Vertical dimensions and age of the Wicklow Mountains ice dome, Eastern Ireland, and implications for the extent of the last Irish Ice Sheet

Abstract

Patterns of erratic distribution show that the Wicklow Mountains formerly supported an independent ice cap or ice dome. Geomorphological mapping of the upper limits of evidence for glaciation (ice-scoured and moulded bedrock, perched boulders) and the distribution of features indicative of prolonged periglacial conditions (tors, frost-shattered rock, blockfields) indicates that along the main axis of high ground erosive warm-based ice buried all but the highest (>725 m) summits and over-ran adjacent lower peaks and cols. The presence of gibbsite in soil samples from above the inferred upper limit of glacial erosion and its absence in all samples from below this limit is consistent with the geomorphological evidence and implies removal of gibbsitic soils below ∼725 m by glacial erosion during the last glacial stage. Cosmogenic 10Be exposure ages for rock outcrops above the inferred upper limit of glacial erosion yield pre-last glacial maximum (LGM) ages of (>) 46.9±3.0 ka to (>) 95.9±6.1 ka, whereas rock outcrops on summits over-ridden by warm-based ice give post-LGM ages of 18.2±1.2 to 19.1±1.2 ka. All geomorphological and dating evidence thus indicates an LGM age for the ice dome. The thickness of the ice dome and limited lateral dispersal of erratics indicate that at the LGM the Wicklow ice was encircled by and confluent with thick, powerful ice streams moving SE from the Irish Midlands and southwards down the Irish Sea basin. This conclusion is irreconcilable with the traditional view that the last Irish ice sheet terminated at the ‘South Ireland end moraine’, but consistent with recent proposals that the last ice sheet was much more extensive than previously believed, and over-ran the south coast of Ireland.