Giant's Causeway, a rock formation on the northern coast of Northern Ireland. It is six miles (10 km) east-northeast of Portrush, in County Antrim, and extends for three miles (5 km). About 40,000 vertical columns of basalt are massed along the coast and out into the sea. Irish legends claim the giant Finn McCool began building a causeway to Scotland here. Many of the columns are six-sided. They range from 15 to 20 inches (38 to 50 cm) in diameter and have flat tops. Some rise as high as 50 feet (15 m). Groups of them have names such as the Wishing Chair, the Organ, and the Loom. The Causeway was formed by lava that fragmented as it cooled.