Pleistocene and Recent, Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Northern Highlands, like other districts of Scotland and northern England, were glaciated during the Pleistocene epoch, and the pre-Pleistocene topography was modified by the widening, straightening and deepening of pre-existing river valleys. On the northern and eastern slopes in particular, freeze-thaw activity combined with glacial erosion produced spectacular corries, and the products of glacial deposition are found on the lower ground in the form of hummocky moraines (P219672) and extensive drift sheets. Some parts of the district, notably the western seaboard and the Outer Hebrides, are largely free of drift, and smoothed and striated rock knobs rise above the peat (P856735). So far, only three in-situ organic deposits are known which are older than the last glaciation, and criteria for the maximum extent of ice rest entirely on geomorphological considerations. Thus, though the area was certainly repeatedly glaciated, there is an element of doubt concerning the extent of the last (Late Devensian) glaciation.
Evidence for events peripheral to or predating the last glaciation
In Lewis, an advance of ice (chiefly local but possibly with a tongue from the Scottish mainland) during what may have been the maximum of the Late Devensian glaciation failed to reach the extreme north of the island (von Weymarn, 1979; Peacock, 1984; Sutherland and Walker, 1984). Here, frostshattering and extensive solifluction flows, perhaps originating during the very cold climate which accompanied the last glaciation, have been preserved, as has a pre-existing raised beach which rests on till deposited during an earlier glacial episode. Interstadial lacustrine deposits on Tolsta Head dated about 27 000 BP (radiocarbon years ago) are found below till of the last glaciation (von Weymarn and Edwards, 1973). A possible interglacial peat has been reported below the raised beach in north Lewis (Sutherland and Walker, 1984). Till older than Late Devensian has been described from St Kilda (D. G. Sutherland and others, 1982), together with interstadial deposits which may date to the same period as those on Tolsta Head. It is perhaps significant that a raised beach overlain by head has been reported from North Rona (Gailey, 1959), suggesting conditions similar to those in north Lewis. This island, likewise, may have been outside the limits of the Late Devensian glaciation. On the Scottish mainland, radiocarbon dating of the remains of reindeer from caves at Inchnadamph in Sutherland has yielded ages of up to 25 000 BP (Lawson, 1984).
Remains of rock terraces, cut by the sea and backed by fossil cliff lines, occur in Rhum, the Hebrides and parts of the west coast including the coastline of Ardnamurchan and Loch Linnhe. These, the ‘pre-glacial’ beaches of the Geological Survey Memoirs, occur mainly at heights between 6 and 45 m OD; some show clear evidence of glaciation in the form of striations and coverings of till. The raised platform and cliff in Rhum, which ranges in height between 20 and 37 m above local datum, is locally strongly glaciated; it has clearly been either faulted or tilted at some stage during the Pleistocene. On the other hand, the well marked platform backed by cliffs on Mull and on the shores of Loch Linnhe, which in the past has been thought to be pre-glacial and which is present even at localities well sheltered from marine action, is gently tilted westwards in a fashion which accords with that of Scottish late- and post-glacial beaches. It has been interpreted as having been formed during a short period of intense marine erosion under periglacial conditions of the Loch Lomond stade some time after the disappearance of the main Devensian ice-sheet (Sissons, 1974; Gray, 1978). Sissons (1982b) has suggested that the ‘pre-glacial’ rock platforms have also been isostatically tilted, and were formed during the last and previous glacial periods by wave and frost action.
Deeply decomposed igneous and metamorphic rocks are found below a cover of till at several localities, such as the Helmsdale area in the north-east, Ratagain and Glenelg in the south-west, and Lewis in the Outer Hebrides. The decomposition has been ascribed by some authors to tropical weathering in late Tertiary times, but the effects of faulting, hydrothermal activity and Pleistocene weathering have probably been responsible in some instances.
It was suggested at one time that interglacial marine beds existed on north Lewis, where shelly, till-like beds are separated by water-sorted deposits including laminated clay, sand and gravel. The mixed fauna and the broken molluscan shells, however, indicate reworking and transportation by the ice-sheet. Other features, such as the presence of numerous debris-flow deposits interbedded with glaciofluvial sands and gravels, support the theory of deposition chiefly peripheral to glacier ice.
The history of the last glaciation can be broadly considered under three headings. During the Devensian maximum, which coincided with an extremely cold climate (probably accompanied by high snowfall) an ice-sheet covered much of Scotland. During the following period of ice wastage, sheets of silt, sand and gravel were laid down; the movement of any active ice was controlled by the local topography. This was followed by the almost complete dissolution of the ice-sheet during the Windermere Interstadial, (13 000–11 000 BP) after which glaciers returned to the mountains of the Highlands and Islands between 11 000 and 10 000 BP. However, climatic considerations (considered briefly below) suggest that some glaciers may have reformed well before 11 000 BP, and a few in the highest corries may have survived throughout the Windermere Interstadial. The well documented readvance between 11 000 and 10 000 BP, termed the Loch Lomond Advance or Readvance, was marked by a brief return of arctic conditions before the rapid amelioration of climate about or somewhat before 10 000 BP.
From an ice-shed situated east of the present principal watershed of northern Scotland, ice flowed westwards into the Minch, eastwards into the Moray Firth and northwards into the Pentland Firth. Subsidary ice-sheds, or perhaps separate centres of dispersal, may have existed in parts of Sutherland (P915504). The Outer Hebrides appear to have supported their own ice-sheet during part if not all of the Devensian maximum (Peacock and Ross, 1978; Flinn, 1979; von Weymarn, 1979). It is likely that the combined mainland and Outer Isles ice occupied much of the Minch and the Sea of the Hebrides but failed to reach St Kilda, which supported only a small valley glacier (D. G. Sutherland and others, 1982). Late-Devensian ice also failed to reach North Rona. In the central areas of the Highlands the ice seems to have been at least 1100 m thick. No sign of glaciation has been noted on the highest summits of mountains such as An Teallach (1061 m) and Ladhar Bheinn (1019 m) which lay west of the ice-shed. Such summits would have suffered minimal erosion and probably stood above the sheet for long periods, if not for the entire Late Devensian.
During the maximum glaciation, a great glacier extended into the Moray Firth where it came up against Scandinavian ice which occupied the North Sea. One arm of the Scottish ice was diverted across the Banffshire coast and the other across Caithness (P915504). The north-westerly flow in Caithness, which was both preceded and succeeded by an ice movement from the west, was responsible for the deposition of stiff, bluish to brownish grey, shelly till which contains, in addition to local rocks, gneisses from Ross-shire, Mesozoic rocks and shells from the bed of the Moray Firth, and fragmentary shells of Pleistocene marine molluscs (most of which are temperate species). The most remarkable erratic is the enormous mass of fossiliferous Lower Cretaceous sandstone and Tertiary clay which was at one time quarried at Leavad (P915496, P915504). Some of the evidence suggests that parts of Caithness may have been subjected to a periglacial climate for much longer than adjoining ground (J. S. Smith, 1978) and were thus deglaciated early.
In the Outer Hebrides ice flowed outwards from ice centres in Lewis and South Harris (von Weymarn, 1979) and from an ice-shed which extended southwards near the west coast of Benbecula and the Uists to Barra (P915504; Flinn, 1979; Peacock, 1984). The eastward-flowing ice on Barra and South Uist reached a thickness of more than 400 m, the basal layers being diverted to the north and south around Beinn Mhor and Hecla. The erratics of Torridonian sandstone and Cambrian quartzite which occur along the west coasts of the more southerly Hebridean islands (Jehu and Craig, 1923–1934) suggest that Scottish mainland ice reached these districts at one time, probably during an earlier glaciation. A fragment of raised beach capped by till on the west coast of Barra may be the same age as the pre-Late Devensian beach of north Lewis.
During retreat, the ice probably remained active for a time, and was only gradually constrained to the valleys as the mountain tops became ice-free. Readvances probably took place from time to time, either as a result of temporary climatic reversals or as a result of local glacier surges. Such a readvance has been identified in Wester Ross (Robinson and Ballantyne, 1979) on the basis of a terminal moraine (P915504). Other readvances have been suggested in the more central areas of the Northern Highlands (Sissons, 1982a) and in the Moray Firth (J. S. Smith, 1978; Synge, 1978) forming, for instance, the gravel and silt deposits at Chanonry Ness and Ardersier. On the east coast, outwash terraces are associated with the high sea levels of late-glacial times, and deltas formed in the sea by glacial meltwaters occur at Beauly and Muir of Ord. Similar features have been recorded elsewhere, including the delta on which Ullapool stands. The most widespread deposits left by the retreat on the wasting ice are the hummocky moraines (P219672) and gravels interspersed with sheets of boulder clay which cover large areas of the lower ground of central Sutherland and the valley bottoms in more mountainous areas.
Loch Lomond Stade
In many valleys and corries in the western Highlands there is abundant evidence for an advance or readvance of ice following the retreat from the glacial maximum (P915504). Fresh-looking terminal moraines mark the readvance limit in many Highland valleys (Sissons, 1979b), as for instance in Glen Moriston and Glen Affric; at the west end of Loch Shiel the ice ploughed through late-glacial marine deposits. In the area around Loch Quoich and Loch Arkaig, small ice-caps developed with an ice-shed east of the present watershed, and the ice reached a thickness of at least 600 m in the Great Glen. The readvance ice in the Glen Moriston area dammed a subsidiary valley to form a glacial lake which is thought to have drained catastrophically during glacier retreat (Sissons, 1977).
In the north, however, the glaciers of the Loch Lomond Readvance were scattered and small (P915504), but their location is marked, as in many localities farther south, by hummocky moraines and morainic features aligned in the direction of ice movement (Sissons, 1979b). Similar landforms occur in southwest Lewis and Harris, but the age of these is as yet uncertain (Peacock, 1984).
Other Late-Glacial and Post-Glacial events
Evidence from lake sediments shows that, as the ice wasted from its maximum position, the ice-free areas of the mainland were colonised by crowberry heath with juniper. Birch woodlands occurred locally in Skye. During the Loch Lomond Stade, however, the vegetation reverted to tundra. Work on the contemporaneous beetle faunas suggests that during the earliest part of the Windermere Interstadial July temperatures were at or above present-day levels in parts of England, but much lower to the north (Coope, 1977). Conditions were generally much colder after 12 000 BP. Marine beds in the inner Moray Firth probably dating from the interstadial show faunal assemblages comparable to those of the present day north of Lofoten in Norway (Peacock and others, 1980). It is likely that, given sufficient precipitation, a few corrie glaciers persisted through the interstadial (it would take only a small drop in temperature for glaciers to reappear in the Highlands at the present day) with a more general rejuvenation of glaciers from about 12 000 BP, and particularly from 11 000 BP onwards, when evidence from many sources suggests a very cold climate (Sissons, 1979b).
During the Loch Lomond Stade, the little-vegetated ground was subjected to intensive frost action; this gave rise to blockfields and stone lobes on the mountains, and to patterned ground and solifluction of till slopes at all levels. Extensive screes formed on suitable hillsides, and the rivers carried great quantities of sand and gravel. They also carried suspensions of silt and clay which were deposited as laminated sediments in the lakes. Of particular interest is the occurrence of isolated ice-wedge casts, such as those in the glaciofluvial gravels of the Oykell valley. The ice-wedge casts, which probably relate to the stadial, would today indicate permafrost and average annual temperatures several degrees below zero celsius.
Many of the numerous landslips in the Northern Highlands probably date from the periods of ice wastage (Plate 31). They would have been triggered by a combination of features, such as unloading of the rock faces as the ice melted, a high water table, the presence of suitably oriented joints and (in flat-lying Tertiary and Mesozoic rocks) the presence of weak horizons such as shales and boles. There is increasing evidence that features resembling the protalus ramparts (ridge of debris formed at the base of a steep snowfield) and rock glaciers of present-day polar regions occur in parts of the Highlands (Sissons, 1979b). A fine example of a fossil protalus rampart has been reported from Baosbheinn, 10 km south of Gairloch in Wester Ross (Sissons, 1976). It is probable that, during the Loch Lomond Stade, interstitial ice also facilitated the renewed movement of some of the older landslips, such as those of Eigg and Skye.
After the final disappearance of the ice, heath was briefly re-established, soon to be replaced by birch and pine forests in many areas and mixed oak forest at more favourable localities in the south and south-west, including southern Skye (Birks, 1977). The windy Outer Hebrides may never have supported forests; however, pine, willow and birch stumps near the base of the peat at various localities show there were stands of trees at various times between 9000 and 4000 BP (Wilkins, 1984).
With the wetter and cooler climate initiated about 6000 years ago, part of the forest was destroyed by blanket peat (assisted to an increasing extent by the hand of man). Such peat still occupies extensive tracts, particularly in Sutherland and Lewis, though the area is being constantly reduced by erosion, exploitation and the demands of agriculture. The deposits left by the ice have been partly eroded and redeposited to form the alluvial haughs seen in many Highland valleys today, and it is probable that these processes were accentuated by the destruction of the forest.
Blown sand occurs at many coastal localities where there are sandy beaches. It is particularly important on the west coast of the Outer Hebrides, where flat ‘machair’, composed of shell sand blown inland, commonly forms almost the only agricultural land. The ‘White Sands of Morar’ are quartz-rich sands with occasional bands of pink garnet. There are extensive stretches of blown sand in the broader bays of Caithness and in north and west Sutherland in the form of dunes and Hat links, the latter only slightly above sea level. Blown sand also occurs at the intakes of some of the inland lochs of Caithness.