geological map

Moine

Grampian Orogeny

Dalradian

Caledonian Orogeny

Devonian

Lewisian Basement

In Shetland the main outcrops of Lewisian basement gneisses of the Hebridean Terrane is on north-west Mainland and are cut in places by foliated pegmatites. These gneisses are acid, banded, high-grade orthogneisses with a minimum age of 2900 Ma and once were granites that had been intruded by smaller bodies of gabbro. Subsequent burial to extreme depths by mountain building events re-crystallised and deformed the granites and gabbros. Since their uplift to higher crustal levels these rocks have been relatively undisturbed and may represent the Caledonian foreland in Shetland.

Inliers of basement gneiss also outcrop within the Caledonian cover rocks and are believed to represent the ancient ‘Laurentian’ continental basement onto which the cover sediments were deposited. Included in the Shetland basement are paragneisses that make up the south coast of St Magnus Bay. These formed from the reworking of the continental basement during the Grenville Orogeny (1100-1000 Ma) when continental convergence formed the Rodinia supercontinent.