Giant's Causeway cases the horizontal fracture has resulted in a bottom face that is convex while the upper face of the lower segment is concave, producing what are called "ball and socket" joints. The size of the columns is primarily determined by the speed at which lava from a volcanic eruption cools. The extensive fracture network produced the distinctive columns seen today. The basalts were originally part of a great volcanic plateau called the Thulean Plateau which formed during the Paleocene. According to legend, the columns are the remains of a causeway built by a giant. The story goes that the Irish
The Holy Cave, Hunterston 19th century. The visible remains suggested a souterrain, but the near absence of records of these in south-west Scotland make this unlikely. The passage is lined with corbelled drystone walling, roofed with capstones, and leads into a natural cave section which may have been formed by wave action as it shows a water-scour likely to have been created by wave action. Some areas of undisturbed stratified deposits have been found, together with evidence of occupation, such as a hearth, bones, and a fragment of glass. A similar cave or rock shelter survives on Holy Isle (off the east coast of