page_title: Orkney
section_title: Geology
hierarchical_section_title: Orkney / Geology
caption_attribution_description: English: Sketch geological map of Orkney taken from various sources including published BGS maps
context_page_description: Orkney, also known as the Orkney Islands, is an archipelago in the Northern Isles of Scotland, situated off the north coast of the island of Great Britain. Orkney is 10 miles north of the coast of Caithness and has about 70 islands, of which 20 are inhabited. The largest island, Mainland, is often referred to as "the Mainland", and has an area of 523 square kilometres, making it the sixth-largest Scottish island and the tenth-largest island in the British Isles. The largest settlement and administrative centre is Kirkwall.
  Orkney is one of the 32 council areas of Scotland, a constituency of the Scottish Parliament, a lieutenancy area, and a historic county. The local council is Orkney Islands Council, one of only three Councils in Scotland with a majority of elected members who are independents.
  The islands have been inhabited for at least 8,500 years, originally occupied by Mesolithic and Neolithic tribes and then by the Picts. Orkney was colonised and later annexed by Norway in 875 and settled by the Norse. The Scottish Parliament then absorbed the earldom to the Scottish Crown in 1472, following the failed payment of a dowry for James III's bride Margaret of Denmark.
context_section_description: The superficial rock of Orkney is almost entirely Old Red Sandstone, mostly of Middle Devonian age. As in the neighbouring mainland county of Caithness, this sandstone rests upon the metamorphic rocks of the Moine series, as may be seen on the Mainland, where a narrow strip is exposed between Stromness and Inganess, and again in the small island of Graemsay; they are represented by grey gneiss and granite.
  The Middle Devonian is divided into three main groups. The lower part of the sequence, mostly Eifelian in age, is dominated by lacustrine beds of the lower and upper Stromness Flagstones that were deposited in Lake Orcadie. The later Rousay flagstone formation is found throughout much of the North and South Isles and East Mainland.
  The Old Man of Hoy is formed from sandstone of the uppermost Eday group that is up to 800 metres (870 yd) thick in places. It lies unconformably upon steeply inclined flagstones, the interpretation of which is a matter of continuing debate.
  The Devonian and older rocks of Orkney are cut by a series of WSW-ENE to N-S trending faults, many of which were active during deposition of the Devonian sequences.  A strong synclinal fold traverses Eday and Shapinsay, the axis trending north-south.
  Middle Devonian basaltic volcanic rocks are found on western Hoy, on Deerness in eastern Mainland and on Shapinsay. Correlation between the Hoy volcanics and the other two exposures has been proposed, but differences in chemistry means this remains uncertain. Lamprophyre dykes of Late Permian age are found throughout Orkney.
  Glacial striation and the presence of chalk and flint erratics that originated from the bed of the North Sea demonstrate the influence of ice action on the geomorphology of the islands. Boulder clay is also abundant and moraines cover substantial areas.
caption: Geology of Orkney
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