INFLUENCE OF MORAINE DEGRADATION ON THE DISTRIBUTION OF BOULDER-DERIVED COSMOGENIC NUCLIDE AGES AT BISHOP CREEK, EASTERN CALIFORNIA

Moraine crests are a useful landform for dating glacial advances and serve as useful markers for determining fault slip rates. A common technique for dating moraines uses cosmogenic nuclide exposure ages from boulders sampled along moraine crests. These ages indicate how long a boulder has been exposed at the top of the moraine, thereby providing a minimum constraint on moraine age. However, boulders currently exposed along the moraine crest may not have been exposed since the moraine formed: as the moraine crest experiences erosion and its topographic profile diffuses, boulders are exhumed from within the moraine to the surface. This means that some of the boulders sampled and dated will have exposure ages that are younger than the deposition of the moraine. As a result, the exposure ages are often younger than the true age of the moraine. By combining the distribution of boulder ages along moraine crests with the predicted change in moraine topographic profile over time, we can more accurately estimate the true age of moraine formation. We assume that the boulders randomly sample depth below the moraine crest and stay at the surface of the moraine crest once exhumed, yielding a distribution of exposure ages approximately proportional to the square root of time. We test this model from the thoroughly dated sequence of moraines along Bishop Creek near Bishop California using airborne lidar topography and discuss the effects of moraine age and height on the age distributions.