Orkney, Pleistocene and Recent
| Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office. |
Introduction
As in Shetland, the first connected account of the glaciation of the Orkneys was produced by Peach and Horne (1880)[1]. They showed that the ice which streamed eastwards from eastern Sutherland was deflected north-westwards and west-north-westwards over Caithness and Orkney by the Scandinavian ice sheet, and that after the retreat of this south-eastern ice, local glaciers lingered for a time in the valleys of Hoy and the higher parts of Mainland. The work of the Geological Survey confirmed Peach and Horne’s principal findings but established that the glacial history was more complex than they had envisaged.
Over the greater part of Orkney the passage of the ice sheets has led to a smoothing out of the pre-existing topography. In the flagstone country the hillsides before glaciation were probably terraced by small rocky escarpments with scree at their base. The passage of ice has in most areas filled the depressions between escarpments with boulder clay and removed any projecting rock, leaving behind only indistinct ledges. In a few areas however, such as the south side of Fitty Hill in Westray and the south side of Rousay, ice advancing along the hill-slope scoured out the debris between escarpments and emphasised the terracing. North-west Hoy, which is the only part of Orkney to have supported local glaciers, has fairly well-developed corries on the east and west slopes of Ward Hill and on the north-west slope of Cuilags. The two glens on either side of Ward Hill, which converge to form the wide valley at Rackwick, have been considerably modified by valley glaciers.
Devensian glaciation
Deposits
The boulder clay of Orkney is largely confined to the low ground, and is exposed in many excellent coast sections, where it ranges in thickness from 3 to 10 m. It generally consists of red or purple sandy clay with abundant polished and striated boulders. In eastern and many northern exposures the matrix is composed largely of material derived from the ‘marls’ and sandstones of the Eday Beds. Traced westward across West Mainland and Westray the red colour is gradually replaced by shades of brown, yellow and grey as the proportion of comminuted rock derived from the Rousay and Stromness Flags increases. Most boulders and smaller stones in the till consist of local material. Clasts derived from outside Orkney include various types of granitic, felsitic and schistose rocks, quartzites and quartzose sandstones, dark limestones with plant remains, calcified fossil wood, chalk and chalk flints. In the northern isles and parts of Mainland much of the till contains broken, smoothed and striated marine shells. Erratic boulders are relatively rare, the most interesting being a large boulder found near Saville in the north of Sanday which measures 2 m x 1.8 m x 0.75 m and consists of an unusual variety of biotite-oligoclase-gneiss (Heddle 1880[2], Flett 1898a[3]). This ‘Saville Boulder’ is now generally considered to be of Scandinavian origin. Other possible Scandinavian erratics have been recorded on the east shore of Flotta. These consist of laurdalite, a variety of nepheline-syenite known to occur in situ in the Oslo district of Norway (Saxton and Hopwood 1919)[4]. The distribution of boulders in the till leaves no doubt that the principal ice movement was in a north-westerly direction. Examples are the abundance in Westray of blocks of red and bull” sandstone of the type exposed on Eday, the presence in north-west Shapinsay of blocks of slaggy basalt which crops out in the south-east corner of the island, and the abundance of blocks of metamorphic rocks from the Stromness and Yesnaby outcrops on the ground west of these inliers. Most other igneous and metamorphic erratics can be matched with rocks cropping out in Sutherland. The Mesozoic rocks and marine shells are most probably derived from the bed of the North Sea to the east of northern Scotland and Orkney.
Striae
Striae are less abundant and more poorly preserved than in Shetland. Their predominant direction is north-westerly but there are also some with northerly, westerly and south-westerly trends (P915594). Some glaciated surfaces in Rousay indicate two or three directions of ice movement. These suggest that the south-west and east~west trends are earlier than the northwesterly and northerly ones (Wilson 1935)[5]. From this it has been inferred that in the earlier stages of the Devensian (Würm) glaciation Scandinavian ice travelled westwards across Orkney, being locally deflected south-westward by the mountains of Hoy. Later the pressure exerted by the eastern ice appears to have decreased and Scottish ice was able to push its way north-westwards across the islands. This relief of pressure may have occurred more than once, and there may even have been times when Orkney was partly or completely free of ice.
Later local ice
Orkney never supported a local ice cap of the type that once covered Shetland, and the only evidence for local glaciers is found in north-west Hoy. Here the corries and glaciated valleys contain, or are associated with, morainic mounds. Hummocky moraines also occur on the hillside south-east of Rackwick (Hoy) and in the valley of the Forse Burn (Hoy).
Glacial retreat features
Morainic mounds, whose origin has not been fully explained, are found in the valley leading west from Finstown (West Mainland) and in the northern part of Mainland, near Loch Harray and Evie. These may be the deposits left by lobes of ice which, during a stage in the deglaciation, re-advanced westward and south-westward from the ice-filled bays and straits up the valleys of West Mainland.
Fluvioglacial deposits have only been recorded in western Hoy, where spreads of sand with gravel lenses occupy the floor of the valleys north of Rackwick.
Changes in sea level
Evidence for the gradual submergence of Orkney in post-glacial times is afforded by the absence of raised beaches, the presence in many bays of peat beds below the high-water mark and the existence at sea level of lochs with freshwater deposits, which are separated from the sea by ayres or accumulations of blown sand. No 14C dates from submerged peats have so far been obtained, and the rate of sea level rise has not been calculated. A study of the bathymetry of the sea floor around Orkney by Flett (1920)[6] has shown that there is a flattening of the slope at 35 Fm (64 m) and that platforms occur at 20 Fm (36 m) and 8 to 10 Fm (15 to 18 m). Flett suggested that the two latter levels may be the submerged representatives of the ‘50 ft’ and ‘l00 ft’ raised beaches of parts of the Scottish mainland. Flinn (1969b)[7] has however suggested that the levelling off at 35 Fm around Orkney and 45 Fm around Shetland may be due to a much earlier phase of marine erosion.
Post-glacial deposits
There are no large areas of freshwater alluvium in Orkney. Blown sand covers about one-third of the island of Sanday and also considerable areas in Westray and North Ronaldsay. The largest accumulations on Mainland are at the Bay of Skaill, Birsay and Sandside Bay, Deerness. The sand consists principally of finely comminuted shell fragments which are blown inland from the beaches by on-shore gales.
High-level storm beaches are best developed along the exposed western shores. At Aikerness on Westray and Sacquoy Head on Rousay, crescent-shaped accumulations of large blocks occur at some distance behind the clifftops, which are respectively 12 and 18 m high.
Peat
The largest areas of peat are those covering the eastern hills of West Mainland and the greater part of central Hoy. Most peat is of the blanket type which ranges from less than 50 cm to 1 m in thickness. Basin bogs are of relatively small extent. Two of the latter have been investigated by the Peat Section of the Department of Agriculture and Fisheries for Scotland. These are White Moss, 6km SE of Kirkwall, and Glins Moss, 2.8 km NE of Dounby. The average depth of peat at White Moss is 3.4 m and at Glins Moss it ranges from 2.3 to 3.2 m. The bulk of the bog peat is formed from the remains of Sphagnum mosses together with cotton grass (Eriophorum) and heather (Calluna). Sedge (Carex) peat is commonly found near the bottom of the peat profile, which at Glins Moss also contains wood remains.
Bibliography
- ↑ PEACH, B. N. and HORNE, J. 1880. The Glaciation of the Orkney Islands. Q. Jnl geol. Soc. Lond., 36, 648-63.
- ↑ HEDDLE, M. F. 1880. The Geognosy and Mineralogy of Scotland. Mainland [Shetland], Foula, Fair Isle. Mineralog. Mag. , 3, 18-56.
- ↑ FLETT, J. S. 1898a. On Scottish Rocks containing Orthite. Geol. Mag., 5, 388-92.
- ↑ SAXTON, W. I. and HOPWOOD, A. T. 1919. On a Scandinavian erratic from the Orkneys. Geol. Mag., 56, 273-4.
- ↑ 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.
- ↑ FLETT, J. S. 1920. The Submarine Contours around the Orkneys. Trans. Edinb. geol. Soc., 11, 42-9.
- ↑ FLINN, D. 1969b. On the development of coastal profiles in the north of Scotland, Orkney and Shetland. Scott. Jnl Geol., 5, 393-9.