During the Palaeogene period, some 50 to 60 million years ago, Antrim was subject to intense volcanic activity. Highly fluid molten basalt intruded through chalk beds to form a large scale lava plateau. As the lava cooled rapidly, contraction occurred. The size of the columns is mostly 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 Paleogene period. Parts of this plateau can be found in the Faroe Islands, Iceland, and Norway, as well as at Fingal's Cave.