This small island is entirely of volcanic origin. It consists of a basement of tuff, underneath colonnades of a black fine-grained Tertiary basalt, overlying which is a third layer of basalt lava. Slow cooling of the second layer of basalt resulted in an extraordinary pattern of hexagonal columns which form the faces and walls of the principal caves. The lava contracted as it cooled and solidified into columns, a process known as columnar jointing.