An extensive Late Pleistocene delta body, related to a water plane at 100 m above sea level, is exposed in the Upper Carey Valley, Northern Ireland. Exposures along the modern river show a lowermost facies association, of massive diamict and poorly-sorted gravel deposited as sediment gravity flows, overlain by a gravelly foreset and topset facies association recording the progradation of a large glacier-fed delta. Diamict facies were emplaced as subaqueous debris flows; associated gravels are chaotically-bedded and were deposited by the downslope slumping of poorly-sorted boulder gravel; their subtle grading characteristics can be linked with a variety of mass flow processes. Sediment gravity flow facies originated upvalley from rapidly deposited sediments which accumulated near the exit of a narrow subglacial channel below an ice margin fronting in water. Subsequent ice retreat allowed the growth of a Gilbert-type delta. Regional evidence shows that the delta top records high relative sea levels in response to isostatic downwarping around the periphery of the last British ice sheet. The sedimentology and structure of the Carey Valley infill may be typical of high-level glaciomarine deltas preserved in topographic lows around the coastline of north Britain.