Amino acid analyses of marine mollusc valves show that the Basement Till of Holderness, Yorkshire, is of Late Devensian age (ca. 20,000 BP) and not pre-Ipswichian (>125,000 BP) as is traditionally supposed. Together with the overlying Skipsea and Withernsea Tills the Basement Till is argued to be a ‘deformation till’ resulting from the repeated onshore surging of Late Devensian ice over a muddy sea floor, and the subglacial transport, attenuation and mixing of marine sediment. Silts on the surface of the Basement Till, yielding 14C dates of 18,500 and 18,240 BP, have been regarded as minimum ages for Late Devensian glaciation in eastern England (e.g. the Dimlington Stadial of Rose, 1985, 1989). The amino acid data presented here indicates that the maximum of the Late Devensian glaciation in eastern England occurred earlier. Sedimentological interpretation of the Late Devensian tills of Holderness as glacially-reworked marine sediments supports previous glaciological models of the British Ice Sheet involving surging of ice lobes along the coast of eastern England. The Basement, Skipsea and Withernsea tills are separated by shallow marine sediments and may be representative of ‘deformable bed’ till facies deposited below Pleistocene ice sheets moving over soft, fine-grained sediments.