Black basalt lava spread across what's now Northern Ireland (as well as Skye and Mull) at the time of the opening of the Atlantic Ocean, a mere 50 million years ago. Some of the basalt cracked as it cooled, into hexagonal columns of the Giant's Causeway, as well as the 'Chimney Tops' seen at the top of the cliff here. The red 'laterite' layer is the top of a lava flow, weathered rusty red in the open air before the next lava flow came in on top.

Chimney Tops, just east of Giant's Causeway, Antrim

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