 It is the remains of a volcanic pipe which cut through the surrounding sedimentary rock, before cooling to form very hard dolerite, a coarser-grained equivalent of basalt.Subsequent glacial erosion was resisted more by the dolerite, which protected the softer rock to the east, leaving a crag and tail formation.The summit of the castle rock is above sea level, with rocky cliffs to the south, west and north, rearing up to from the surrounding landscape.