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
St. Michael's Cave receiving almost 1,000,000 visitors a year. The cave was created by rainwater slowly seeping through the limestone rock, turning into a weak carbonic acid which gradually dissolved the rock. Through this process, tiny cracks in The Rock's geological fault grew into long passages and large caverns over thousands of years. The numerous stalactites and stalagmites in the cave are formed by an accumulation of traces of dissolved rock deposited by water dripping from the ground above. In 1974 a Neolithic bowl was discovered in the cave, one of many examples which prove that the cave was known to prehistoric humans.