MOVEMENT OF AN ICE-CEMENTED ROCK GLACIER OVER ANNUAL TO MULTI-MILLENNIAL TIMESCALES, UINTA MOUNTAINS, UTAH

Rock glaciers, common geomorphic features in high-mountain landscapes, are important mechanisms of debris transport and comprise a potentially significant water resource. Because rock glaciers are rarely associated with radiocarbon-dateable organic matter and typically move much slower than alpine glaciers, less is known about their age and motion over time. In this study, we evaluated the movement and age of a representative rock glacier in the Uinta Mountains (Utah, USA) through repeat GPS surveying and cosmogenic surface-exposure dating with ¹⁰Be. A 100-MHz ground penetrating radar survey reveals that the rock glacier we studied is an ice-cemented feature lacking a massive ice core. Real time known (RTK) GPS measurements indicate that 32 boulders on the surface of the rock glacier displaced an average of 10.8 cm (±2 cm, range 6.2 to 14.9 cm) in a consistent downslope direction between September 2021 and September 2022. Six large (>1 m on a side) boulders on the rock glacier surface have cosmogenic ¹⁰Be surface-exposure ages ranging from 1 to 10 ka, suggesting that the rock glacier has existed throughout the Holocene. Exposure ages increase linearly with distance from the headwall; if we assume the rock glacier is a conveyor transporting material downslope over time, these exposure ages constrain a long-term average flow rate of 3 cm yr^-1. The apparent mismatch in velocities between our single year of GPS measurements (10.8 cm yr^-1) and the long-term average (3 cm yr^-1, as determined with cosmogenic dating) may indicate that the rock glacier is currently accelerating. Alternatively, the behavior of this rock glacier over time may be dominated by episodes of dormancy interrupted by short-lived intervals of relatively rapid movement. These are the first cosmogenic surface-exposure ages to constrain movement of a North American rock glacier, and together with the GPS measurements, they illuminate that rock glaciers may have multi-millennial histories with complicated patterns of changing flow rate over time.