A lithostratigraphical framework has been erected for Quaternary deposits of the Sellafield district in west Cumbria. It has enabled a sequence of glacial and deglacial events to be established that is one of the most detailed for the last glacial-inter-glacial cycle onshore in Britain and provides new constraints on models of the evolution of the Irish Sea basin during the last 120 000 years. The products of at least one major pre-Devensian (Oxygen Isotope Stage 6 or older) glaciation have been recorded in cored boreholes around Drigg and Nether Wasdale. Ice flowing from Wasdale terminated in proglacial lakes on two occasions during OIS 3 and 4 and an intervening cold-water marine incursion occurred that reached a height of at least 20 m below present sea level, probably during OIS 3. Glaciers emerged again from the Lake District during the build-up of the main Late Devensian ice sheet, which appears to have reached its maximum extent early in OIS 2, when the whole district was glaciated. Scottish ice flowing into the northern Irish Sea basin eventually deflected local ice southwards. The Irish Sea ice stream became dominant again during several readvances that followed a significant deglacial event. The first readvance (Gosforth Oscillation) over-rode most of the coastal plain. The last major one (Scottish Readvance) proglacially tectonized the glacigenic sequence along the coast, forming a push moraine at St Bees. Thick sequences of glaciolacustrine sediment accumulated within the major valleys of the district, both before and after the Gosforth Oscillation, when ice at the coast caused considerable ponding of meltwater inland.