The Irish Sea Glacier was a huge glacier during the Devensian Ice Age that, probably on more than one occasion, flowed southwards from its source areas in Scotland and Ireland and across the Isle of Man, Anglesey and Pembrokeshire. It was forced through a relatively narrow constriction in St George's Channel by ice flowing from the Irish Ice Cap (to the west) and the Welsh Ice Cap (to the east). At one point the glacier was probably no more than 80 km wide.