The Last Irish Ice Sheet: Extent and Chronology

Abstract

Radiocarbon ages on sub-till organic material suggest that Ireland was largely ice-free until ~32 ka, after which the last Irish Ice Sheet (IIS) thickened to cover all mountain summits. Offshore moraines and related evidence indicate that the IIS extended westward to the Atlantic shelf break, eastwards into the Irish Sea Basin and southwards to the Celtic Sea shelf edge. Expansion of the IIS to its offshore limits was asynchronous: it apparently reached its limits on the Malin Sea shelf by ~27 ka, but after ~25 ka on the Porcupine Spur and possibly as late as ~24–23 ka on the Celtic Sea shelf. The ice-sheet margin appears to have retreated to the present coastline of Ireland by ~20 ka, and probably earlier in SE Ireland. Nested offshore and onshore moraines and the alignment of successive generations of subglacial bedforms demonstrate that retreat was interrupted by readvances and characterised by migration of ice divides and switching flow directions. Only two readvances are securely dated: the Clogher Head Readvance (~18.4 ka) in northern Ireland and the Killard Point Readvance (~17.3–16.6 ka) along the NE coast. Following the Killard Point Readvance, Scottish ice advanced across the north coast (the East Antrim Coastal Readvance) sometime around or after ~16.5 ka. It is likely that low ground was completely deglaciated following rapid warming at ~15.4–15.0 ka or slightly later. Small glaciers formed in some mountain areas during the Nahanagan (Younger Dryas) Stadial of ~12.9–11.7 ka, but some cirque moraines formed much earlier.