Rock Glacier Movement and Debris Transport Over Annual to Multi‐Millennial Timescales

Rock glaciers are common in alpine landscapes, but their evolution over time and their significance as agents of debris transport are not well‐understood. Here, we assess the movement of an ice‐cemented rock glacier over a range of timescales using GPS surveying, satellite‐based radar, and cosmogenic 10Be surface‐exposure dating. GPS and InSAR measurements indicate that the rock glacier moved at an average rate of ∼10 cm yr−1 in recent years. Sampled boulders on the rock glacier have cosmogenic surface‐exposure ages from 1.2 to 10 ka, indicating that they have been exposed since the beginning of the Holocene. Exposure ages increase linearly with distance downslope, suggesting a slower long‐term mean surface velocity of 3 ± 0.3 cm yr−1. Our findings suggest that the behavior of this rock glacier may be dominated by episodes of dormancy punctuated by intervals of relatively rapid movement over both short and long timescales. Our findings also show that the volume of the rock glacier corresponds to ∼10 m of material stripped from the headwall during the Holocene. These are the first cosmogenic surface‐exposure ages to constrain movement of a North American rock glacier, and together with the GPS and satellite radar measurements, they reveal that rock glaciers are effective geomorphic agents with dynamic multi‐millennial histories. Plain Language Summary Rock glaciers are slowly moving mixtures of ice and rock debris found in many cold mountain environments. Although they are common, questions remain about their rates of movement, age, and effectiveness in shaping mountain landscapes. We studied a typical rock glacier in the Uinta Mountains of Utah (USA), demonstrating that it formed about 10,000 years ago and is currently moving at a rate of about 10 cm per year. This is about 3 times faster than the long‐term average calculated from the age and length of the rock glacier, meaning that it is now moving faster than usual. We were also able to estimate that this rock glacier is transporting a volume of debris equivalent to about 10 m of erosion from the cirque headwall above it, confirming that rock glaciers, although slow moving, are important agents of erosion. Key Points We investigated an active rock glacier currently moving at ∼10 cm yr−1 The rock glacier formed at the start of the Holocene (∼10 ka) The rock glacier exhibited a long‐term average flow rate of ∼3 cm yr−1