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 Mainland, where a narrow strip is exposed between Stromness and Inganess, and again in the small island of Graemsay; they are represented by grey gneiss and granite.
The Middle Devonian is divided into three main groups. The lower part of the sequence, mostly Eifelian in age, is dominated by lacustrine beds of the lower and upper Stromness Flagstones that were deposited in Lake Orcadie. The later Rousay flagstone formation is found throughout much of the North and South Isles and East Mainland.
The Old Man of Hoy is formed from sandstone of the uppermost Eday group that is up to 800 metres (870 yd) thick in places. It lies unconformably upon steeply inclined flagstones, the interpretation of which is a matter of continuing debate.
The Devonian and older rocks of Orkney are cut by a series of WSW-ENE to N-S trending faults, many of which were active during deposition of the Devonian sequences. A strong synclinal fold traverses Eday and Shapinsay, the axis trending north-south.
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.
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