The Giant's Causeway is made up of around 40,000 columns that fit snugly together. They were formed during the Paleogene period. This geological phenomenon is basalt, which is solidified lava from a flow that filled a river valley. It cooled and cracked, but the cracks went right through the depth of the flow, which is what created the columns we see today. The lava from the Causeway cooled and shrank, splitting apart to create a pavement-like surface and tall columns, most of which have five sides.