Giant's Causeway

There is a small element of truth in the legend of Finn McCool, who is said to have built the causeway to fight a much larger giant in Scotland, in that the causeway was formed in the Scottish Midland Valley which extends to Northern Ireland. The Midland Valley developed some 400 million years ago as a consequence of the collision between two land masses containing the Northern and Southern fragments that formed the British Isles. About 60 million years ago, as America and Europe separated, several million years of basaltic magmatism covered the whole area; leaving the remnants seen today.

There is much speculation about the processes which formed these unusual basalt columns. They are often said to be hexagonal but in fact various polygonal shapes are present. Several papers have been published on the formation of such patterns. The formation of the columns is associated with the cooling process but only a very few flood basalts cool in this way. An interesting mystery.

However other natural structures sometimes cool in a similar fashion:

Ice flows in Arctic Canada.