Editing Outer Hebrides (section)

== Geology ==
[[File:Hebridean Terrane.png|thumb|left|Geological map of the [[Hebridean terrane]]]]
[[File:Barra Head Lighthouse cliffs.jpg|thumb|right|upright=0.50|alt=A small white building is barely visible on top of dark and imposing cliffs with deep blue water at their base.|The "[[Lewisian complex|Old Boy]]" — the [[gneiss]] cliffs of {{lang|gd|Sloc na Bèiste}}, [[Barra Head]], the southernmost point of the Outer Hebrides]]

Most of the islands have a bedrock formed from [[Lewisian complex|Lewisian gneiss]]. These are amongst the oldest rocks in Europe, having been formed in the [[Precambrian]] period up to three billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the [[Moine Thrust Belt|Moine Thrust]] and on the islands of [[Coll]] and [[Tiree]].<ref>Gillen (2003) p. 44</ref> These rocks are largely igneous in origin, mixed with metamorphosed [[marble]], [[quartzite]] and [[mica schist]] and intruded by later basaltic [[Dyke (geology)|dykes]] and granite magma.<ref>McKirdy ''et al.'' (2007) p. 95</ref> The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy".<ref name="Murray 1966 p. 2">Murray (1966) p. 2</ref>{{#tag:ref |Lewisian gneiss is sometimes described as the oldest rock found in Europe, but trondhjemite gneiss recently measured at Siurua in Finland has been dated to 3.4–3.5 Ga.<ref>{{cite conference |author1=Lalli, Katja |author2=Huhma, Hannu |year=2005 |url=http://arkisto.gtk.fi/idc5/idc5_pre_excursion/idc5_pre_excursion_pages_57_58.pdf |title=The oldes ''(sic)'' rock of Europe at Siurua |conference=Fifth International Dyke Conference |location=Rovaniemi, Finland |access-date=9 July 2010 |url-status=dead |archive-url=https://web.archive.org/web/20110720182359/http://arkisto.gtk.fi/idc5/idc5_pre_excursion/idc5_pre_excursion_pages_57_58.pdf |archive-date=20 July 2011}}</ref>|group="Note"}}

Granite intrusions are found in the parish of [[Barvas]] in west Lewis, and another forms the summit plateau of the mountain [[Roineabhal]] in Harris. The granite here is [[anorthosite]], and is similar in composition to rocks found in the mountains of the [[Moon]]. There are relatively small outcrops of [[Triassic]] sandstone at [[Broad Bay (Western Isles)|Broad Bay]] near Stornoway. The Shiant Islands and St Kilda are formed from much later tertiary [[basalt]] and basalt and [[gabbro]]s respectively. The sandstone at Broad Bay was once thought to be [[Torridonian]] or [[Old Red Sandstone]].<ref name="Thompson 1968 p. 13"/><ref>Gillen (2003) p. 42</ref><ref>McKirdy ''et al.'' (2007) p. 94</ref>