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 o f 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 i s selectively preserved along a gully at Malin Beg and comprises mud/m uddy diamict, sand and diamict. (1) The mud, which was deposited from suspended sediment plumes, passes upwards into muddy diamict, which re flects an increase in ice-rafting. The muds contain a redeposited fora miniferal assemblage of ice-proximal origin pumped into the water colu mn by jets reworking previously deposited outwash. The presence of Mid dle Devensian shells have been identified from the mud by amino-acid a nalysis. This suggests a Late Devensian age for the succession. (2) Th e overlying sand indicates an increase in traction current activity at the site. (3) There is a sharp planar contact between the sands and t he overlying tabular diamict unit. (4) A surficial sheet of coarse-gra ined 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 isos tatic 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-wes tern sector of the ice sheet was drained by major ice-streams. These e nded in calving bays. Ice marginal response to ice wastage depended la rgely on catchment factors such as the magnitude of the ice reservoirs and the length of flow lines. Ultimately calving led to accelerated i ce flow and areal down draw and collapse of the north-western sector o f the ice sheet.