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Abstract Rock glaciers dominate the cryosphere in mid‐latitude alpine settings, yet their activity and their histories remain challenging to constrain. We focus on the Thomas Lake rock glacier on Mt. Sopris in Colorado, USA. We measure surface velocities by feature tracking of image pairs and document Holocene¹⁰Be exposure ages on surface debris. The surface speeds average 0.8 m/yr and peak at 2 m/yr in a steep reach. Exposure ages range from 1.4 to 13.2 kyr and monotonically increase down‐glaciers. Ages exceeding 6 kyr occur in the bottom quarter of the landform, coinciding with sporadic tree cover. These constraints constrain a numerical model of Holocene rock glacier activity. In our model, surface velocity is entirely explained by the deformation of the ice‐rich core with the extra load of the rocky carapace. Surface mass balance is simplified to an accumulation area of ice and debris equivalent to the avalanche cone, and very low, uniform ablation in the remaining rock glacier where rock cover minimizes melt. Climate drives the activity through a history of ice accumulation in the avalanche cone. Matching the observed age and speed structure requires: (a) Early Holocene growth of the rock glacier, (b) low accumulation during the middle Holocene warm period (Hypsithermal), and (c) two Neoglacial accumulation pulses, the most recent being the Little Ice Age. Pulses travel down the valley as kinematic waves, re‐activating the landform. The headwall retreat rate of 4 mm/yr, inferred from rocky layer thickness and surface speed, far outpaces bedrock down wearing rates. 

Noetzli, J., Christiansen, H.H, Guglielmin, M., Hrbáček, F., Hu, G., Isaksen, K., Magnin, F., Pogliotti, P., Smith, S. L., Zhao, L. and Streletskiy, D. A. 2024. Permafrost temperature and active layer thickness. In: State of the Climate in 2023. Bulletin of the American Meteorological Society, 105 (8), S43–S44, https://doi.org/10.1175/BAMS-D-24-0116.1