Shetland, Foula and Fair Isle, Pleistocene and Recent
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
- 1 Shetland - main island group
- 2 Foula
- 3 Fair Isle
- 4 Bibliography
Shetland - main island group
The first comprehensive account of the glaciation of the Shetland Islands was produced by Peach and Home (1879b) who concluded that during the ‘primary glaciation’ Shetland was overridden by an ice sheet which originated in Scandinavia and approached the island group from the north-east. As it crossed the axis of Shetland its course was deﬂected to the north–west, probably by the pressure of ice moving northwards from the Scottish mainland. They showed that, after the pressure of the Scandinavian ice had eased a local ice cap was established over the island group and that this ﬂowed off the land in all directions. They recognised a final phase of glaciation, when small local glaciers occupied the valleys and left numerous moraine heaps sprinkled over the area. Peach and Home also drew attention to the absence of ‘kames’ (i.e. the deposits left by glacial meltwaters) and the complete absence of raised beaches.
Support for the presence of Scandinavian ice in Shetland was provided by the discovery in 1900 of a large glacial erratic weighing over 2 tons at Dalsetter in south-east Mainland (P915593). This consists of tönsbergite, a rock known to occur in situ only in southern Norway (Finlay 1926a).
Since Peach and Horne’s pioneering studies there has been much additional information about the glaciation of these islands, and their conclusions have been considerably modified. It is recognised that Shetland, like the rest of Scotland, must have been covered by ice during all four glacial maxima of the Pleistocene Period, but that most of the superficial deposits and ice-formed features are attributable to the last (Devensian or Weichselian) glacial episode.
The ice sheets have greatly modified the Shetland landscape. In most of eastern Mainland and in the eastern and northern isles the passage of ice has smoothed out the original relief, giving rise to rounded hills on which many of the original irregularities are now covered by a skin of boulder clay. U-shaped, glacially overdeepened valleys are rare. Possible examples occur at Dales Voe and Colla Firth in Delting and at Quarff, 8 km SW of Lerwick, where a major east-west trending valley cuts through the Clift Hills range, but the glacial origin of these is still in doubt. In Northmaven, Lunnasting, Muckle Roe and the northern part of the Walls Peninsula the craggy topography shows the effects of glacial moulding which has given rise to roches moutonnées and less well-defined ridges with intervening ice-gouged depressions. The glacial scouring has produced numerous hollows, most of now form small inland lochs. The straits between peninsulas and islands, such as Ronas Voe and the Swarbacks Minn (90 m deep) between Vementry and Muckle Roe, may also have been deepened by ice scouring. It is also likely that the deep sea-basins close to Shetland, such as St Magnus Bay (160 m deep) and the basin between Whalsay and Yell (145 m) may have resulted from glacial overdeepening, though Flinn (1970c) has suggested that these may represent earlier meteor impact craters which were cleared out by ice during the Pleistocene.
Deposits known to be older than Devensian have been recorded at only two localities in Shetland. On the west coast of Fugla Ness (HU 312 913), North Roe, a bed of peat, up to 1-4 m thick, occurs between two layers of boulder clay (see Chapelhow 1965) and at Sel Ayre (HU 177 541) on the west coast of the Walls Peninsula a 7.5 m thick deposit of bedded sand and gravel with up to 45 cm of peat near its base, and 3 m of boulder clay above it, occupies a preglacial or interglacial valley. Pollen analyses of the Fugla Ness (Birks and Ransome 1969) and Sel Ayre peats suggest that the deposits were formed during the Hoxnian interglacial stage, but, as there are no other known interglacial deposits within several hundred kilometres, any correlation must be tentative. Radiocarbon dates from Fugla Ness have given ages ranging from 35 000 to 40 000 BP (Page 1972), and a similar age was obtained from Sel Ayre (Mykura and Phemister 1976).
The Shetland Islands are partly covered by a generally thin, irregular layer of stony till, with a matrix and pebble content which varies according to the character of the underlying bed rock and the type of rock over which the ice sheet has passed. Excellent temporary sections are seen from time to time in freshly opened roadside quarries and road cuttings but good permanent coast and stream sections are rare, the best being found on the shores of a few open voes and sounds, in drift-filled gullies and at the heads of some geos. Ice-transported erratics, some up to several metres in diameter, are common in many parts of Shetland. Mounds and ridges of morainic material, which are so common in the Scottish Highlands, are rare and generally small. Probably the best moraine belt extends for over 1.5 km across the centre of Papa Stour; smaller patches of hummocky moraine occur throughout central and southern Shetland.
Direction of ice movement
The pebble content of the boulder clay and moraine and the composition of the erratics have been used in conjunction with striated rock faces, chatter markings, roches moutonnées and other glacially moulded features to determine the directions and phases of ice movement (P915593). Data from some areas such as Yell are still very scarce and only a few areas have been investigated in detail. The interpretations put forward in this page must therefore be regarded as tentative.
Maximum stage of glaciation
Peach and Horne’s concept of an uninterrupted westward ice ﬂow across Shetland during the maximum stage of the last glaciationhas not been substantiated, except perhaps in the extreme north and south of the island group. In Unst boulders and pebbles of serpentine of the type cropping out in the east and centre of the island are present on the high western Valla Field ridge and along the west coast to the south of Wood Wick, indicating ice ﬂow from the east-south-east. In North Roe (north Mainland) a boulder clay exposed along the east coast, which has been linked with either north-north-east trending or north-west trending striae, contains pebbles which most probably come from north-easterly and south-east to easterly directions. In South Mainland there is evidence that south of the latitude of Quarff ice moved westward from the North Sea across the high backbone of the island. Blocks of Old Red Sandstone conglomerate of the type cropping out along the east coast, for example, were recorded by Peach and Home along the west shore near Wester Quarff and occur all along the hilltops extending south from Quarff. Erratics of both Clift Hills phyllite and Old Red Sandstone are common on St Ninian’s Isle. It is also probable that ice coming from the north-east covered part of Bressay, as in the northern half of the island all the ice-transported pebbles and boulders consist of Old Red Sandstone sediment and there are no stones of metamorphic rock of the type found on Mainland.
In the central part of Shetland the evidence points to the existence of a local ice sheet during the maximum stage of the Devensian. West of the central backbone of Mainland the overall ice movement was to the west. The actual direction was north-westward in North Roe, westward to west-north-westward in Northmaven and Muckle Roe, almost radially outward from the Walls Peninsula, and south-westward in the Scalloway-Burra Isle area. There is an abundance of indicator stones throughout the area which confirm this direction of movement, but there are no stories which could with certainty have been derived from a more easterly source than the main Scallafield ridge of central Mainland. In eastern Mainland the prevailing orientation of striae swings from east–south–east and east-west in northern Tingwall, Nesting, Laxo and on the south coast of Whalsay, to north–east and even north–north–east in Lunnasting, central and north Whalsey and the Out Skerries. In an unpublished report, now in an Institute open file, Robertson (1935) has shown that, although the orientation of these striae is in apparent accord with those farther west, the stones in the till and the erratics invariably indicate an eastward. or north-eastward direction of ice transport. This contradicts the conclusions of Peach and Horne, who visualised a south-westward ice movement in these areas. Rocks on which Robertson’s evidence is based include the migmatite and granite of the Colla Firth permeation belt (P915596), boulders of which abound on the peninsulas between Dales Voe and Wadbister Voe and in the South Nesting Peninsula, and the Stava Ness Granite which provides many boulders in Whalsay, and rarer boulders as far east as the Out Skerries. Boulder-trails are also provided by smaller outcrops of distinctive rocks, such as the small bosses of serpentine in Lunning and west Whalsay, blocks of which can be found for some distance to the north-east. Farther north on the island of Fetlar, Phemister has recorded many blocks of metamorphic rocks from the Lamb Hoga Peninsula resting on the serpentines occupying the centre of the island, and abundant serpentines in the till overlying the Funzie conglomerate in the east. As no boulders of the staurolite-schist from the main Scallafield Ridge of central Mainland or of rocks cropping out even further west have been found in the eastern areas, Robertson concluded that central Shetland was covered by a local ice cap which had its ice-shed in the vicinity of Weisdale and Pettadale, whence it moved out both westward and eastward. The eastward-ﬂowing ice encountered resistance, presumably by Scandinavian ice, a short distance east of the coast of Mainland and then turned north-eastward past Whalsay and the Out Skerries.
A similar conclusion was reached independently by Flinn (1964, l967b), who stated that, though eastern ice was carried westward across the watershed in the extreme north and the south of Shetland, in east-central Shetland a strong stream ﬂowed eastward and then swung north as it extended farther east. The ice crossing Unst from the east may have been either deﬂected local ice or Norwegian ice. Hoppe (1970) however, though not disputing the facts enumerated above, has reverted to Peach and Horne’s concept that Shetland was overridden by the Scandinavian ice sheet during the maximum stage of the Wiirm (i.e. Devensian) Period. He based this conclusion on the fact that there are striae on Bressay and on Weisdale Hill which indicate an early ice movement from the north-east and south-east respectively. It must be pointed out, however, that Weisdale Hill lies to the west of Robertson’s postulated ice-shed and that the striae on Bressay could have been formed by ice which did not override Shetland Mainland. The present writer concludes that, though eastern ice may well have overridden Shetland at one period, the evidence for this has been obliterated by later ice, and that all the available evidence can be reconciled with Robertson’s and Flinn’s concept that during the Devensian maximum a local ice cap covered central Shetland and deﬂected the Scandinavian ice sheet around it.
Later ice cap
The weakening of the Scandinavian ice sheet and its subsequent break-up over the North Sea appear to have resulted in first a release of pressure on the locally generated ice and then its complete isolation. This enabled the local ice cap to expand and brought about an outward ﬂow of ice throughout Shetland. The local ice eventually covered the part of southern Shetland which had earlier been overridden by eastern ice. Good evidence for an eastward ice ﬂow in south-east Mainland is found in the Helli Ness Peninsula, where there are several large erratics of ‘schist’ and vein quartz derived from the Clift Hills and where the till contains many boulders of Cunningsburgh Spilite. Other evidence for eastward ice movement is found at Scousburgh, where large blocks of the basal Old Red Sandstone breccia occur up to 100m E of the breccia outcrop, in the No Ness Peninsula near Sandwick where the drift contains both quartz and schist pebbles, and on the east shore of the Bay of Quendale near Sumburgh Airfield as well as on Sumburgh Head, where the till contains boulders and debris of Spiggie Granite. In the north-east part of North Roe Chapelhow has recognised a second till which contains boulders whose source lies to the south and west and which occupies much the same ground as the earlier till derived from the east (see Maximum stage of glaciation). In west Shetland it has not been possible to separate the deposits and striae of the later ice from those formed during the glacial maximum. There was here probably neither a time break nor a major directional change in the ice ﬂow, though minor changes in direction may have taken place as the confining pressure exerted by the eastern ice diminished. As the ice broke up in the deeper bays around Shetland ice from the land would tend to ﬂow directly into the bays, producing the ice ﬂow at right angles to the present coast line.
It is not certain if at this period Unst and northern Yell were covered by the local ice sheet, as the evidence for eastward ice movement in Unst is weak. Nor is it certain that Bressay was covered by ice coming from Mainland. Peach and Home have suggested that the Mainland ice was deﬂected south-eastward along the Sound of Bressay and that the Ward of Bressay may at this stage have supported its own ice cap.
Little is known about the stages in the melting and break-up of the Shetland ice cap. Its final remnants consisted of a number of small immature corrie glaciers and ice patches whose existence can be recognised by the presence of rudimentary corries and small morainic mounds and ridges (Charlesworth 1956). The largest of these are centred on Ronas Hill, and there are less well developed ones on the hills of central Mainland between Dales Voe to Weisdale Hill, on the eastern slope of the Clift Hills west of Cunningsburgh, and on the north and west slopes of Sandness Hill.
Glacial retreat features
Deposits and landforms produced by the melting of land ice are extremely rare in Shetland. Mounds, ridges, or spreads of glacioﬂuvial sand and gravel are virtually absent and glacial overﬂow channels are rare and poorly developed. In the Walls Peninsula there are a number of small isolated dry channels, which may have been formed by glacial meltwaters. The relative sea level around Shetland has risen considerably since the retreat of the ice, and, as the products and features of ice wastage in most upland areas are concentrated on the lower ground, it can be assumed that in Shetland they are now largely submerged.
Periglacial action on the higher plateaux has given rise to the blockfields (Felsenmeere) which cover Ronas Hill and the adjoining high plateaux and, to a less extent, Sandness Hill and the Ward of Culswick.
The best evidence relating to the date at which Shetland became free of ice has been obtained by the radiocarbon dating of the earliest late-glacial deposits taken from several Shetland lochs (Hoppe and others 1965, Hoppe 1970). These have given 14C ages of about 10 000 years BP, which suggests that the greater part of Shetland became free of ice during the Allerod interstadial or possibly slightly earlier. The small corrie glaciers may have lingered on considerably longer.
Changes in sea level
The presence of submerged peat beds, the lack of raised beaches and well-defined submerged shore platforms, the characteristic drowned river valleys now forming many of the Shetland voes, and the local traditions of submergence in historic times have all long been taken as evidence for the continuous submergence of the land since the Ice Age (see Finlay 1930). Submerged peat is common in the sheltered voes and sounds and has also been encountered in excavations in the harbours at Lerwick, Scalloway, Bressay, Graven (Sullom Voe) and Symbister (Whalsay). At Lerwick harbour peat was found at a depth of 6-4 m below high-water mark and at Symbister at between 8-6 and 8-9 m. The latter gave 14C dates which range from 5455 BP to 6970 BP, indicating that some 5500 years ago sea level must have been at least 9 m lower than at present (Hoppe 1965). The subsequent rise in sea level may be partly accounted for by the world-wide eustatic rise in sea level which may have been as much as 6 m, and partly by the isostatic sub— mergence of the land. It is probable that the isostatic depression of the Scottish mainland and Scandinavia during the glacial maxima was accompanied by uplift along the marginal zone of the ice sheets, which includes the Shetland area, and that the post-glacial submergence of Shetland is the result of recovery from this peripheral uplift (Flinn 1964).
Flinn (1964) has recognised submerged platforms around Shetland at the following levels (below OD): (1) 45 Fm (82 m), (2) 25 Fm (45 m), (3) 13 Fm (24 m), and possibly also (4) 5 Fm (9 m). Flinn believes that these shelves are not drowned post-glacial wave-cut platforms, but may represent remnants of earlier erosion surfaces. He suggests that in post-glacial times there have been no major pauses in the rise of sea level which would have permitted the cutting of wave-cut platforms.
As there are no major rivers, freshwater alluvium occupies only a very small proportion of the Shetland land surface. Blown sand covers a considerable area around the Bay of Quendale in south Mainland and somewhat smaller areas at Scousburgh, Bigton, Meil (Burra Isle), Melby and Papa Stour. There are also small areas of blown sand at West Wick and Inna Ness on the island of Yell and on Balta (Unst). Storm beaches are a feature of the exposed coasts. They are particularly well developed along the clilfs of Stenness and Esha Ness (Northmaven) where the Old Red Sandstone lavas and tuﬂ‘s are readily broken up into blocks by wave action. At the Grind of the Navir, for instance, there is an arcuate accumulation of blocks on a waveswept platform at 15 m above sea level. Fine storm beaches are also seen along the cliff-tops on the south coast of the Out Skerries and, at a lower level, at Fugla Ness, near Hamnavoe on the Burra Isle.
Peat, mainly of the blanket bog type, covers a considerable part of Shetland and forms an almost unbroken cover over Yell, western Unst and large parts of central and western Mainland. It is virtually absent from the serpentine and greenstone areas of Unst and Fetlar and occupies only small patches in the rugged terrain ﬂoored by the diorite and granophyre of Northmaven and North Roe, the metamorphic rocks of the Walls Peninsula and the metamorphic rocks of the Scalloway area and the Burra Isle. On some islands, such as Papa Stour, all the peat has been removed for fuel.
The blanket bog does not normally exceed 1-5 m thickness, but in hollows and valleys up to 6 m of bog peat have been recorded. On Yell the peat reaches a thickness of 3 m, even on steep slopes. The stratigraphy and vegetational history of the Shetland peat was studied by Lewis (1907, 1911) who established the following sequence of vegetational zones:
- B. Upper unstratified peat. (This has a greater extent than the lower peat, and rests in many places directly on weathered boulder clay.)
- A. Lower stratified peat—consisting of:
- 5. Second Arctic Bed
- 4. Lower Peat Bog (up to 2 m+ thick)
- 3. Forest Bed
- 2. First Arctic Bed
- 1. Basal peat (very local in extent), with aquatic plants.
These zones have not, as yet, been correlated with the pollen zones established in the peats of the Scottish mainland. According to Lewis the peat of the Forest Bed extends right up to the western seaboard of the Walls Peninsula, which indicates that the climatic regime at the time of its formation was very different from the present climate with its prevalent strong westerly winds.
Foula, with its relatively mountainous topography, was probably never completely overridden by ice from the east. Striated pavements with east to east-south-east trending scratches are seen on the north and east coasts of the island. The boulder clay exposed along these shores contains boulders of sandstone similar to that cropping out on the island, as well as metamorphic and igneous rocks. Erratics of epidotic granite, like the Spiggie Granite, are common. The island has three corries which face north-east to east and a U—shaped valley, the Daal, which may originally have terminated in a corrie just west of the present island. These features were formed by local ice which was probably already established at an early stage of the main Devensian glaciation and remained until the period of the Corrie Glaciation on Shetland Mainland. Ice ﬂowing eastward from these corries was deﬂected by ice moving westward from Shetland Mainland both to the north and the south along the depression extending north and south from Ham. When the pressure from the eastern ice diminished local ice may have extended from the corries eastward beyond the limits of the island, thus accounting for the presence of sandstone boulders in some of the east coast till. At a later stage ice did not extend far beyond the higher corries, one of which has a fine arcuate terminal moraine at its mouth.
Fair Isle was overriden by ice from the east-south-east, and this has obliterated all traces of any possible earlier glaciation. The westward ice movement appears to have been very powerful and has produced a strongly ice-moulded topography. This is particularly impressive in the southern half of the island, where the ice has gouged out depressions along east-south-east trending faults. The drift of Fair Isle is reddish brown in colour, has a relatively sandy matrix and contains a large number of small boulders and fragments of soft red sandstone. According to Flinn (l970a) both the till and the sandstone clasts contain millet-seed sand grains and he has suggested that the sediment from which these are derived may be of Permo—Triassic age. Flinn has also found evidence for an ice ﬂow in a south-easterly across Fair Isle. There is no indication that Fair Isle has at any time had a local ice cap.
- FINLAY, T. M. 1926a. A Töngsbergite Boulder from the Boulder-clay of Shetland. Trans. Edinb. geol. Soc., 12, 180.
- CHAPELHOW, R. 1965. On Glaciation in North Roe, Shetland. Geogrl. Jnl, 131, 60-70.
- BIRKS, H. J. B. and RANSOM, MAREE, E. 1969. An interglacial peat at Fugla Ness, Shetland. New Phytol., 68, 777-96.
- MYKURA, W. and PHEMISTER, J . 1976. The geology of western Shetland. Mem. geol. Surv. Gt Br.
- ROBERTSON, T. 1935. The Glaciation of Aithsting, South Nesting, Whalsay and the Out Skerries. Geological Survey Records, (unpublished).
- FLINN, D. 1964. Coastal and Submarine Features Around the Shetland Islands. Proc. geol. Ass., 75, 321-39.
- FLINN, D. 1967b. Ice front in the North Sea. Nature, Lond., 215, 1151-4.
- HOPPE, G. 1970. The Würm ice sheets of northern and arctic Europe. Acta. geogr. Univ. Lodz., 24, 205-15.
- CHARLESWORTH, J. K. 1956. The Late-glacial History of the Highlands and Islands of Scotland. Trans. R. Soc. Edinb., 62, 769-928.
- HOPPE, G. 1965. Submarine peat in the Shetland Islands. Geogr. Annlr., 47A, 195-203.
- FINLAY, T. M. 1930. The Old Red Sandstone of Shetland. Part II. North-western Area. Trans. R. Soc. Edinb., 56, 671-94.
- LEWIS, F. J. 1907. The Plant Remains in the Scottish Peat Mosses. III. The Scottish Highlands and The Shetland Islands. Trans. R. Soc. Edinb., 46, 33-70.
- LEWIS, F. J. 1911. The Plant Remains in the Scottish Peat Mosses. IV. The Scottish Highlands and Shetland, with an Appendix on Icelandic Peat Deposits. Trans. R. Soc. Edinb., 47, 793-833.
- FLINN, D. 1970a. The Glacial Till of Fair Isle, Shetland. Geol. Mag., 107, 273-6.