Glaciomarine facies from the western sector of the last British ice sheet, Malin Beg, County Donegal, Ireland

The coastal margins and mountainous core of County Donegal, Republic of Ireland have been areally scoured and deeply-eroded along structural lineaments by major Late Devensian ice streams which moved generally westwards onto the continental shelf. A raised glaciomarine sequence is selectively preserved along a gully at Malin Beg and comprises mud/muddy diamict, sand and diamict. (1) The mud, which was deposited from suspended sediment plumes, passes upwards into muddy diamict, which reflects an increase in ice-rafting. The muds contain a redeposited foraminiferal assemblage of ice-proximal origin pumped into the water column by jets reworking previously deposited outwash. The presence of Middle Devensian shells have been identified from the mud by amino-acid analysis. This suggests a Late Devensian age for the succession. (2) The overlying sand indicates an increase in traction current activity at the site. (3) There is a sharp planar contact between the sands and the overlying tabular diamict unit. (4) A surficial sheet of coarse-grained breccia is attributed to intense periglacial weathering along the slopes of Leahan mountain. This sequence and those at other sites in western Ireland (e.g. Belderg, Co. Mayo) provide a record of deep isostatic depression and sedimentation in a peripheral trough at the north-western margin of the last British ice sheet. Directional indicators including drumlin fields and major troughs indicate that the north-western sector of the ice sheet was drained by major ice-streams. These ended in calving bays. Ice marginal response to ice wastage depended largely on catchment factors such as the magnitude of the ice reservoirs and the length of flow lines. Ultimately calving led to accelerated ice flow and areal down draw and collapse of the north-western sector of the ice sheet.