Many karst areas in the UK have been glaciated one or more times during the last 0.5 Ma, yet there are few documented examples of caves in these regions being affected by glacial processes other than erosion. The karst of South Wales is one area where sub or pro-glacial modification of pre-existing caves is thought to occur. Evidence from the Ogof Draenen cave system suggests that caves can sometimes act as subterranean glacial ‘underspill’ channels for melt-water. This cave, one of the longest in Britain with a surveyed length of over 70 km, underlies the interfluve between two glaciated valleys. Sediment fills and speleo-morphological observations indicate that melt-water from a high level glacier in the Afon Lwyd valley (>340m asl) filled part of the cave and over-spilled into the neighbouring Usk valley, temporarily reversing non-glacial groundwater flow directions in the cave. It is suggested that this may have occurred during a Middle Pleistocene glaciation
