The Giant’s Causeway is situated in the north of Northern Ireland, in County Antrim, not far from the Protestant town of Bushmills. In earlier days it was known to the Irish as Clochán na bhFómharach, or the Fomorians’ stepping-stones. The Fomorians were a mythical race of godlike—though not necessarily gigantic—beings who supposedly inhabited Ireland before the arrival of humans. The more recent name is connected with the legendary Irish giant Fionn mac Cumhaill, or Finn McCool, who was said to have built the causeway in order to cross the sea to Scotland, where a similar rock formation is found on the island of Staffa. Science tells us that the polygonal columns were formed by an unusually slow cooling of basalt, a common volcanic rock. As the molten basalt cooled and shrank, it cracked along approximately regular lines into an arrangement of hexagonal and other polygonal shapes. The same kind of phenomenon can be seen in the polygonal cracks that often appear in sun-baked mud flats.

Wikipedia has several fine pictures of the Giant’s Causeway and of the columnar basalt at Staffa. The two sites are linked not just in legend but in geology, as they resulted from the same lava flow. Wikipedia has shorter entries on columnar basalt formations elsewhere in the world, such as Sheepeater Cliff in Wyoming, Garni Gorge in Armenia, and Devils Postpile in California. (I would write the last name as “Devil’s Postpile” if it weren’t for the U.S. Board on Geographic Names and its bizarre policy of stripping apostrophes from place names.) The National Park Service gives a non-technical yet fairly detailed explanation of how polygonal cracks form in cooling lava. One way in which scientists have studied these phenomena is with the aid of laboratory simulations, such as the miniature columns of corn starch described in the 2006 research paper “An Experimental Investigation of the Scaling of Columnar Joints” (PDF).