About 60 million years ago, the continents of Europe and America were moving away from one another as the Atlantic Ocean opened. Hot magma welled up through the rift between the continents and spilled on to the surface of the Earth, covering what is now Antrim in layers of basalt.
Activity paused for a few million years, and the action of wind and weather on the top layer of basalt leached out soluble minerals from the top layers, leaving behind a surface layer of bright red lateritic rocks rich in iron, which is relatively insoluble.
Then eruptions resumed, and an outpouring of basaltic lava flowed down into a valley in the earlier rocks. As it cooled on contact with the air, the lava contracted and cracks developed along planes of least resistance, creating the hexagonal columns which make up the Giant's Causeway. The columns develop perpendicular to the nearest cool place, so those on the sides of the valley tended to develop on a slant. They also contracted lengthwise, forming the joints which cut across the columns at intervals.
The result is the Giant's Causeway, a World Heritage Site.