Late-glacial and postglacial features, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
As climatic conditions gradually changed, rocks that had been shattered by permafrost conditions over thousands of years thawed out, releasing fragments and providing much surface detritus. Collapse of over-steepened glaciated valley walls was commonplace, and widespread landslipping and rock falls took place, both of which continue to the present day. Throughout the Inner Hebrides, landslips are especially common where thick successions of Paleocene lavas overlie incompetent Mesozoic sedimentary rocks. On Skye, landslips occur at intervals all along the east-facing lava scarp from near the mouth of Loch Sligachan to the northern end of the Trotternish peninsula. In the spectacular examples at Quiraing (P580491), many of the slipped masses at lower elevations are ice-sculpted and may predate the Main Late Devensian Glaciation. However, the angular, unmodified outlines of those landslips closest to the present-day scarp, with their steep spires, cliffs and rock pinnacles, could not have survived ice action and must postdate the Main Late Devensian Glaciation. Some of the landslips remain active, as at Flodigarry. In central Skye, there are a few smaller, less spectacular landslips, for example on the north-east side of Glas Bheinn Mhór, where the landslip is superimposed on deposits that might be attributed to the Loch Lomond Stadial. Large landslips, and rock falls occur in eastern Raasay, where a major fall was reported at Hallaig in 1934. Deep fissuring of the thick sandstone at the top of the Middle Jurassic, Bearerraig Sandstone Formation on the summit of Beinn na' Leac can probably be attributed to, or has been accentuated by, landslipping.
Small landslips are present along the granite cliffs of south-west Rum, but the most spectacular examples in the Small Isles occur around the edges of the lava escarpments in northern Eigg. There, a major postglacial landslip on the coast at Talm is likely to be still active, but west of the Bay of Laig, postglacial beach deposits are banked against an older rotational slip. On the north-east coast of Eigg, both ancient scree-covered landslips and more recent landslips can be seen, and there is abundant evidence of recent rockfalls, usually involving the Middle Jurassic Valtos Sandstone Formation and the overlying Paleocene lavas. In the south of the island, at Grulin Cottage, numerous scattered large blocks of columnar pitchstone are part of a rockfall from the south face of An Sgùrr. On Mull, landslips of both pre-Late Devensian and postglacial or Late-glacial age are recognised at Gribun, and modern examples occur in The Wilderness on the Ardmeanach peninsula. Landslips are prominent south of Loch Teacuis and south of Loch Arienas in Morvern, where Paleocene lavas overlie a thin Mesozoic succession. The well-known Fallen Rocks on Arran are the site of landslips involving Old Red Sandstone conglomerate and farther north, at An Scriadan east of the Cock of Arran, large blocks of Permian sandstone collapsed less than 300 years ago, with a noise that was heard over a wide area.
Talus and other upland features
The present-day environment on the mountain tops of the Inner Hebrides and Arran is dominated by copious rain and high winds. The meagre vegetation is removed by the wind and by grazing animals, laying bare to erosion the silt and fine-sand in the poorly developed soil. This leaves relatively coarse lag deposits and leads to the formation of low, turf-banked terracettes which are very noticable on the higher hills. Talus (scree) deposits, comprising unmodified rockfall accumulations, are well represented in the mountainous areas and along sea cliffs. Fine examples are found within the Cuillin of Skye, where the coarse, mobile debris forms 'stone chutes', the best known of which is the Great Stone Chute at the head of Coire Lagan. Elsewhere on Skye, the light coloration of the Red Hills (Plate 26) arises in large part from their mantle of granite debris and scree, of which the coarse, bouldery scree on Glamaig and Beinn na Caillich are of particular note. Scree accumulations are common in the mountains and on the coasts of Rum, Mull and Arran. It is not unusual to be able to distinguish older scree, stabilised by soil and vegetation, from younger, mobile scree, for example in the scree banked against the microgranite cliffs of south-west Rum.
Boulders and shingle make up a large proportion of the present-day coastal beach deposits, for example on the shoreface at the base of the lava cliffs of Skye and Mull. Sandy beaches are commonly made of material washed out of glacial deposits, as at Brodick Bay, Arran, and sandy detritus that has been derived from the weathering of nearby bedrock comprises other beaches, for example Torridonian sandstone at Kilmory on Rum, New Red Sandstone at Kildonnan and Blackwaterfoot on Arran and Middle Jurassic Valtos Sandstone at the Bay of Laig, Eigg. At Camas Sgiotaig, in the Bay of Laig, the 'Singing Sands' emit shrill squeaks when the dry sand is crushed underfoot. Beaches formed largely of shelly debris also occur, for example at Calgary Bay and Port Langamull in north-west Mull, near Canna Harbour and at Gaillinach on Muck. At Duntulm, in northern Skye, olivine from the weathering of nearby dolerite sills is a prominent constituent of the beach sand. At a number of localities there are gleaming white beaches composed of fragments of the calcareous algae Lithothamnion calcareum. On Skye, examples of these so-called 'coral sands' are found north of Rubha na Gairbe on the east shore of Loch Dunvegan and north of Ord in Sleat. Coastal sand dunes back a number of the bays, for example at Drumadoon Point on Arran, Kilmory on Rum, Laig on Eigg and at the east end of Staffin Bay on Skye, where the adjoining dolerite cliffs are polished by sand blasting.
High atmospheric moisture levels during the fifth millenium BP led to the decline of woodlands, as sites became waterlogged, and promoted the growth of blanket peat, which covers many hillsides to the present day. Extensive spreads of peat occur on the lava plateaux of Skye and Mull, and in broad, ill-drained glens, for example Kinloch Glen on Rum. Peat was formerly an important source of fuel (p. 175). The pollen stratigraphy of peat provides a valuable record of climate change during the Quaternary (e.g. Lowe and Walker, 1986, 1991).
Most streams in the mountainous areas have gravelly beds, and deposits of fine-grained alluvium are not extensive, partly due to the absence of major rivers. Small, elongate patches of alluvium occur next to many of the streams and rivers, and some of the broader valleys have more extensive deposits. Relatively large areas of alluvium are gradually encroaching on certain lochs, for example Loch Bà on Mull, and at Loch Cill Chroisd in Strath, Skye, where alluvium, lacustrine mud and vegetation are gradually filling the loch. Also on Skye, there is an extensive tract of alluvium on the site of a former shallow loch at Eilean Chaluim Chille, north of Uig, which was drained in 1947 to give a large area of new pasture. Terraces are cut into the alluvium at more than one level, for example at the bottom of Glen Iorsa, Arran; farther up the glen, alluvium covers the valley floor for a distance of over 5 km above Loch Iorsa. Small placer deposits of heavy minerals occur in some rivers, with chromite-rich examples found on Rum in the Abhainn Rangail, Glen Harris.
Lacustrine deposits of diatomaceous earth are found in many places on the Trotternish peninsula, northern Skye, where they were worked in the past (p. 176). The diatoms are the skeletal remains of unicellular plants that were laid down in freshwater lakes, probably under warmer climatic conditions than at present. Extensive deposits of diatoms mixed with clay and peaty material occur at Loch Cuithir, about 6 km north-north-west of The Storr, where they rest on moraine, and hence postdate the last glaciation, but are overlain by peat (Anderson and Dunham, 1966). Diatomaceous earth is also known from near Knock on Mull and has been recorded on Eigg (Haldane et al., 1940).