Lateglacial deglaciation of the McGillycuddy’s Reeks, SW Ireland through 10Be surface-exposure dating of glacial deposits in the Gaddah Valley: Implications for late glacial climate variability.

Reconstructing the precise timing and geometry of the British-Irish Ice Sheet (BIIS) is critical for understanding the sensitivity of the North Atlantic climate system to abrupt perturbations. We present preliminary results from a cosmogenic beryllium-10 (¹⁰Be) surface-exposure chronology of glacial deposits in the Gaddagh valley, situated on the northern flank of the McGillycuddy’s Reeks, southwest Ireland. Our geomorphological and chronological transect reveals a detailed history of deglaciation and ice-margin fluctuations since the Late Pleistocene.

Initial results indicate that high-elevation areas (300–350 m asl) adjacent to the main valley were ice-free by ~28 ka (n=2), suggesting an earlier onset of local thinning than previously modelled. The prominent "Hag’s Tooth Moraine”, the primary geomorphological feature in the valley, appears to represent a culmination during the Last Glacial Maximum (LGM). Following this peak, the deposition of a barely preserved subdued moraine impounded Loch Callee at ~19 ka (n=3), marking a significant phase of ice retreat at the onset of Termination 1. The final pulse of glacial activity is recorded 200 m higher in the catchment by a complex of five latero-frontal moraines. These landforms mark the former extent of a small cirque glacier, with the final abandonment of these positions occurring at ~12.7 ka (n=3).

Thus far, our findings implicate the following. First, the data do not support a complete ice cover over the McGillycuddy’s Reeks during the LGM as previously proposed; instead, we suggest that ice was topographically restricted to the main valleys, with the front of the Gaddah glacier not below 150 m asl. Second, our chronology indicates that terminal deglaciation occurred during a period traditionally associated with relatively cold climate conditions. This pattern of glacier recession during inferred year-round cold climate aligns with recent ¹⁰Be chronologies from Scotland (Bromley et al., 2018, 2023), central East Greenland (Kelly et al., 2025), southernmost Greenland (Carlson et al., 2021) and Norway (Putnam et al., 2023; Wittmeier et al., 2020), which demonstrate glacier shrinkage during the Younger Dryas. These results contribute to the ongoing discussion about glacier extension in the area and the evolving paradigm of North Atlantic climate dynamics, emphasizing the role of summer temperature as the primary driver of glacial mass balance during millennial-scale stadials.