The columns 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 Northern Ireland, on the island of Ulva and Ardmeanach on the Isle of Mull. The 'Staffa Group' is the name given to the series of olivine basalts found in and around Mull. The volcanic eruptions took place 55–58 million years ago.