TESTING THE RELATIONSHIP OF HEIGHT AND COSMOGENIC EXPOSURE AGE OF MORAINE BOULDERS

When choosing boulders from a moraine to sample for cosmogenic exposure dating, a traditional approach involves preferential sampling of tall boulders. This is based on the assumption that tall boulders are less likely to have been covered by snow or buried under the surface of the moraine crest after being deposited. Surface cover of moraine boulders decreases the energy of cosmic radiation received by the boulder surface and would result in an exposure age of the boulder that is younger than the age of the moraine. Few, if any, reported datasets of cosmogenic exposure ages of moraine boulders have demonstrated that taller boulders yield exposure ages closest to the true moraine age. This study tests the assumption that taller boulders yield older exposure ages than younger boulders by exploring a large number of exposure ages and heights of moraine boulders available in the literature. Using reported heights and cosmogenic ¹⁰Be exposure ages of boulders atop Pleistocene moraines in the Uinta Mountains, Utah, USA and Gorkha Himal and Langtang Himal, Nepal and Pamir, Tajikistan, linear regression analysis was used to evaluate the relationship of boulder height verses exposure age for each moraine. The slope of the best-fit line is considered an indicator of whether taller boulders yield older exposure ages than shorter boulders, and therefore yield exposure ages that are closest to the true age of the moraine. For the moraines we studied, the slopes of the best-fit lines cluster around zero (with low chi-squared values), suggesting that taller moraine boulders yield the same exposure age as shorter boulders. This finding is similar to that of Briner (2009), who found indistinguishable exposure ages of pebbles and boulders from atop Pleistocene moraine crests in the Colorado Rocky Mountains. Despite the similarity of these findings, the question of whether they are evidence of a geomorphically stable moraine crest or significant degradation of a moraine crest needs further consideration. We discuss this issue in the context of our statistical analysis and previous statistical analyses of the distribution of cosmogenic exposure ages of moraine boulders. [Briner, J.P., 2009. Moraine pebbles and boulders yield indistinguishable ¹⁰Be ages: A case study from Colorado, USA. Quaternary Geochronology 4, 299-305.]