In pre-historic times Staffa was covered by the ice sheets which  spread from Scotland out into the Atlantic Ocean beyond the  Outer Hebrides. After the last retreat of the ice around  20,000 years ago, sea levels were up to 125 m lower than at  present. Steadily rising sea levels since that time then further  isolated this little island, which 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  basaltic lava lacking a crystalline structure. By contrast, slow  cooling of the second layer of basalt resulted in an extraordinary  pattern of predominantly hexagonal columns which form the faces and walls of the principal caves.  The lava contracted towards each of a series of equally spaced centres as it cooled and solidified  into prismatic columns. The columns typically have three to eight sides, six being most common.  The columns are also divided horizontally by cross joints.  Similar formations are found at the Giant`s Causeway in Nothern Ireland, on the island of Ulva and  at Ardmeanach on the Isle of Mull. Grooves in the roof of MacKinnon's cave indicate either a  pyroclastic flow or a series of eroded ash falls in the rock above the columnar basalt.The 'Staffa  Group' is the name given to the series of olivine tholeiite basalts found in the vicinity of Mull which  erupted 55–58 million years ago.  GoogleEarth Staffa Location: 53.15° N, 6.23° W Elevation:  42 m  Photo: Rolf Cosar Photo: Rolf Cosar Photo: Rolf Cosar Photo: Rolf Cosar Photo: Rolf Cosar Staffa, September 1993 HOME Photo: Rolf Cosar