This is a list of Orkney islands in Scotland. The Orkney archipelago is located 9.9 mi north of mainland Scotland and comprises over 70 islands and skerries. In addition to the Orkney Mainland there are three groups of islands. The North and South Isles lie respectively north and south of Mainland. The Pentland Skerries are a group of small islands in the Pentland Firth, a dangerous stretch of water between mainland Scotland and the larger islands of Orkney, through which run the strongest tidal streams in Britain. The Isle of Stroma is often mistakenly included with the Orkney Islands, but is part of Caithness. Orkney islands mapMost of the islands have a bedrock formed from Old Red Sandstone, which is about 400 million years old, and was laid down in the Devonian period. The islands have good agricultural qualities and have been continuously inhabited for thousands of years, as evidenced by the World Heritage Site of the Heart of Neolithic Orkney. The archipelago is exposed to wind and tide, and there are numerous lighthouses as an aid to navigation. The European Marine Energy Centre, which is located at Stromness, is currently testing various wave and tidal energy devices from Billia Croo on the Mainland and Eday. The islands all fall within the Orkney Islands Council area. Most of the larger islands have development trusts that support the local economy.