Northern Isles - Geology

Geology

The superficial rock of Orkney is almost entirely Old Red Sandstone, mostly of Middle Devonian age. As in the neighbouring mainland county of Caithness, this sandstone rests upon the metamorphic rocks of the Moine series, as may be seen on the Orkney Mainland, where a narrow strip of the older rock is exposed between Stromness and Inganess, and again on the small island of Graemsay.

Middle Devonian basaltic volcanic rocks are found on western Hoy, on Deerness in eastern Mainland and on Shapinsay. Correlation between the Hoy volcanics and the other two exposures has been proposed, but differences in chemistry means this remains uncertain. Lamprophyre dykes of Late Permian age are found throughout Orkney. Glacial striation and the presence of chalk and flint erratics that originated from the bed of the North Sea demonstrate the influence of ice action on the geomorphology of the islands. Boulder clay is also abundant and moraines cover substantial areas.

The geology of Shetland is quite different. It is extremely complex, with numerous faults and fold axes. These islands are the northern outpost of the Caledonian orogeny and there are outcrops of Lewisian, Dalriadan and Moine metamorphic rocks with similar histories to their equivalents on the Scottish mainland. There are also small Old Red Sandstone deposits and granite intrusions. The most distinctive feature is the ultrabasic ophiolite, peridotite and gabbro on Unst and Fetlar, which are remnants of the Iapetus Ocean floor. Much of Shetland's economy depends on the oil-bearing sediments in the surrounding seas.

Geological evidence shows that at around 6100 BC a tsunami caused by the Storegga Slides hit the Northern Isles, (as well as much of the east coast of Scotland), and may have created a wave of up to 25 metres (82 ft) high in the voes of Shetland where modern populations are highest.