Geology of the Causeway

The Giants Causeway geologically is an astonishing complex of basalt columns packed together, whose tops form 'stepping stones' leading from the cliff foot and disappearing under the sea. Similar columns appear on the island of Staffa, in the Hebrides.

During the Paleogene period (62-65 million years ago), Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded through chalk beds to form an extensive lava plateau. As the lava cooled rapidly, contraction occurred. While contraction in the vertical direction reduced the flow thickness (without fracturing), horizontal contraction could only be accommodated by cracking throughout the flow. The extensive fracture network produced the distinctive columns seen today.

Over the Causeway as a whole, there are about 37,00Q of these stone columns or rock formations, mostly 6-sided, but some 4, 5, 7 and 8 sided. They were formed about 60 million years ago by the cooling and shrinking - along regular lines of force - of molten lava from a vast volcanic eruption that formed the Antrim plateau. The tallest columns, in the Giant's Organ, are 12m. high. But the solidified lava in the cliffs is at places 24m, thick. Light-coloured patches in the cliffs are the residue of bubbles in the boiling lava. Plant fossils show that the lava erupted over vegetation and that the climate of Ireland was semi-tropical.

In addition to the Grand Causeway and the Giant's Organ, other formations are called: The Honeycomb, The Wishing Chair, The Giant's Granny, Lord Antrim's Parlour, The King and his Nobles, The Keystone, The Chimney Pots, The Fan, The Punchbowl, etc.

At the west end of the cliffs Portcoon Cave is a large sea cavern which may be entered from the landward side and has striking underwater colour reflections. Runkerry Cave, 0.5km. west of Portcoon Cave, is over 200m. long but only accessible from the sea. Some of the structures in the area, having been prone to several million years of weathering, resemble objects, such as the Organ and Giant's Boot structures pictured here. Other features include many reddish, weathered low columns known as Giants Eyes, created by the displacement of basalt boulders; the Shepherd's Steps; the Honeycomb; the Giant's Harp; the Chimney Stacks; the Giant's Gate and the Camel's Hump.