The Bill consists of gently south-dipping Portland Freestone, the base of which is at about sea-level here at the extreme southern point of the promontory. Because the dip is southward this junction is a little higher in the cliffs just northwest of the lighthouse, as at Pulpit Rock, seen here. The oolitic limestone has been much quarried, modifying the coast in places, and where it is near sea-level it been much undercut by sea erosion into broad caves. The Portland Cherty Series lies underneath the Portland Freestone. It is at sea-level downwards at the Bill itself but gradually rises northward to form resistant vertical cliffs. At Pulpit Rock it forms the lower part of the cliff. Above the Portland Cherty Series and the Portland Freestone are the thinner-bedded limestones and shales of the Purbeck Formation. Only the basal part of this unit is present at Portland Bill, although more is seen to the north. A broad raised beach platform cuts across the Purbeck strata and this unconformable sediment is Pleistocene in age. It has been classified into two raised beaches, both largely consisting of shelly and sandy pebble deposits.