page_title: Outer Hebrides
section_title: Geology
hierarchical_section_title: Outer Hebrides / Geology
caption_attribution_description: English: Geological map of the Hebridean Terrane, NW Scotland, modified after maps in the Geology of Scotland and other published maps
context_page_description: The Outer Hebrides or Western Isles, sometimes known as Innse Gall or the Long Isle/Long Island, is an island chain off the west coast of mainland Scotland. The islands are geographically coextensive with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland. They form part of the archipelago of the Hebrides, separated from the Scottish mainland and from the Inner Hebrides by the waters of the Minch, the Little Minch, and the Sea of the Hebrides. Scottish Gaelic is the predominant spoken language, although in a few areas English speakers form a majority.
  Most of the islands have a bedrock formed from ancient metamorphic rocks and the climate is mild and oceanic. The 15 inhabited islands have a total population of 26,830 and there are more than 50 substantial uninhabited islands. From Barra Head to the Butt of Lewis is roughly 210 kilometres.
  There are various important prehistoric structures, many of which pre-date the first written references to the islands by Roman and Greek authors.
context_section_description: Most of the islands have a bedrock formed from 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 and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy".
  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 near Stornoway. The Shiant Islands and St Kilda are formed from much later tertiary basalt and basalt and gabbros respectively. The sandstone at Broad Bay was once thought to be Torridonian or Old Red Sandstone.
caption: Geological map of the Hebridean Terrane
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