Over two hundred “Trap Dykes” of igneous origin cut the Devonian sediments of Orkney in a NE-SW direction, but a small group in southeast Orkney runs N-S. This “Camptonite – Monchiquite Suite” of dykes is part of a set of nine similar dyke swarms found in the Highlands and Islands, widely distributed both in space and time.
These hard, black rocks often follow the line of fractures. They are notable in occasionally exhibiting an exotic collection of mantle-derived xenolith inclusions, indicating rapid transport of the host liquids from great depths. It is possible that these dykes never reached the surface to form volcanoes. The age of the Orkney dykes is 252+/-10 million years, placing the time of intrusion in the late Permian, a period of renewed tectonic movement in the sedimentary basins to the west of Orkney.