The Giant's Causeway, at the foot of basalt cliffs in Northern Ireland, is made up of 40,000 black basalt columns jutting out of the ocean. Volcanic activity 50 million to 60 million years ago created these step-like columns on the edge of the Antrim plateau.

Three periods of volcanic activity created the different parts of the causeway, also referred to as the Lower, Middle and Upper Basalts; the Middle Basalt portion features the famous hexagonal columns of the Causeway.

-- Kelsey Ramos, Los Angeles Times