In defence of the Little Ice Age glacier hypothesis in the Cairngorm Mountains

Ballantyne [2026. Did the Cairngorm Mountains support glaciers during the Little Ice Age? Scottish Geographical Journal (this issue)] expresses scepticism of the evidence for glacier ice during the Little Ice Age (LIA) in Coire an Lochain (Cairngorm Mountains), arguing that the Holocene exposure ages of moraine boulders [Kirkbride, M.P., Everest, J.D., Benn, D.I., Gheorghiu, D. M. & Dawson, A.G. (2014). Late-Holocene and Younger Dryas glaciers in the northern Cairngorm Mountains, Scotland. The Holocene, 24 (2), 141–148] reflect exhumation by snow avalanches, and suggests that a LIA glacier could not have formed due to removal of snow by avalanching. He argues that our temperature estimates used for glacier-climate modelling [Harrison, S., Rowan, A., Glasser, N. F., Knight, J., Mills, S. C., & Plummer, M. (2014). Little Ice Age glaciers in the British mountains. The Holocene, 24(2), 135–140] overestimate LIA cooling, and that the moraines formed in the Loch Lomond Stadial c. 12 kyr BP. We stand by our LIA interpretation of the Great Slab moraine as the most parsimonious integration of evidence. Ballantyne’s (2026) interpretation is problematic for four reasons. First, there is no evidence of avalanche erosion of the 10Be sample sites on the moraines, nor do nearby avalanches appear to be erosive. Second, the avalanche-exhumation hypothesis requires removal of an unrealistically thick debris layer from the moraine for exposure ages to be around ten millennia younger than the Loch Lomond Stadial. Third, degradation would have removed most of a 12,000 year-old moraine, but not one deposited during the LIA. Finally, summer temperature estimates from tree-ring reconstruction will underestimate winter and mean annual temperature depressions, and extrapolations to high elevations using free-air lapse rates ignore cirque microclimate. Historical observations from the Ben Nevis observatory support a modelled 1.5⁰C LIA temperature depression, consistent with other published estimates. Ballantyne’s scenario of landscape evolution seems less likely than the LIA glacier hypothesis in which a small glacier reworked regolith cover from the Great Slab to create the moraines, much of the down-slope debris apron, and an environment enhancing present-day avalanching from the Great Slab.