Editing Staffa (section)

==Geology and pre-history==

In prehistoric times ([[Pleistocene]]) 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&nbsp;years ago, sea levels were up to {{convert|125|m|ft|0}} lower than at present. Although the [[isostasy|isostatic]] rise of land makes estimating post-glacial coastlines a complex task, around 14,000 years ago it is likely that Staffa was part of a larger island, just off the coast of mainland Scotland, which would have included what are now Mull, [[Iona]] and the [[Treshnish Isles]].<ref>Murray (1973) p. 69</ref>

Steadily [[sea level rise|rising sea levels]] 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 without 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.<ref name=Smith/> The lava contracted towards each of a series of equally spaced centres as it cooled and solidified into prismatic columns, a process known as [[columnar jointing]]. The columns typically have three to eight sides, six being most common. The columns are also divided horizontally by cross joints.<ref>Mitchell, Colin and Mitchell, Patrick (2005) ''Landform and Terrain: The Physical Geography of Landscape'' Brailsford Press. {{ISBN|1-904623-56-5}} [http://www.brailsfordpress.com/chapters%201-3.pdf 3.pdf] {{webarchive|url=https://web.archive.org/web/20080407142126/http://www.brailsfordpress.com/chapters%201-3.pdf |date=7 April 2008 }}</ref> These columnar jointed sections represent the tops and bottoms of individual lava flows. Between these sections lie regions of much more chaotic jointing, known as the entablature. The origin of the entablature is unknown, but could be due to flooding of the lava flow, causing much more rapid cooling, or the interaction of stress fields from the two regions of columnar jointing as they approach one another.<ref>Phillips et al. (2013) ''The formation of columnar joints produced by cooling in basalt at Staffa, Scotland'' Bulletin of Volcanology 75:715</ref>

Similar formations are found at the [[Giant's Causeway]] in [[Northern Ireland]], on the island of [[Ulva]] and at Ardmeanach on the Isle of Mull.<ref name=Keay>Keay & Keay (1994) p. 894</ref> 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.<ref>[http://www.sat.dundee.ac.uk/arb/gsg/news/128.html "To Staffa with Ladder"] {{Webarchive|url=https://web.archive.org/web/20080319123058/http://www.sat.dundee.ac.uk/arb/gsg/news/128.html |date=19 March 2008 }} (September 2006) ''Grampian Speleological Group Newsletter'' Number 128. Retrieved 9 September 2008.</ref> The 'Staffa Group' is the name given to the series of [[olivine]] [[tholeiite basalt]]s found in the vicinity of Mull which erupted 55–58&nbsp;million years ago.<ref>Bell, B.R. and Jolley, D.W. (1997) ''Application of palynological data to the chronology of the Palaeogene lava fields of the British Province: implications for magmatic stratigraphy''. Journal of the Geological Society. London. Vol. 154, pp. 701–708.</ref>