Calendonian mountains

Map to show the extent of the Caledonian MountainsZoom Map to show locations of Shetland's plutonic intrusionsZoomWhen the Iapetus Ocean closed between 480 and 390 million years ago to form Euramerica (and eventually Pangaea) the continents on either side collided and were forced upwards to build an enormous mountain chain which would have been similar in size to the Himalayas. The remains of this Caledonian mountain chain can be traced from North America through Greenland, Ireland, Scotland and Scandinavia and form the spine of Shetland today.

Shetland’s Dalradian rocks - psammites, schists, quartzites and limestones - were originally laid down horizontally but were tilted up almost vertically so that the limestones form a series of bands running roughly north-south through the Central Mainland. The Clift Hills phyllites which run down the western side of South Mainland are the youngest in Shetland’s sequence of Dalradian rocks.

Rocks of the Moine Supergroup were thrust up over Lewisian basement rocks.  The line of the Moine thrust can be seen in the North Mainland. The Moine thrust was first identified on the Scottish Mainland at Knocken Crag.  Its discovery in the nineteenth century by Ben Peach and John Horne was of international significance as it helped geologists to understand the tectonic processes involved in mountain formation.

Part of the Earth’s crust and mantle beneath the Iapetus Ocean were forced up over the top of the Laurentian continent. A section of this ocean floor can be seen in the eastern parts of Unst and Fetlar where it was driven up along two major thrusts.  An exposed ocean crust such as this is called an ophiolite and Shetland is one of the best places in the world to see one.

Between 450 and 350 million years ago rocks melting in the roots of the mountain chain generated a magma that forced its way up into the colder crust to solidify below the surface into granitic bodies called plutons.  These plutons were later exposed on the surface of the Earth through tectonic uplift and the erosion of the overlying rocks.  Much of Northmaven was formed in this way, including Shetland’s highest point Ronas Hill.