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In addition to the Mainland, most of the islands are in two groups, the North and South Isles, all of which have an underlying geological base of Old Red Sandstone.
The climate is mild and the soils are extremely fertile, most of the land being farmed.
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.
Other structures from this period include underground storehouses, and aisled roundhouses, the latter usually in association with earlier broch sites.
The largest island, Mainland, is often referred to as "the Mainland".
It has an area of 523 square kilometres (202 sq mi), making it the sixth-largest Scottish island and the tenth-largest island in the British Isles.
Both Orkney and Shetland saw a significant influx of Norwegian settlers during the late 8th and early 9th centuries.
Vikings made the islands the headquarters of their pirate expeditions carried out against Norway and the coasts of mainland Scotland.
Other remains from that era include the Standing Stones of Stenness, the Maeshowe passage grave, the Ring of Brodgar and other standing stones.