Devensian Ice Sheet Research Articles

A complex slope failure on Beinn nan Cnaimhseag, Assynt, Sutherland

A rare, multi‐phased form of slope failure is described from the Assynt area of NW Scotland. Although possessing some of the characteristics of a rock glacier, detailed analysis of the clastic deposits involved in the slope failure suggest that it is best interpreted as a complex landslide. After the downwastage of the Late Devensian ice sheet, a rockfall talus sheet developed on the southern slopes of Beinn nan Cnaimhseag. The removal of confining pressure imposed by the ice sheet, together with the presence of mylonised facies and fissuring due to tectonic movement along the Ben More thrust plane which cuts across the top of this hill, led to rock sag and collapse of the central part of the slope. Holocene rockfall talus cones have subsequently formed, and partly bury the upper slopes of the blocky apron of debris created by the landslide. Large boulders found on the upper surface of the blocky debris are due to later toppling processes from the free face below the summit of the hill.

Devensian organic interstadial deposits and ice sheet extent in Buchan, Scotland

Pre-Late Devensian organic deposits in the Buchan area of northeast Scotland were investigated for their geomorphological and palaeoecological (pollen, plant macrofossils, coleoptera) properties. Close ecological agreement exists between fossil indicators and allows the inference that the environment in the vicinity of the deposits was a dwarf shrub tundra of the type met today in high latitude areas of Scandinavia and arctic Russia. The latest in a series of radiocarbon dates from the site produced determinations beyond the limits of the method, although the geomorphological and fossil evidence appears to point to an interstadial date within Oxygen Isotope Stages 5a or 5c. The site has special significance for arguments concerning the much-debated concept of ‘Moraineless Buchan’; indeed, evidence is presented which supports the concept of extensive ice sheet glaciation during the Late Devensian for this crucial geographical area. If Buchan is to be seen as a further casualty amongst other disputed ice-free enclaves, then a return to earlier models of extensive ice sheet glaciation in the Late Devensian of Scotland would seem to be necessary. © 1998 John Wiley & Sons, Ltd.

Late Devensian ice sheet characteristics: a palaeohydraulic approach

Glacial meltwater channels are incised into bedrock and diamicton along much of the length of the Mid-Cheshire Ridge. Detailed mapping of one such system near the town of Helsby reveals a dendritic channel network developed in the opposite direction to the regional ice flow during the last (Late Devensian) glaciation. The channels formed subglacially, under atmospheric and not hydrostatic pressure, presumably as the ice sheet downwasted during deglaciation. Morphological and palaeohydraulic evidence suggests that not all of the network was necessarily active contemporaneously. Former water levels in the channels can be estimated due to the presence of bar surfaces, giving a calculated palaeodischarge of at least 111 m3 s−1. The ablation rates required to account for this large discharge are an order of magnitude greater than those obtained from theoretical calculations and those observed in modern glacial environments. This implies that some form of high-magnitude discharge, such as a seasonal flood event, must have taken place in this area during deglaciation. This picture of the Late Devensian ice sheet suggests that during recession the ice sheet was static, crevassed and relatively thin (<50 m). This study also shows that there is no simple relationship between meltwater channel direction and ice dynamics, and that care is required when using the former to make inferences about the latter. Copyright © 1998 John Wiley & Sons, Ltd.

Subglacial meltwater channels at Thurstaston Hill, Wirral and their significance for Late Devensian ice sheet dynamics

Small-scale bedrock features of glacial meltwater erosion such as Nye channels are rare in Britain and these features do not appear to have been previously described from England. In this paper we describe the range of glacial meltwater features developed at Thurstaston Hill, Wirral. The forms at this site are carved in Triassic sandstone and include a spectacular three-dimensional linked plunge pool-Nye channel cavity system. Across the site as a whole, Nye channels are the most common landform. The channels are mostly linear and incised in bedrock to a depth of up to 1 m. These features show two prominent orientations, one parallel to regional ice flow during the last (Late Devensian) glaciation and another transverse to ice flow. There are three important implications for the Late Devensian glaciation of the area: (i) the ice sheet was relatively thin and heavily crevassed at the time when the system was created; (ii) deglaciation was characterized by active ice with no evidence for ice stagnation; and (iii) deglaciation was achieved under warm-based conditions.

Maximum altitude of the Late Devensian ice sheet on the Isle of Rum

[Dahl et al. (1996)][1] have recently published a reconstruction of the upper surface of the last (Late Devensian) ice sheet on Skye. This shows that at its maximum thickness the ice surface descended from 750–870 m in the Cuillins to c. 450 m in north and NW Skye, and raises the possibility that the highest summits on the neighbouring island of Rum also escaped glaciation by the Late Devensian ice sheet, which is thought to have built up after c. 25 ka BP but to have retreated to the mountains of the Western Highlands by c. 13 ka BP ([Sutherland 1991][2]). We here present evidence for the upper limit of Late Devensian glaciation on Rum, and assess its wider significance. Evidence provided by striae, ice-moulded rock and erratics demonstrates that at the last glacial maximum ice from the Scottish mainland crossed Rum in a westerly direction, but was deflected northwards and southwards around mountains in the south of the island ([Harker 1908][3]; [Peacock 1976][4]; [Le Coeur 1994][5]; Fig. 1). [Harker (1908)][3] noted that erratics occur up to 550 m on Barkeval, and believed that ice had covered the highest summits, a view echoed by [Ryder (1968)][6] and [Ryder and McCann (1971)][7]. However, [Godard (1965)][8] depicted nunataks on Rum above c. 570 m, an interpretation favoured by [Le Coeur (1994)][5] on the grounds that ice-moulded bedrock occurs only below this altitude, whereas rock pinnacles, protruding dykes and ‘couloirs de gelivation’ (frost-weathered gullies) occupy higher ground. [Wilson (1969)][9] demonstrated the presence . . . [1]: #ref-9 [2]: #ref-24 [3]: #ref-13 [4]: #ref-20 [5]: #ref-17 [6]: #ref-21 [7]: #ref-22 [8]: #ref-11 [9]: #ref-25

Periglacial trimlines, former nunataks and the altitude of the last ice sheet in Wester Ross, northwest Scotland

High-level weathering limits separating ice-scoured topography from frost-weathered detritus were identified on 28 mountains in Wester Ross at altitudes of 700–960 m, and a further 22 peaks support evidence of ice scouring to summit level. Weathering limits are defined most clearly on sandstone and gneiss, which have resisted frost shattering during the Late Devensian Lateglacial, but can also be distinguished on schists and quartzite. Schmidt hammer measurements and analyses of clay mineral assemblages indicate significantly more advanced rock and soil weathering above the weathering limits. The persistence of gibbsite above weathering limits indicates that they represent the upper limit of Late Devensian glacial erosion. The regular decline of weathering-limit altitudes along former flowlines eliminates the possibility that the weathering limits represent former thermal boundaries between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines that define the maximum surface altitude of the last ice sheet. Calculated basal shear stresses of 50–95 kPa are consistent with this interpretation. Reconstruction of ice-sheet configuration indicates that the former ice-shed lay above 900 m along the present watershed, and that the ice surface descended northwestwards, with broad depressions along major troughs and localised domes around independent centres of ice dispersal. Extrapolation of the ice surface gradient and altitude suggests that the ice sheet did not overrun the Outer Hebrides, but was confluent with the independent Outer Hebrides ice-cap in the North Minch basin. Erratics located up to 140 m above the reconstructed ice surface are inferred to have been emplaced by a pre-Late Devensian ice sheet (or ice sheets) of unknown age. © 1997 John Wiley & Sons, Ltd.

The last glaciation of earth: Size and volume of ice-sheets

The volume and size of the late glacial ice-sheets in the Northern Hemisphere has been estimated, based on geomorphic and chronostratigraphic data. Limited glacial extents during the late glacial maximum characterized both Arctic Canada and northern Eurasia, in contrast to the presence of the large Laurentide, Scandinavian and Devensian ice-sheets to the south. Glacial maxima were not synchronous throughout the Northern Hemisphere: for example, the Novaya Zemlya ice-sheet reached its maximum extent ca. 39,000 BP, approximately 20,000 years earlier than the maximum extent of the contiguous Scandinavian ice-sheet. Western and central Siberia, and the Chukhui Peninsula, were marked by limited glaciation during the last glacial maximum, although pre-Sartanian events were more extensive. Mountain glaciation in northeast Asia was similarly restricted during the last glacial maximum. Spatial and dynamic characteristics of the last glaciation developed in response to climatic factors. The establishment and strength of high-pressure systems over Siberia limited precipitation and glacier formation throughout northeastern Eurasia.

Glacial striae and former ice movement: the evidence from Assynt, Sutherland

Synopsis The mapping of an extensive pattern of glacial striae on quartzite bedrock has enabled the reconstruction of basal flow patterns beneath the Late Devensian ice sheet. There is evidence for an earlier phase of ice movement in certain limited localities, believed to relate to ice build-up. Ice flow beneath a thick ice sheet that covered all but the highest parts of the terrain was followed by a phase in which basal flow was strongly influenced by topography. The ice sheet appears to have been warm-based throughout much of its existence.

Late Devensian meltwater movement and storage within the Ochil Hills, central Scotland

Synopsis This study examines the origin and significance of spectacular bedrock channel forms and sedimentary sequences in the Ochil Hills, central Scotland. Channels and terraces form a discontinuous system extending 4 km from the main Forth valley into central Glen Devon via Glen Quey. It is suggested that as the Ochil Hills emerged above the Late Devensian ice sheet, large amounts of glacial meltwater flowed into the ice free area mainly from the margins of the Forth valley glacier. Meltwater entered Glen Devon via a series of deeply incised meltwater channels passing into a large ice-dammed lake with an overflow into Glen Eagles. Boulder-sized sediment forming a large delta confirms the tremendous erosional and transport capacity of flows entering the former ice-dammed lake. Glaciofluvial and glaciolacustrine sedimentation within the Ochil Hills was very sensitive to changes in ice-margin position and ice thickness due to the configuration of outlet spillways and cols. The Ochil Hills therefore acted as a major storage area for meltwater during deglaciation. The release of such stored meltwater may have taken place suddenly resulting in high magnitude floods or jökulhlaups whose erosional and depositional effects may have extended beyond the immediate area.

Ice-proximal glaciomarine sedimentation and sea-level change in the inverness area, Scotland: A review of the deglaciation of a major ice stream of the British Late Devensian ice sheet

Evidence of both rising and falling relative sea levels and glacitectonic movements is preserved in two formations of raised glaciomarine deposits that were laid down in front of an oscillating ‘grounded’ tidewater glacier in the Inverness Firth. These changes occurred during the latter stages of the disintegration of the Moray Firth ice-stream, one of the major ice streams that drained the British main Late Devensian ice sheet. Most of the glaciomarine deposits antedate a sequence of glacio-isostatically tilted Late Devensian marine shorelines and associated littoral and estuarine deposits. The shorelines began forming at about 13,000 BP and record a progressive fall in relative sea level. A new model for the deglaciation of the Moray Firth region is proposed after a critical appraisal of published accounts of both onshore and offshore Quaternary sequences. The disintegration of the Moray Firth ice stream involved several rapid phases of retreat to pinning points, caused by iceberg calving and triggered by rising global sea level. Each retreat was followed by minor readvances or stillstands, possibly caused by short-lived accelerated periods of glacio-isostatic rebound and concomitant temporary falls in relative sea level. Two such events occurred in the Inverness Firth: the Ardersier Oscillation and the Alturlie Stillstand. Substantial differences (lower relative sea levels, later deglaciation) are apparent between the pattern of ice-retreat in the Moray Firth region and published accounts of the deglaciation of the Irish Sea basin. These differences require a reassessment of some current hypotheses concerning the disintegration of major ice streams associated with high relative sea levels. Furthermore, geological and geomorphological evidence suggesting both rising and falling sea levels in the Inverness area, prior to ca. 13,500 BP, is not fully compatible with recently published computer simulations of the dissolution of the British main Late Devensian ice sheet.

Strandflat development and Quaternary shorelines on Tiree and Coll, Scottish Hebrides

AbstractThe islands of Tiree and Coll in the Scottish Inner Hebrides are dominated by staircases of glaciated rock‐platform surfaces in Lewisian gneiss that are interpreted here as areas of strandflat. The rock platform surfaces exhibit differential glacio‐isostatic uplift from east to west and are considered to indicate at least four separate periods of strandflat formation during prolonged intervals of Quaternary cold climate. It is suggested that many of the well‐known areas of high rock platform located farther east in the Scottish Inner Hebrides represent eroded remnants of the strandflat surfaces described here. The existence of the Tiree and Coll strandflat surfaces in conjunction with their formation during periods of Quaternary glaciation implies that they were produced during periods when mainland Scotland was ice‐covered yet when the western margin of the ice sheet was normally located east of these islands. A prominent Late‐glacial raised shoreline also occurs on both islands and declines in altitude to the west, from 22 m in northeastern Coll to 9 m in western Tiree. It is believed that this shoreline, whose regional gradient is 0.39 m km−1, may represent part of the Main Wester Ross Shoreline that was formed in the Northwest Highlands of Scotland during a period of glacial advance that interrupted the decay of the last (Late Devensian) ice sheet. The general similarity in altitude between the Late‐glacial shoreline features and rock platform surfaces implies that during each period of Quaternary strandflat formation, relative sea‐level returned to the same approximate position. This would appear to indicate that the glacio‐isostatic and glacio‐eustatic components affecting the positions of relative sea level in Scotland have remained in approximate equilibrium throughout the duration of the Quaternary.

Drumlins in ice sheet reconstructions, with reference to the western Pennines, northern England

Drumlin morphology and distribution can be used to determine the palaeoglaciological conditions under which subglacial bedforms were created by Late Pleistocene ice sheets. Detailed field mapping of part of the large drumlin fields in northern England, in particular the identification of areas of superimposed drumlins, has been shown to be of significance in the delimitation of the ice divide within the area. The overall distribution of the subglacial bedforms has also been used to define the extent of ice streams within this part of the Late Devensian (Dimlington Stadial) ice sheet. Based on the physical principles which underlie the deformational model of drumlin formation and present-day glaciological information on subglacial ice sheet conditions, variations in drumlin morphology have allowed a first attempt at reconstructing areas of high (> 100 kPa) and low (< 20 kPa) basal driving stresses for this part of the ice sheet.

A shallow marine emergent sequence from the northwestern sector of the last British ice sheet, Portballintrae, Northern Ireland

At Portballintrae, Northern Ireland, a well-preserved emergent facies sequence formed around 17 kyr B.P. as the Late Devensian ice sheet withdrew from the continental shelf. Three main lithofacies assemblages are recognised: (1) At the base of the sequence a massive muddy diamict was formed by resedimentation of an unstable sedimentary apron deposited in a tidewater setting. (2) Rhythmically-bedded sand and mud units contain a wide range of wave diagnostic features and are separated from the underlying diamict by both transitional and wave-eroded planar contacts. Palaeowave calculations suggest deposition in a coastal tract open to Atlantic waves. The preservation and range of boreo-arctic foraminifera present are also typical of a lower shoreface, storm-influenced setting. (3) Interbedded gravel and sand facies are separated from the underlying rhythmically-bedded facies by a planar wave-cut surface. This facies occurs as crudely organised beds within cross-cutting, shallow scours which are associated with off-shore directed, storm-induced sediment pulses.Overall the coarsening-upwards sequence is explained by tidewater sedimentation during ice wastage, rapid isostatic rebound and isostatic deceleration in the late glacial. An isostatic-tectonic model comprising a forebulge on the outer continental shelf and a peripheral trough on the inner shelf is presented to resolve the apparent discrepancies between onshore and offshore sea-level data.

The stratigraphic and sedimentological significance of Late Devensian Ice Sheet surging in Holderness, Yorkshire, U.K.

Amino acid analyses of marine mollusc valves show that the Basement Till of Holderness, Yorkshire, is of Late Devensian age (ca. 20,000 BP) and not pre-Ipswichian (>125,000 BP) as is traditionally supposed. Together with the overlying Skipsea and Withernsea Tills the Basement Till is argued to be a ‘deformation till’ resulting from the repeated onshore surging of Late Devensian ice over a muddy sea floor, and the subglacial transport, attenuation and mixing of marine sediment. Silts on the surface of the Basement Till, yielding 14C dates of 18,500 and 18,240 BP, have been regarded as minimum ages for Late Devensian glaciation in eastern England (e.g. the Dimlington Stadial of Rose, 1985, 1989). The amino acid data presented here indicates that the maximum of the Late Devensian glaciation in eastern England occurred earlier. Sedimentological interpretation of the Late Devensian tills of Holderness as glacially-reworked marine sediments supports previous glaciological models of the British Ice Sheet involving surging of ice lobes along the coast of eastern England. The Basement, Skipsea and Withernsea tills are separated by shallow marine sediments and may be representative of ‘deformable bed’ till facies deposited below Pleistocene ice sheets moving over soft, fine-grained sediments.

Late Devensian and Flandrian palaeoenvironmental changes on the Scottish continental shelf west of the Outer Hebrides

AbstractExamination of two radiocarbon‐dated vibrocores taken from south of St Kilda at a water depth of about 155 m, a short distance within the maximum position of the Late Devensian (Dimlington Stadial) ice sheet, suggests that the St Kilda Basin became free of glacier ice after 15250 yr BP. Sedimentation in a shallow, low energy, high arctic, muddy environment continued until after 13500 yr BP. There followed a higher energy temperate episode during which water depths were roughly about 40 m: this is correlated with the latter part of the Windermere Interstadial and with the warmer interval known in shallow Scottish seas about or a little before 11 000 yr BP. The Loch Lomond (Younger Dryas) Stadial is marked in the vibrocores by the return of muddy sediments and a cold‐water fauna. Relatively shallow water conditions seem to have persisted into the earliest Flandrian, when the water depth was still roughly 60 m, corresponding to a sea‐level in the area 90–100 m below present. It is suggested that pack ice was widespread in the northeast Atlantic before the Windermere Interstadial and also during the Loch Lomond Stadial, when it transported shards of Icelandic volcanic ash into the St Kilda basin. Estimates of sea‐surface temperature for the last part of the Windermere Interstadial are close to those derived from the deep‐sea record for the same period.

The extension of the Late Weichselian/Late Devensian ice sheets in the North Sea Basin

AbstractThe distribution of large channel‐like features, comprising Weichselian/Devensian incisions, in the western North Sea provides evidence for a much larger extension of the last ice sheet than currently assumed. Morphological comparison of the incisions with those in North Germany and Poland reveals a striking similarity in shape and distribution. The features on the North Sea floor are interpreted as being formed by meltwater erosion within the margin of the ice sheet. The widespread absence of Weichselian/Devensian till in the area under consideration may be attributed to later erosion. Large‐scale reworking and redistribution of sediments is indicated by the complete sediment infill of the majority of the incisions.

Glacial rebound and sea‐level change in the British Isles

ABSTRACTObservations of sea levels around the coastline of the British Isles for the past 10,000–15,000 years exhibit a major regional variation and provide an important data base for testing models of glacial rebound as well as models of the Late Devensian ice sheet. A high‐resolution rebound model has been developed which is consistent with both the spatial and temporal patterns of sea‐level change and which demonstrates that the observations are the result of (i) the glacio‐isostatic crustal rebound in response to the unloading of the ice sheet over Britain and, to a lesser degree, of the ice sheet over Fennoscandia, and (ii) the rise in sea‐level from the melting Late Pleistocene ice sheets, including the response of the crust to the water loading (the hydro‐isostatic effect). The agreement between model and observations is such that there is no need to invoke vertical crustal movements for Great Britain and Ireland of other than glacio‐hydro‐isostatic origin. The rebound contributions are important throughout the region and nowhere is it sufficiently small for the sea‐level change to approximate the eustatic sea‐level rise. The observational data distribution around the periphery as well as from sites near the centre of the former ice sheet is sufficient to permit constraints to be established on both earth model parameters specifying the mantle viscosity and lithospheric thickness and the extent and volume of the ice sheet at the time of the last glaciation. Preliminary solutions are presented which indicate an upper mantle viscosity of (3–5)1020 Pas, a lithospheric thickness of about 100 km or less, and an ice model that was not confluent with the Scandinavian ice sheet during the last glaciation and whose maximum thickness over Scotland is unlikely to have exceeded about 1500 m.

Former ice movement in Assynt, Sutherland, as shown by the distribution of glacial erratics

Synopsis The distribution of glacial erratics shows that at some time in the past the Assynt area has been glaciated by an ice sheet of considerable thickness which, at the glacial maximum, probably covered all but the highest mountains. Whether this was the Late Devensian ice sheet or some earlier glacial phase cannot be determined from the evidence of glacial erratics alone. A model for the development of an ice sheet in this part of Scotland is proposed, whereby initial ice development occurred predominantly on the eastern side of the mountains, with glacial build-up continuing until the ice was thick enough to flow westwards as well as eastwards. The ice divide thus established always lay to the east of the Assynt mountains. The dispersal of erratics appears to have been limited in the ice-shed zone.

A reappraisal of the supposed Ardersier Readvance, inner Moray Firth

Synopsis Previous authors have proposed that the deglaciation of the Late Devensian ice sheet in the inner Moray Firth area was interrupted by the “Ardersier Readvance”. The morphological and lithological evidence indicates that the Ardersier features were not produced by a major readvance of ice but more probably represent glaciomarine deposits formed while ice occupied the Inverness Firth. Similarly the Late Devensian shoreline sequence of the area does not support the readvance hypothesis; instead it suggests that the sediments were deposited as relative sea level fell and the ice mass retreated westward.

Reconstructing past glacier dynamics and erosion from glacial geomorphic evidence: Snowdon, North Wales

AbstractBedrock surfaces exposed around Llyn Llydaw, North Wales demonstrate contrasting styles of erosion beneath a Late Devensian ice sheet and a Loch Lomond Stadial (LLS) valley glacier. Ice sheet erosion involved lee‐side fracturing, surface fracture wear and abrasive wear, while LLS erosion was primarily by abrasive wear. Preservation of ice sheet erosional features indicates limited rates of erosion during the LLS. Analysis of the geometry and distribution of erosional markings suggests that the low erosional capacity of the LLS glacier was due to a low basal sliding velocity. This prevented the formation of lee‐side cavities, reduced the debris flux over the bed and minimised particle‐bed contact loads. Reconstructions of the mass balance and geometry of the LLS glacier indicate that most of its balance velocity could be achieved by internal deformation alone. A combination of low subglacial water pressures and an unusually rough substrate explain the low sliding velocities. High bed roughness is due to the absence of leeside cavities and a change in flow orientation between ice sheet and LLS times, which meant that the LLS glacier was in contact with roughness elements which were generated in cavities beneath the ice sheet.