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
Cave S sea level and below the Great Gibraltar Sand Dune that was once used to capture rainwater which served as Gibraltar's main water supply. The cave is about from the north end of the Rock and opens on the east face of the Rock just above Holy Boy's Cave. The cave provides evidence of marine life indicating that despite its current altitude it was once a sea cave. This is not the only evidence of rising and falling sea levels in Gibraltar which have historically varied greatly. Wynfrid Duckworth described the cave's floor as falling in height towards the entrance and