The Giant's Causeway is an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic fissure eruption. Most of the columns are hexagonal, although there are also some with four, five, seven, or eight sides. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Geologically, these columns formed 50 to 60 million years ago when highly fluid molten basalt intruded through chalk beds; as the lava cooled rapidly, contraction occurred, resulting in the geometric pillars. According to legend, the causeway was built by the giant Finn MacCool so he could cross the North Channel to fight a Scottish rival. It is Northern Ireland's only UNESCO World Heritage site and a prime example of the column jointing process in igneous rock.