This is one of the few places in the world where volcanic basalt has cooled in a columnar formation. The columns approximately form a hexagonal tessellation and tend to break off to produce a pavement with this pattern. A calculation by a Mr. Mallet (c1900?) shows that the hexagonal tessellation requires least energy, but I wonder how valid this is. One cannot see the full length of the columns, but it is estimated that they may be 20 ft in height before merging into the underlying irregular basaltic mass. It is estimated that 99% of the columns are hexagonal; only one triangular column is known. Though many of the hexagons are pretty regular, some have a side twice as long as their smallest side. Sides vary from 8 in to 18 in. The pillars break up into sections of length 6 in to 36 in, with a concavo-convex junction rather than a plane junction. Other examples are at Kirkjubaejarklaustri, Iceland, and Devil's Postpile, California.
See THIS LINK

An extract from
*The Mathematical Gazetteer of the British Isles*
created by David Singmaster
The original site is at
THIS LINK