Sixty million years ago Antrim was subject to intense volcanic activity, when highly fluid molten rock was forced up through fissures in the chalk bed to form an extensive lava plateau.

The dramatic cliff like edge of the plateau forms the Causeway coastline. The larger fissures, through which the lava flowed, can be clearly seen as bands of dark rock which cut down the cliff faces and jut out to sea. There were three periods of volcanic activity which resulted in the flows, known as the Lower, Middle and Upper Basalts.

It is the Middle Basalts rocks which forms the columns of the Giants Causeway. The rapidly cooling lava contracted and variations in the cooling rate resulted in the world famous columnar structure.

The columns are mainly hexagonal though there are some with up to eight sides. Weathering of the top of of the lower Basalts formed the Inter Basaltic Bed - the band of reddish rock which is a feature of the area. The same action of the weather created circular formations round a nugget of basalt which are known locally as "giants eyes".

Some other formations with popular names are the Chimney Stacks, The Harp, The Organ and the Camel's Hump.