Devensian glaciations, 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.|
The continental European record indicates that the Ipswichian Interglacial was halted by rapid climatic deterioration at about 116 ka BP. This was followed by a warmer period at about 100ka BP (MIS 5c), which probably correlates with the Chelford Interstadial, when mixed birch, pine and spruce forest developed over the region. Cooling recommenced at about 90 ka BP, followed by another warmer period at about 80 ka BP (MIS 5a), the so-called Brimpton Interstadial. Significant cooling then occurred from about 70 ka BP (MIS 4), when an Early Devensian glaciation may possibly have affected the region. However there is no definitive record and an alternative model indicates that an ice-free, tundra-like environment developed between 90 and 45 ka BP during which there were short-lived, warmer interludes at 76, 57 and 50 ka BP. The main features of the Devensian glacial–interglacial cycles are summarised in P912390.
Most of the glacigenic deposits preserved in the south of Scotland were laid down during the Main Late Devensian (MLD) Glaciation, between about 29 and 14.7 ka BP, when the district was overwhelmed entirely by ice. There is growing evidence from north-west Europe that the Last Glacial Maximum (LGM) occurred relatively early in the Late Devensian, from about 27 ka BP to 22 ka BP, but the British ice-sheet appears to have achieved its maximum southerly position on the east coast of Britain shortly after 21.6 ka BP, between Humberside and The Wash. An instructive coastal section is seen at Dimlington, to the east of Hull in Holderness, and as a result this glacial episode has been referred to as the Dimlington Stadial. It was followed by a period of glacial retreat before significant readvances, particularly involving coastal ice streams. One such event, the so-called Scottish Readvance, affected the Solway area (P912370). It followed a dramatic reorganisation of ice-flow patterns within the Vale of Eden and Solway lowlands, and may have been contemporaneous with a readvance in north-east Ireland at about 17 ka BP. The whole of southern Scotland was probably ice free by 14 ka BP (P912367).
Directions of ice flow have been obtained mainly from the distribution of erratics and the orientation of striae, drumlins and other ice-moulded landforms. However, the ice-flow indicators clearly relate to more than one phase of active glaciation, and generalised directions of ice flow commonly conflict with local data derived from detailed mapping, till fabric analysis, satellite imagery or digital terrain models.
The sequence of events that occurred during the MLD glaciation is not fully understood since there is insufficient geochronological control on the formation of landforms and deposits that clearly result from more than one phase of glaciation, problems compounded by a stratigraphical record beset with difficulties of regional correlation. Local centres of ice accumulation over the Galloway Mountains, the Moffat Hills and the Cheviot Hills formed important elements of the MLD ice-sheet. In these areas, the ice largely remained relatively sluggish, depositing little till and causing only limited glacial erosion, but ice emanating from them coalesced with more active ice flowing south from the Grampian Highlands. Overall, the MLD ice-sheet was dynamic with migrating ice divides, corridors of fast-flowing, topographically constrained ice streams and fluctuating margins that locally surged into proglacial lakes and across the adjacent sea bed (e.g. Irish Sea Basin and Tweed Basin extension). Good evidence for fast flowing palaeo-ice streams is provided by features of the Lochmaben and Tweed basins (P912371).
MLD glaciation of the Southern Uplands
The evidence in the south of Scotland for glacigenic deposits of possible Early or Mid Devensian age is equivocal. Commonly in the Southern Uplands basal tills rest directly on bedrock. Locally, pockets of frost-shattered rock and rubbly, periglacial deposits occur beneath tills of presumed Main Late Devensian age, and may have formed in the predominantly cold, Mid Devensian substage. Rare examples where intercalated peat is preserved, as at Sanquhar (P001449), may enable a maximum age of the overlying till to be estimated, but no radiocarbon age determinations are currently available. Likewise, where sequences of two or more tills are preserved, there is no direct evidence of the age of the lowest units. In general, the earliest tills (of presumed Late Devensian age) were laid down by ice flowing from the Grampian Highlands. At Nith Bridge (NS 594 141), New Cumnock, immediately north of the Southern Upland Fault, three tills are recorded, the lower two of ‘Midland Valley’, northern provenance and the upper sourced from the Southern Uplands to the south. On the Rhins of Galloway at Clanyard Bay (NX 102 380) and Port Logan Bay (NX 098 405), a blue-grey lodgement till contains bivalves, foraminifera and crustacea, derived from the bed of the Firth of Clyde. It is overlain by a sandy diamicton with a high proportion of locally derived sandstone clasts that possibly formed as ice melted in situ.
Basal tills locally present in the Gretna district of the Solway estuary are probably no older than Devensian, and are currently assigned to the Caledonia Glacigenic Group. At Chapelknowe (NY 3110 7181), the lower of two tills (the Chapelknowe Till Formation (Irish Sea Coast Glacigenic Subgroup): see below, P912372 and Plate 55) could be the product of an early phase of the Main Late Devensian glaciation or of an earlier event during the Early or Mid Devensian. In the higher parts of the Southern Uplands, tills of the Southern Uplands Glacigenic Subgroup are thin and patchy, especially across interfluves. Locally thicker accumulations occur in the lee of bedrock highs and within steep-sided valleys. In the Langholm area and parts of the valleys of the Nith and Annan, the till of the MLD glaciation is referred to the Langholm Till Formation (P912372), a stiff, pale yellowish brown to pale grey, stony, sandy silty clay diamicton containing subangular to subrounded clasts of wacke sandstone and siltstone. Near Hoghill Farm (NY 3820 8905), north of Langholm, the base of the Lang-holm Till Formation is represented by 2.5 m of dense clast-supported diamicton with angular to subangular wacke and siltstone clasts held in pale, yellowish brown, silty sand. This unit, the Hoghill Gravel Bed is interpreted as a gelifractate (scree), and rests on reddish brown diamicton that possibly correlates with the Chapelknowe Till Formation. A granite-rich variant of the Langholm Till Formation, the New Abbey Till Member, is defined for tills deposited in the vicinity of the Criffel Pluton. Tills of the western and eastern Southern Uplands remain unnamed, their compositions in large part reflecting local bedrock lithologies.
Scottish Readvance in the Solway lowlands
Two major glacial readvances of Scottish ice occurred across the Solway lowlands and the coast of west Cumbria during the later stages of the last glaciation, with the second event referred to as the Scottish Readvance (P912367 and P912370). In the Solway area evidence for this readvance and deglaciation is provided by landforms and by the preservation of ‘tripartite sequences’ comprising ‘lower’ and ‘upper’ tills separated by a ‘middle’ unit of sands, silts and clays. The sedimentary sequences (shown schematically in P912372) are assigned to the Irish Sea Coast Glacigenic Subgroup and are best preserved in south-trending valleys, cut in Permian and Triassic strata, which lie at right angles to the general direction of ice flow, as demonstrated at Chapelknowe (Plate 55) and Plumpe Farm, Gretna (P543597).
At the Chapelknowe section (Plate 55) the uppermost unit is 2.5 m of very stiff, reddish brown, massive to crudely stratified, matrix-supported, sandy silty clay diamicton with some large boulders and slabs of red sandstone (Gretna Till Formation). The till grades down over 10 cm into very dense, clast-supported gravel with a matrix of reddish brown, clayey fine to coarse-grained sand and secondary infillings of red clay (Loganhouse Gravel Member of the Plumpe Sand and Gravel Formation). The gravel rests unconformably on at least 2 m of extremely stiff, reddish brown and orangey yellow mottled, matrix-supported, sandy silty clay diamicton of the Chapelknowe Till Formation. Constituent clasts include wacke sandstone, siltstone, red sandstone, granodiorite and amygdaloidal basalt. Many of the clasts of siltstone and basalt in the lower till are weathered.
The section at Plumpe Farm (NY 3344 6813), Gretna (P543597) shows the upper part of the tripartite sequence and also provides evidence for its wet-based, subglacial deformation. The upper unit comprises 1.5 m of very compact, reddish brown, massive, matrix-supported, clayey, fine sandy diamicton (Gretna Till Formation). At its base, there is a 0.5 m thick unit of very compact, interlaminated sand, silt, clay and diamicton. This subglacially sheared unit rests on over 2.5 m of reddish brown, silty fine-grained sand that coarsens downwards (Plumpe Farm Sand Member of Plumpe Sand and Gravel Formation).
In broad terms, the ‘tripartite sequences’ can be explained by a model involving a dynamic ice-sheet with shifting ice divides. Early flow of Scottish ice was up the Vale of Eden and subsequently eastwards across the Solway lowlands and through the Tyne Gap of Northumberland. This was followed by ice flow westwards from the Vale of Eden towards the Irish Sea, after partial deglaciation and a substantial glacial reorganisation. Next came the Scottish Readvance, with a minor subsequent readvance or stillstand in the vicinity of Powfoot (P912370).
Fine-grained glaciolacustrine deposits of the Cullivait Silt Formation accumulated in ‘Glacial Lake Carlisle’, which occupied the Carlisle area during the Scottish Readvance when ice blocked drainage within the Solway lowlands. Glaciofluvial sands and gravels deposited in and around the lake to the north have been assigned to the Plumpe Sand and Gravel Formation. Both these and the glaciolacustrine sediments of the Cullivait Silt Formation have been locally disturbed glacitectonically (P543597) and are partially capped by the Gretna Till Formation, a red diamicton dominated by Scottish clasts (P912372).
Deglaciation of south-west Scotland
The products of deglaciation following the LGM comprise predominantly moundy, glaciofluvial sand and gravel, together with glaciolacustrine silts and clays. In the Stranraer district Scotland extensive ice-contact kamiform deposits are fringed to the south by outwash sandur deposits which locally were laid down as prograding delta foresets. South of Stranraer, at the former Clashmahew tile-works (NX 062 594) the sands and gravels are overlain by shelly marine laminated clay with dropstones. There is insufficient evidence to confirm whether these and similar clays at the former Terally Brickworks (NX 121 406), north of Drummore, correlate with the Errol Clay Formation of eastern Scotland (pre-13 ka BP) or the younger Clyde Clay Formation (about 13–11.5 ka BP) of the Firth of Clyde.
The most extensive morainic, glaciofluvial, or glaciolacustrine deposits occur in the valleys of the Rivers Nith, Annan and Esk (P912310). There, the glaciofluvial sands and gravels are referred, on clast content, to the Kilblane Sand and Gravel Formation (Irish Sea Coast Glacigenic Subgroup) with clasts predominantly of Permo-Triassic sandstones, and the Kirkbean Sand and Gravel Formation (Southern Uplands Glacigenic Subgroup), which contains clasts predominantly of Lower Palaeozoic sandstone. Locally, esker gravels developed in subglacial channels close to an ice margin. Glaciolacustrine, interlaminated red brown silts and clays of the Cullivait Silt Formation are present around Dumfries. Many morainic ridges, closely associated with ice-marginal glacial drainage channels, are composed of mixed diamicton, boulder gravel, silt and clay. The deposits are referred to the Kerr Moraine, Mouldy Hills Gravel and Dalswinton Moraine formations (P912389 and P912372).
Cheviot Hills and Berwickshire
At the LGM, an independent ice cap was positioned over the Cheviot Hills, where it was almost encircled by a combination of ice flowing from the south-west and a substantial ice stream flowing from the north-west through the Tweed Basin. The Tweed Valley palaeo-ice stream, together with ice from central Scotland, became deflected south-eastwards, parallel to the coast of north-east England. The strongly drumlinised land surface of the lower Tweed basin is interpreted as the product of a fast flowing palaeo-ice stream (P912371). It is mainly underlain by the Norham Till Formation (Borders Glacigenic Subgroup: P912369) which forms a gently undulating, 5 to 10 m thick sheet covering predominantly Upper Palaeozoic strata of the Tweed Basin over much of lowland Berwickshire. Blocky periglacial deposits and decomposed bedrock are particularly common within the Cheviot Hills, where there has been relatively little glacial erosion. Till of the Kale Water Till Formation (Cheviot Glacigenic Subgroup: P912369), typically preserved in valley floors, is characterised by a high proportion of volcanic clasts derived from the Cheviot massif.
Ice-contact proglacial sands and gravels were laid down in parts of Berwickshire during deglaciation following the LGM. Situated on the southern flanks of the Lammermuir Hills, north of Greenlaw Moor, the remarkable Bedshiel ‘Kaims’ is an upland esker at 210 m above sea level. This 3 km long, isolated ridge rises up to 15 m above the surrounding moorland (P912373) and records the early stages of englacial and subglacial meltwater drainage of the Tweed Valley ice stream. Gravels of this esker are referred to the Greenlaw Gravel Formation (Borders Glacigenic Subgroup). Later stages of deglaciation are recorded by moundy, glaciofluvial sand and gravel, together with glaciolacustrine silts and clays within river valleys draining to the east. The most extensive of these deposits lie in the wide valley floor of the River Tweed. Gravels of the highest river terraces were probably laid down by meltwater, whereas lower terraces and alluvium are the product of subsequent Holocene fluvial processes.
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