The rock is estimated to have formed some 350 million years ago during the early Carboniferous period.

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.

[1] This means that the only readily accessible route to the castle lies to the east, where the ridge slopes more gently.

The defensive advantage of such a site is clear, but the geology of the rock also presents difficulties, since basalt is an extremely poor aquifer.