Due to its abundance of bays, inlets and offshore islands, Mayo has the longest coastline of any county in Ireland, at or approximately 21% of the total coastline of the State. The geology and geomorphology of the county and its islands is among the most varied and complex in Ireland. Islands such as those at Clew Bay are low-lying, formed when rising sea levels drowned a vast field of drumlins, while others like Clare Island and Achill are characterised by mountains and towering sea-cliffs, of which Croaghaun are the third-highest in Europe, at