The Giant's Causeway

[Wiki] The Giant's Causeway is an area of about 40,000 interlocking basalt columns resulting from a volcanic eruption. It is located on the North East coast of Northern Ireland, about 3 kilometres (2 miles) north of the town of Bushmills. Legend has it that the Irish giant Finn McCool built the causeway to walk to Scotland to fight his Scottish counterpart Benandonner.

The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Most of the columns are hexagonal, however there are some with four, five, seven and eight sides. The tallest are about 12 metres (36 ft) high, and the solidified lava in the cliffs is 28 metres thick in places.

During the Paleogene period, Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded through chalk beds to form an extensive lava plateau. As the lava cooled rapidly, contraction occurred. While contraction in the vertical direction reduced the flow thickness (without fracturing), horizontal contraction could only be accommodated by cracking throughout the flow. The extensive fracture network produced the distinctive columns seen today.

Although the basaltic columns of the Giant's Causeway are impressive, they are not unique. Basalt columns are a common volcanic feature, and they occur on many scales (faster cooling produces smaller columns). See other basalt columns here.