Giant's Causeway

On the north coast of Northern Ireland, around 40,000 interlocking basalt columns step down into the sea. They are the eroded remains of a lava plateau laid down in the Palaeogene, roughly 50 to 60 million years ago.

Most of the columns are hexagonal. Some have four, five, seven or eight sides, which is the ordinary untidiness described in Columnar Jointing.

The site is a useful illustration of why Basalt behaves the way it does: a flow thick enough and uniform enough to cool slowly produced unusually regular jointing.