ABSTRACTDrumlins in central Ulster, Northern Ireland show complex variation in composition and internal structure. These can be resolved by facies modelling into four major till facies associations, reflecting either a particular depositional process or environment. They show that drumlin formation resulted from the interplay of a number of processes. Subglacial erosion, localised till lodgement, widespread subglacial till melt- out subglacial fluvial-lacustrine sedimentation, subglacial cavity sedimentation, superglacial till superimposition and resedimentation all contributed to drumlin formation in this area. The associations reveal contributed to drumlin formation in this area. The associations reveal spatial and temporal changes in processes and patterns of subglacial sedimentation, prior to, during and following drumlinisation. One of the most important changes identified in this study was the infilling and blockage of subglacial drainage networks prior to drumlinisation. This caused a reduction in hydraulic transmissibility at the ice-substrate interface and may have initiated widespread subglacial till melt-out. During and following drumlinisation, large quantities of meltwater were stored at the ice-substrate interface, contributing to ice sheet instability. Drumlinisation in central Ulster may have occurred under surging conditions. Associated with periods of high subglacial meltwater expulsion. This study suggests that subglacial hydraulic processes may have played an important role in drumlinisation and at least some types of drumlins could form as features of subglacial meltwater erosion.