Geology and the scientific explanation
During the Paleogene period, 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. The basalts were originally part of a great volcanic plateau called the Thulean Plateau which formed during the Paleogene period.
The Irish version of events
Legend has it that the Irish giant Fionn mac Cumhaill (Finn McCool) built the causeway to walk to Scotland to fight his Scottish counterpart Benandonner. One version of the legend tells that Fionn fell asleep before he got to Scotland. When he did not arrive, the much larger Benandonner crossed the bridge looking for him.
To protect Fionn, his wife Oonagh laid a blanket over him so he could pretend that he was actually their baby son. In a variation, Fionn fled after seeing Benandonner’s great bulk, and asked his wife to disguise him as the baby. In both versions, when Benandonner saw the size of the ‘infant’, he assumed the alleged father, Fionn, must be gigantic indeed. Therefore, Benandonner fled home in terror, ripping up the Causeway in case he was followed by Fionn.
Another variation is that Oonagh painted a rock shaped like a steak and gave it to Benandonner, whilst giving the baby (Fionn) a normal steak. When Benandonner saw that the baby was able to eat it so easily, he ran away, tearing up the causeway.
The “causeway” legend corresponds with geological history in as much as there are similar basalt formations (a part of the same ancient lava flow) at the site of Fingal’s Cave on the isle of Staffa in Scotland.