The last ice sheet in Snowdonia

In Snowdonia there is a marked contrast between weathered summits, often with well‐developed blockfields or tors, and lower ice‐moulded terrain. The boundary is interpreted as a trimline marking the upper surface of the last ice sheet. This interpretation is supported by the presence of gibbsite, an end‐product of prolonged weathering, at the base of soils above but not below the trimline. The reconstructed ice surface reaches about 850 m above present sea‐level along an ice divide running NE–SW through the massif. There is no evidence to support the popular view that ice centred further south extended over Snowdonia, and breaching to form the major glacial troughs can be explained by the action of local ice. The field evidence presented here demonstrates that most models of the southern British and Irish Sea ice sheets are significantly flawed, the earliest being far too thick and the most recent far too thin. Copyright © 2000 John Wiley & Sons, Ltd.     

Gibbsitic soils on former nunataks: implications for ice sheet reconstruction

X-ray diffraction analyses of soils above and below a periglacial trimline developed across the basalts of the Trotternish Escarpment (Isle of Skye, Scotland) demonstrate that gibbsite is restricted to soils above the trimline. This suggests that the gibbsite is a relict of pre-Late Devensian weathering, and that the trimline did not develop after the last ice sheet achieved its maximum thickness. The sharpness of the boundary between frost-weathered regolith and gibbsitic soils upslope and ice-scoured bedrock associated with gibbsite-free soils downslope suggests that the trimline represents the altitude of the last ice sheet at its maximum thickness rather than a former boundary between passive cold-based ice and erosive warm-based ice. These findings illustrate how identification of high-level periglacial trimlines and associated contrasts in clay mineralogy provide a means for constraining reconstructions of the form of the last ice sheets.     

Periglacial trimlines and nunataks of the Last Glacial Maximum: the Gap of Dunloe,southwest Ireland

During the last main phase of glaciation (26–13 ka) an ice‐cap developed in southwest Ireland and ice, from a dispersal centre in the vicinity of Kenmare, flowed north through the Gap of Dunloe in the Macgillycuddy's Reeks. On surrounding hillsides a weathering limit separates ice‐moulded bedrock, on low ground, from frost‐weathered terrain above. Assessment of bedrock dilation joint characteristics, Schmidt hammer R‐value data and clay‐sized mineral contents of basal soil samples, demonstrate significant contrasts in the degree of weathering above and below this limit. The weathering limit declines in altitude along former ice flow‐lines and is confluent with morainic deposits on the eastern side of the Gap. This supports the assertion that the high‐level weathering limit is a periglacial trimline that marks the former maximum upper limit of the body of ice which occupied the Gap of Dunloe during the Last Glacial Maximum (LGM). Reconstruction of the former ice‐surface profile from periglacial trimline limits on the eastern side of the Gap yields a mean estimate for basal shear stress of 106.5 kPa. This value suggests that the ice mass which occupied the Gap of Dunloe at the LGM was warm based and flowed on a bedrock substrate. Copyright © 2004 John Wiley & Sons, Ltd.     

The vertical dimensions of Late Devensian glaciation on the mountains of Harris and southeast Lewis,Outer Hebrides,Scotland

On North Harris and southeast Lewis a weathering limit separates glacially-moulded bedrock on low ground from frost-shattered bedrock and blockfields on high plateaux. Analysis of the depths of horizontal stress-release joints demonstrates significant contrasts in bedrock weathering above and below this boundary, and the survival of gibbsite only in soils above the weathering limit indicates that it represents the upper limit of Late Devensian glacial erosion. The weathering limit declines regularly in altitude on either side of the former ice shed, and is therefore interpreted as a periglacial trimline defining the upper limit of a locally-nourished ice mass at its maximum extent, rather than a former thermal boundary between protective cold-based and erosive warm-based ice. Calculated basal shear stress values are consistent with this interpretation. The configuration of the trimline indicates that at the last glacial maximum the area supported an ice cap that achieved a maximum altitude of ca. 700 m above present sea level and declined in altitude to the west-northwest and east-southeast at an average gradient of ca. 20 m km^?1. Extrapolation of the dimensions of this ice cap suggests that it terminated ca. 7–10 km west of the present coast of Harris, and was confluent with mainland ice a short distance east of the present coastline.     

Nunataks of the last ice sheet in northwest Scotland

High-level weathering limits separating ice-scoured topography from an upper zone of frost-weathered detritus were identified on 17 mountains in NW Scotland at altitudes of <600 m to< 900 m, and a further 6 peaks were found to support evidence of ice scouring to summit level. Weathering limits are most clearly defined on Torridon Sandstone, which is resistant to frost shattering, but can also be mapped on Cambrian Quartzite, Lewisian Gneiss and Moine Schist. Contrasts in degree of rock surface weathering above and below the weathering limits were evaluated using measurements of joint depth and rock surface hardness, and through X-ray diffraction analyses of clay mineral assemblages. The results indicate significantly more advanced rock and soil weathering above the weathering limits. Widespread persistence of gibbsite above the weathering limits suggests that they represent the upper limit of Late Devensian glacial erosion, and the regularity of the decline in weathering limit altitude along former flowlines eliminates the possibility that it represents a former thermal boundary between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines defining the maximum surface altitude of the last ice sheet around former nunataks. Calculated basal shear stresses of 50–78 kPa are consistent with this interpretation. The altitude of the trimlines implies that the former ice shed lay at 900–930 m in the Fannich Mountains and descended gently northwards, and that the ice surface descended NW from the ice shed to >500 m over the extreme NW tip of Scotland and to 700–730 m at the head of Little Loch Broom.     

Cosmogenic 10Be insights into the extent and chronology of the last deglaciation in Wester Ross,northwest Scotland

Cosmogenic ¹⁰Be surface exposure ages for bedrock sites around Torridon and the Applecross Peninsula in Wester Ross, northwest Scotland, provide new insights into the Lateglacial transition. Accounting for postglacial weathering, six statistically comparable exposure ages give a late Younger Dryas (G‐1) exposure age of 11.8 ± 1.1 ka. Two further outliers are tentative pre‐Younger Dryas exposure ages of 13.4 ± 0.5 ka in Torridon, and 17.5 ± 1.2 ka in Applecross. The Younger Dryas exposure ages have compelling implications for the deglaciation of marginal Loch Lomond Stadial ice fields in Torridon and Applecross. Firstly, they conflict with predictions of restricted ice cover and rapid retreat based on modelling experiments and climate proxies, instead fitting a model of vertically extensive and prolonged ice coverage in Wester Ross. Secondly, they indicate that >2 m of erosion took place in the upper valleys of Torridon and Applecross during the Younger Dryas, implying a dominantly warm‐based glacial regime. Finally, the exposure ages have clarified that corrie (cirque) glaciers did not readvance in Wester Ross, following final deglaciation. Copyright © 2011 John Wiley & Sons, Ltd.     

Geomorphological signature and flow dynamics of The Minch palaeo‐ice stream,northwest Scotland

Large‐scale streamlined glacial landforms are identified in 11 areas of northwest Scotland, from the Isle of Skye in the south to the Butt of Lewis in the north. These ice‐directional features occur in bedrock and superficial deposits, generally below 350 m above sea level, and where best developed have elongation ratios of >20:1. Sidescan sonar and multibeam echo‐sounding data from The Minch show elongate streamlined ridges and grooves on the seabed, with elongation ratios of up to 70:1. These bedforms are interpreted as mega‐scale glacial lineations. All the features identified formed beneath The Minch palaeo‐ice stream which was ca. 200 km long, up to 50 km wide and drained ca. 15 000 km² of the northwest sector of the last British‐Irish Ice Sheet (Late Devensian Glaciation). Nine ice‐stream tributaries and palaeo‐onset zones are also identified, on the basis of geomorphological evidence. The spatial distribution and pattern of streamlined bedforms around The Minch has enabled the catchment, flow paths and basal shear stresses of the palaeo‐ice stream and its tributaries to be tentatively reconstructed. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

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.     

Evolution of a periglacial landscape in the Late Devensian: environments and palaeobotany of the Mepal area,Cambridgeshire, England

The landscape evolution of the Mepal area from Late Devensian Block Fen Terrace times to the beginning of the Flandrian, a period of ca. 8000 radiocarbon years, is reconstructed. Stratigraphy is based on borehole transects and single boreholes, centred on a depression between the Block Fen Terrace and the Isle of Ely. Within the depression is a Devensian late‐glacial sequence, with the Windermere Interstadial represented by radiocarbon‐dated organic sediments. Pollen and plant macroscopic remains of the late‐glacial sediments are analysed. Plant communities with Betula developed in the interstadial. Before and after the interstadial there is much reworked pollen in the inorganic sediments, derived from local pre‐Devensian Pleistocene sediments, including temperate Ipswichian Stage sediments, and from mass‐wasting of the local Jurassic bedrock. Periods of such mass‐movement occur before and after the deposition of the late‐glacial lake sediments. Deposition of aeolian sediment occurs later than the main period of mass movement, but before the Windermere Interstadial. The relationship of the aeolian sediments in time and space to permafrost, indicated by local contraction polygons and cracks, is discussed. Solifluction diverted the flow of the River Great Ouse from a northeast direction in Block Fen Terrace times to a southwest direction as a channel developed to the west of the Chatteris–March ‘island’. This led to a drainage divide in Flandrian times. Copyright © 2002 John Wiley & Sons, Ltd.     

Late Devensian and Holocene relative sealevel changes at Loch nan Eala,near Arisaig,northwest Scotland

Pollen, diatom, radiocarbon and lithostratigraphical data from isolation basins in northwest Scotland are used to quantify the reference water (tide) level, indicative range and age of differenty types of isolation and connection contacts. Tendencies of sea-level movement and relative sea-level changes from the mid-Lateglacial Interstadial (11.8 ka BP) to the late Holocene are constructed from these data. Relative sea-level fell continuously from + 17.8 m OD at 11.8 ka BP to ca. + 5.2 m OD at 10.1 ka BP. From an unkown minimum between 10 ka and 9 ka BP relative sea-level then rose to +6.3 m OD at 8.3 ka BP. The maximum Holocene sea-level occurred within the range +6.6 m OD to +9.3 m OD between 6.6 ka BP and 4.0 ka BP before falling to present. Isobase, age-altitude and quantitative rebound models for northwest Scotland are tested using these sealevel data, but none of the published models shows close agreement with the new results.     

A reinterpretation of the Lateglacial environmental history of the Isle of Skye,Inner Hebrides,Scotland

This paper presents a major revision of the Late Devensian Lateglacial environmental history of the Isle of Skye, Scotland, based upon a combination of geomorphological, biostratigraphical and radiocarbon evidence. The distribution of glacial and periglacial landforms, and of raised shorelines, suggests that there was only one extensive readvance of local glaciers in southern Skye following the wastage of the Late Devensian ice sheet. Pollen-stratigraphic evidence from 10 sites inside and 4 sites outside the mapped ice limits indicates that this readvance occurred during the Loch Lomond Stadial. At that time over 180km² of the uplands of south-central Skye were covered by glacier ice, a much more extensive glaciation than previously envisaged. Palynological evidence from four Lateglacial profiles implies that degree of exposure to strong westerly winds was the principal factor determing vegetational contrasts on the island, and that regional differences in vegetational type were less pronounced than has hitherto been suggested. The glacial and palaeobotanical reconstructions reported here are more compatible with Lateglacial data from the Scottish mainland and Hebridean islands than were the previously-published accounts for the Isle of Skye.     

High-resolution reconstruction of the last ice sheet in NW Scotland

Reconstructions of the last (late Devensian) British ice sheet have hitherto been based on assumptions regarding its extent and form. Here we employ observational evidence for the maximum altitude of glacial erosion (trimlines) on mountains that protruded through the ice (palaeonunataks) to reconstruct the form of the ice sheet over ≈ 10 000 km² of NW Scotland. Contrasts in the clay mineralogy of soils and exposure ages of rock surfaces above and below these trimlines confirm that they represent the upper limit of late Devensian glacial erosion. The reconstruction yields realistic values of basal shear stress and is consistent with independent evidence of ice movement directions. The ice sheet reached ≈ 950 m altitude over the present N–S watershed, descended northwards and north-westwards, was deflected around an ice dome on Skye and an independent Outer Hebrides ice cap, and probably extended across the adjacent shelf on a bed of deforming sediments.     

Glacial meltwater erosion of the Mid-Cheshire Ridge: implications for ice dynamics during the Late Devensian glaciation of northwest England

Mapping of glacial meltwater channels along the length of the 25-km Mid-Cheshire Ridge reveals evidence for four distinctive channel morphologies, which are used to establish the pattern of meltwater flow during the Late Devensian glaciation. A key characteristic of all channels is an abrupt change in morphology between inception on the Mid-Cheshire Ridge and the downstream continuation on the surrounding Cheshire Plain, with large reductions in channel cross-sectional area at this point. The interpretation of this evidence is that meltwater flowing off the bedrock ridge was absorbed into a layer of permeable sediment beneath the Late Devensian ice sheet. This permeable sediment is significant because it would have acted as a deforming layer beneath the former ice sheet in this area. Reconstruction of the Late Devensian ice sheet based on information from the meltwater channels and using values of shear stresses typical of ice sheets resting on deformable beds (ca. 20 kPa) suggests an ice surface elevation over the Irish Sea of ca. 700 m. This value is considerably less than previous estimates of the vertical extent of the ice sheet of ca. 1000–1200 m and has important implications for the rapidity and mode of deglaciation during the Late Devensian. Copyright © 1999 John Wiley & Sons, Ltd.     

黑龙江省东部山地河谷地区冰缘现象及其形成时代

本文根据堆积物具有的多方面共同特点,确认黑龙江省东部河谷地区(河床、河漫滩和一级阶地)堆积物为流水成因。又根据岩石地层、年代地层、磁性地层及孢粉资料综合分析与评价该地区发育的冰缘现象的形成时代,特别是对嘉荫县团结沟大青剖面同时发育的四层冰缘层内卷划分为四个形成期:第一形成期称中团结沟冰缘阶,时代47400±2800—22000±250年;第二形成期称晚团结沟早冰缘阶,时代14800±740—13000年;第三形成期称晚团结沟晚冰缘阶,时代11370±100—10000年;第四形成期称大青冰缘期,时代约6500年。     

The high-level marine shell beds of Scotland and the build-up of the last Scottish ice sheet

It is argued that high-level shell beds buried by till at various localities around the Scottish coast are in situ and represent a marine transgression immediately prior to and consequent upon loading of the earth's crust by the build-up of the last Scottish ice sheet. The high-level rock platforms of the Hebrides may also have been eroded at this time. A relationship between the build-up of the last Scottish ice sheet and world sea-level is suggested and it is further argued that the Scottish ice sheet was a more sensitive indicator of the onset (and termination) of a period of northern hemispheric glaciation than either the Laurentide or Scandinavian ice sheets. It is suggested that the build-up of the last Scottish ice sheet took place in the Early Devensian and a tentative correlation is proposed between the Scottish evidence and the deep-sea evidence for glacier build-up at ca. 75,000 years B.P     

Late devensian ice sheet in the western Grampians,Scotland

A radial pattern of ice flow of the last ice sheet in the largest source area of ice (c. 5000 km²) in the British Isles in western Scotland is demonstrated by the dispersal of indicator erratics and by patterns of striae, friction cracks and ice-moulded landforms. Three major ice domes and the principal ice divide are identified in the western Grampians. The ice domes coincided with the highest mountain blocks while it is inferred that the alignment of many of the pre-existing valleys controlled much of the outflow of ice, forming ice streams within the ice sheet. The importance of the Rannoch Moor basin as a radial provider of ice to surrounding areas was apparently less significant than has hitherto been considered. Ice flowed into and across the southern part of the basin from the principal ice divide located to the west and south. No evidence has yet been found that would support a model of an eastward-migrating ice divide either during the build-up or during the deglaciation of the Late Devensian ice sheet.     

Extent of the last ice sheet in northern Scotland tested with cosmogenic 10Be exposure ages

The extent of the last British–Irish Ice Sheet (BIIS) in northern Scotland is disputed. A restricted ice sheet model holds that at the global Last Glacial Maximum (LGM; ca. 23–19 ka) the BIIS terminated on land in northern Scotland, leaving Buchan, Caithness and the Orkney Islands ice‐free. An alternative model implies that these three areas were ice‐covered at the LGM, with the BIIS extending offshore onto the adjacent shelves. We test the two models using cosmogenic ¹⁰Be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas. Our results indicate that the last BIIS covered all of northern Scotland during the LGM, but that widespread deglaciation of Caithness and Orkney occurred prior to rapid warming at ca. 14.5 ka. Copyright © 2008 John Wiley & Sons, Ltd.