Late glacial period, Quaternary, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Late Glacial Period
The period of time between the diachronous retreat of the MLD ice-sheet from any given area, and the beginning of the Holocene at 11.7 ka BP, is referred to as ‘late glacial’. It includes the Windermere Interstadial and the Loch Lomond Stadial, both periods of climatic instability. Deglaciation commenced in a cold, arid environment and high parts of the district probably witnessed several thousand years of ice-free, periglacial conditions before the onset of rapid warming at about 14.7 ka BP, the beginning of the Windermere Interstadial. The abrupt amelioration in climate occurred when temperate waters of the Gulf Stream returned to the western coasts of the British Isles. River terraces and alluvial fans formed across the district by paraglacial processes, which swept away loose glacial debris before soils became stabilised by vegetation. Masses of ice buried within glacigenic sediments during all phases of the MLD glaciation melted out relatively slowly to form kettleholes. Organic sequences preserved within kettleholes, loch basins and bog sites may contain pollen, spores and the remains of beetles (coleoptera) and midges (chironomids), and hence provide a valuable record of environmental change through the late glacial and Holocene. At Whitrig Bog (NT 624 349), west of Smailholm in south-east Scotland, the establishment of a late glacial chironomid stratigraphy has enabled mean July air temperatures to be inferred. The thermal maximum occurred early in the interstadial when the temperature reached about 12°C. There was then a downward trend to about 11°C punctuated by four distinct cold oscillations. At the beginning of the Loch Lomond Stadial the mean July temperature fell to about 7.5°C, but gradually increased to about 9°C before a rapid rise at the onset of the Holocene.
Good sections in late glacial sediments in the Solway district are known from Bigholms Burn (NY 316 812), 6 km south of Langholm, and Redkirk Point (NY 302 650) on the coast of the Solway Firth, 11 km east of Annan. At Bigholms Burn, grey sandy silt with disseminated organic material from the Windermere Interstadial is overlain by stratified gravel with lenses of peat. These deposits, together with the overlying peat of Holocene age, are referred to the Blelham Peat Formation, originally defined in west Cumbria and a division of the Britannia Catchments Group (P912389).
Freshly deglaciated ground was first colonised by a pioneer vegetation of open-habitat, Alpine species, followed by the immigration of crowberry heath, juniper and dwarf varieties of birch and willow. Eventually open birch woodland developed with juniper and isolated stands of Scots pine locally. An oscillatory climatic deterioration occurred throughout the Windermere Interstadial (P912367 and P912368) and it is possible that glaciers had already started to build up in the Galloway and Moffat hills before more sustained cooling began at about 13 ka BP. Giant deer flourished during the Interstadial, but then became extinct. The Gulf Stream current retreated abruptly to the latitude of northern Portugal at about 12.9 ka BP, heralding the start of the Loch Lomond Stadial, known in much of continental Europe as the Younger Dryas stage (named after Dryas octopetala, a late glacial tundra flower). On the basis of evidence provided by well-formed, hummocky moraine fields, small corrie glaciers of Loch Lomond Stadial age are thought to have formed in Galloway (P774195) and the Moffat hills. Across southern Scotland a tundra environment was ubiquitous and only plant communities tolerant of the Arctic conditions were able to colonise the unstable soils. The stadial witnessed the demise of large herbivores, including the woolly mammoth and woolly rhinoceros, but whether or not hunting by the Upper Palaeolithic human population played any part in the extinctions is still uncertain.
Periglacial processes destroyed the immature soils that had developed during the Windermere Interstadial, creating a range of rubbly deposits collectively known as ‘Head’ that are especially well developed in the Cheviot Hills. There was extensive development of permafrost and ground ice, the latter melting to create fossil ice wedge casts and pingos (circular depressions) in lowland areas, particularly within spreads of glaciofluvial sand and gravel (e.g. at Lochmaben and Stranraer) (P774196). Both fluvial and debris-flow activity were enhanced, especially during springtime snowmelts, and slopes were particularly prone to failure. The sand and gravel that underlies the loamy floodplain alluvium in many of the larger valleys was probably deposited from braided rivers during the stadial. Fossil periglacial landscape features are widespread, particularly on slopes where intermittent viscous flow of seasonally thawed soils (gelifluction) has occurred. On steep, upland slopes, tear-shaped gelifluction lobes have been washed out to form arcuate lobes of boulders. The lower slopes of valleys and headwater basins commonly take the form of smooth, gently sloping gelifluction terraces that terminate in bluffs along the streams. The periglacial climate was particularly severe in upland areas, causing many rock falls and producing abundant frost-shattered rock debris. Many talus fans formed on hillslopes during the stadial though most have since become inactive and vegetated.
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