Periglacial trimlines and the extent of the Kerry-Cork Ice Cap, SW Ireland

Periglacial trimlines on the mountains of SW Ireland demarcate a contrast between glacially scoured bedrock on lower ground and thick periglacial regolith covers on summits and plateaus, and have previously been interpreted as marking the maximum altitude of the Kerry-Cork Ice Cap (KCIC). Glacially eroded bedrock extends to the highest points on the Beara peninsula, but the highest summits on the Iveragh Peninsula support thick regolith covers. Trimline altitudes on the Iveragh Peninsula decline radially from a former ice divide near the head of Kenmare River, from ∼700 m altitude near the divide to ≤470 m near Dingle Bay. Contrasts in the clay mineral content of above- and below-trimline soils and cosmogenic isotope exposure ages indicate a LGM age for the trimlines. Assumption that the trimlines represent the maximum thickness of the KCIC implies an ice divide altitude of ∼825 m. However, modelling of ice-surface profiles demonstrates that an ice divide at 825 m is irreconcilable with stratigraphic evidence for extension of the KCIC at least as far as Garryvoe, 115 km east of the divide. We infer that the Iveragh trimlines represent a former englacial transition between warm-based sliding ice on low ground and cold-based ice on high ground rather than delimiting nunataks as previously interpreted. Modelling of ice-surface altitudes suggests that during the LGM the KCIC probably overtopped all of the mountains in SW Ireland. The trimlines of SW Ireland thus define the minimum rather than maximum altitude of the KCIC at the LGM, as well as the sites of former ‘cold patches’ within the KCIC.

► Trimlines in SW Ireland exhibit radial decline in altitude from former ice divide.
► Clay mineralogy and cosmogenic isotope dating demonstrate LGM trimline age.
► Trimline interpretation resolved by reconstructing implied ice extent.
► Modelled ice extent invalidates palaeonunatak interpretation of trimlines.
► Demonstrates that trimlines demarcate cold patches within LGM ice cap.