffusion, and models have been developed to predict this degradation. We studied the slope angles of moraines from glaciations in the eastern Sierra Nevada Range, including the Tahoe (~140,000-80,000 years ago), Tioga (26,000-18,000 years ago) and Little Ice Age glaciations (1350-1850 A.D), to determine the extent to which a correlation exists between slope angle and age.

We surveyed 7 moraines (1 Little Ice Age, 3 Tioga, and 3 Tahoe, in McGee Canyon, along Convict Creek, and in Glacier Canyon below Mt. Dana) using a TruPulse laser rangefinder. We used these transects to calculate a slope angle for each side of each moraine. The steepest moraine we surveyed is the Little Ice Age moraine non-ice side with a slope angle of 33.0°. The 6 slope angle values of the Tioga-age moraines fall between 17.8° and 31.7°, whereas the 5 slope angle values of the Tahoe-age moraines fall between 12.6° and 26.2°. The steepest angles from each glacial episode decrease linearly with increasing age. When comparing the ice and non-ice sides of each moraine, we found that the non-ice sides of the terminal moraines are on average 14.2° steeper than the ice sides, whereas the ice sides of the lateral moraines are on average 7.3° steeper than the non-ice sides.

Younger moraines have steeper slope angles than older moraines, consistent with erosion and weathering causing slopes to degrade as the age of the moraine increases. Our data show that the initial angle from the horizontal of a moraine is close to the angle of repose (34.0°), and over time erosion and gradual settling cause the angles to decrease. We also found that the ice sides of lateral moraines tend to be steeper than the non-ice sides, whereas the non-ice sides of terminal moraines tend to be steeper than the ice sides. Our research demonstrates that maximum slope angles can provide a method for assigning ages to moraines.