Orkney and Shetland, an introduction
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
The Northern Isles of Scotland consist of the Shetland and Orkney Islands (P915565). The Shetland Isles lie about 165 km NE of the Scottish mainland and about 340 km W of Bergen in Norway. The island group extends for 109 km from north to south, has a total land area of 1426 km2 and consists of over a hundred islands, of which 13 are inhabited. It includes the two outlying islands Foula and Fair Isle, which lie respectively 23 km W and 39 km S of Mainland, the largest island of the group. The Orkney Islands lie about 125 km SW of Shetland and are separated from the mainland of Scotland by the Pentland Firth, which at its narrowest is only 10 km wide. They are formed of about 90 islands and skerries, of which 14 are inhabited. The land area is 956 km2 and the islands extend for 80 km from north to south and 47 km from west to east.
The two island groups have strongly contrast (P915566) is its north—south elongation and the presence in the eastern half of the island group of smooth north to north-east trending ridges with intervening partly drowned valleys. These features reﬂect the underlying rock formations which consist of metamorphic rocks and partially recrystallised granites, in both of which the foliation is vertical or steeply inclined and trends north or, in places, north-east. In the western half of Shetland the topography is more rugged and diverse than in the east. This is due to the varied bedrock geology of the area, which includes several large masses of granite and diorite, some belts of metamorphic rock and a large area of highly folded sandstone and lava of Old Red Sandstone age. Slightly less undulating terrain occurs along the south-eastern coastal strip of Mainland and on the adjacent islands as well as on the western seaboard of the peninsulas forming north and west Mainland. These areas are formed of gently inclined sandstones, ﬂagstones and conglomerates of Old Red Sandstone age and, in Papa Stour and Esha Ness, of Old Red Sandstone lavas. The outlying islands Foula and Fair Isle both consist mainly of sandstone which has been eroded into prominent sea cliffs on their western coasts. The cliffs on the west coast of Foula attain a height of 372 m.
The Orkney Islands (P915567), with the exception of western Hoy, have a more subdued topography than Shetland, though in West Mainland, Rousay and Westray hills ranging from 170 m to nearly 275 m in height have small escarpments or less well-defined terrace features on their sides which reﬂect the alternation of hard and soft layers in the Old Red Sandstone ﬂagstones that form the greater part of the island group. Considerable portions of the islands of Eday, Sanday, Burray and South Ronaldsay and parts of East Mainland are underlain by thick beds of sandstone, which give rise to ridges and escarpments on Eday and South Ronaldsay. The topography of western Hoy stands out in striking contrast to that of the rest of Orkney. This is because it consists of massive sandstones which form rounded but steep-sided hills up to 477 m high. On the west coast of the island some of the hills are abruptly truncated by magnificent sea cliffs. At St John’s Head these reach a height of 335 m.
The Orkney Islands are separated from the Scottish mainland by the east-south-east trending Pentland Firth. Another depression, which trends south-east, separates the Mainland—Rousay—Shapinsay group of islands from the northern isles of Orkney. Smaller south-east trending depressions form the straits between Rousay and Mainland, between Mainland and Hoy and the hollow which is now in part occupied by the lochs of Harray and Stenness and which continues eastwards as Scapa Flow. It has been suggested (Wilson and others 1935, pp. 6-7) that these depressions are the remnants of a late-Tertiary river system which drained to the east-south-east and which was only slightly modified by ice during the Pleistocene Period.
No similar remnants of Tertiary river systems can be recognised in Shetland, though some of the breaks in the north—south trending ridges, such as the Quarff gap 9 km SW of Lerwick, have by some authors been taken as evidence for the existence of ‘pre-glacial’ or even earlier river valleys.
The ice sheets which covered the islands in the Pleistocene Period and the subsequent rise of sea level around Shetland and Orkney have been major factors in the shaping of the present landscape. In the more rocky parts of Shetland the ice has produced a strongly ice-moulded topography. In Orkney it appears in most areas to have smoothed off and partly obliterated the terrace features formed by the earlier sub-aerial erosion of the ﬂagstones, but there are some hillsides on which ice-gouging may have emphasised these features. The ice has been responsible for the overdeepening of some of the straits between islands and the deepening of some major sea basins such as St Magnus Bay on the west side of Shetland. The ice sheets also scooped out many shallow basins which are now occupied by inland lakes. In Shetland there is evidence that at a fairly late stage in the Pleistocene Period the island group had its own ice-sheet. Even later, after most of the ice had melted from the islands, the higher hills of Shetland Mainland, Foula and Hoy retained small local glaciers. These produced corries and small terminal moraines and the former are prominent features in the landscape of Foula and the north-west corner of Hoy.
The rise of sea level since the last major glaciation is responsible for the drowned landscape topography of Shetland and Orkney. The most obvious signs of this submergence are the many long open inlets or ‘voes’ along the coast of Shetland, which mark the courses of former valleys. The submergence of the land coupled with the frequent strong winds of the area has also been responsible for the rapid marine erosion along the exposed coasts which has produced the impressive cliffs with their ‘geos’ (long narrow openings along joints or faults), ‘gloups’ (caves opening out landward as vertical chimneys or gullies), natural arches and stacks along the exposed shores of both island groups.
The power of the sea during westerly gales is vividly demonstrated by the presence of high level storm beaches. Along certain exposed parts of the coast crescentic mounds composed of large blocks of rock occur on the rocky platform behind a vertical cliff up to 18 m high. Excellent examples of these are to be found in Esha Ness and the Out Skerries, Shetland, and on the west coast of Aikerness in Westray, Orkney. The mounds are up to 5 m high and consist of blocks which have been torn by the waves from the cliff top and from the rocky platform immediately behind.
Along many parts of the coast of these islands one finds long, narrow spits of shingle or sand which are locally known as ‘ayres’. Most of these are formed across a shallow bay or across a voe, usually near its landward end, cutting off, either partly or completely, a sheet of water from the sea. Where the spit extends right across the bay the enclosed stretch of fresh water, known as an ‘oyce’, may eventually silt up to become a fertile stretch of land. Some ayres form tombolos which join off-shore isles to a larger island. Good examples of such tombolos are the three ayres which join the island of Fora Ness to the Mainland of Shetland at Delting, the sand spit linking St Ninian’s Isle to south Mainland, Shetland (P574612) and the ayre at the head of Long Hope, which joins the island of South Walls to Hoy (Orkney).
- WILSON, G. V., EDWARDS, W., KNOX, J., JONES, R. C. B. and STEPHENS, J. V. 1935. The Geology of the Orkneys. Mem. geol. Surv. Gt Br.