Drumlins are large hill-sized oval mounds caused by glaciers dropping their basal debris load as a result of friction between the ice and the underlying geology. As the glacier continues to advance around the mound of deposited material they are narrowed and straightened. They are typically found in clusters, known as ‘drumlin swarms’ with the blunter end (stoss) facing the direction from which the ice advanced and a tapering end (lee) in the direction the ice was going. The interior composition of drumlins varies; some have unsorted cores of silts, sands and boulders. Others have fluvial deposits indicating that some may have been formed by fluvioglacial processes rather than simply glacial deposition.