Significance of snowblow in the generation of Loch Lomond Stadial (Younger Dryas) glaciers in the western Pennines,northern England

Geomorphological mapping has identified the former existence of five cirque glaciers that formed during the Loch Lomond Stadial in the western Pennines, northern England. Landforms in Mallerstang, which previously have been interpreted as moraine ridges, are shown to be better explained as large complex landslides. Reconstruction of these former glaciers has allowed the calculation of the former equilibrium line altitudes (ELA) by a number of different methods. Values for the ELA show a range across the area from 311 m to 608 m OD, but with low values (311 m and 428 m) for the two western glaciers. These are explained by the existence of large plateau areas adjacent to the former glaciers, which contributed additional mass to the glaciers by snowblow on to the glacier surfaces. Delimitation of the potential snowblow area for each glacier shows that it has a distinct orientation, with the western sector (225–315°) being statistically significant with the ELA indicating the importance of winds from this direction in determining the existence of these palaeoglaciers. The significance of snowblow for the generation of small glaciers in marginal areas means that such local factors must be investigated before regional firn line trends are determined in former glaciated areas.     

The last glaciers (Loch Lomond Advance) in Snowdonia,N. Wales

Evidence for 35 former cwm glaciers has been mapped in Snowdonia, N. Wales. Glacial limits are based on end moraines, boulder and drift limits, and the down-valley extent of hummocky moraine. Protalus ramparts indicate that at least 16 semi-permanent snow beds down to altitudes of 150 m also existed. Radiocarbon dates and pollen stratigraphy indicate that the glaciers and snow beds belong to the Loch Lomond (Younger Dryas) Stadial. The existence of a large number of ‘Older Series Moraines’ is questionned. Glacier reconstruction and contouring allows calculation of fan line altitudes, and trend surface analysis indicates that they rise from 450 m in the southwest to 700 m in the northeast. It is inferred that mean July temperature at sea-level was around 8·5°C. Two differences between north and south Britain are recognized. Firstly in Snowdonia, as in the Lake District and Southern Uplands, the dominance of southerly air streams appears less than in the Scottish Highlands. Secondly, whereas in the Scottish Highlands, the glaciers appear to have stagnated in situ following initial active retreat, some of those farther south were active during much of the time their margins retreated.     

A high‐resolution seismic stratigraphy from a Scottish sea loch and its implications for Loch Lomond Stadial deglaciation

A detailed high‐resolution seismic stratigraphy, calibrated by core data and terrestrial geomorphological mapping, has been constructed for Loch Ainort, Isle of Skye. This study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the Loch Lomond Stadial ice‐field on Skye. The Ainort Glacier reworked pre‐Loch Lomond glacial deposits terminating in a grounded tidewater ice‐front potentially 800 m beyond the previously extrapolated limit. The first stage of deglaciation was characterised by the formation of De Geer moraines indicative of a period of interrupted retreat. The second phase, by contrast, produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands/readvances. Paraglacial reworking of terrestrial slopes resulted in the deposition of thick, subaqueous, debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits. The identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record. However, the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat, occasional stillstands/ readvances did occur. Copyright © 2000 John Wiley & Sons, Ltd.     

Palaeoclimatic reconstruction from Loch Lomond Readvance glaciers in the West Drumochter Hills,Scotland

Geomorphological mapping in the West Drumochter Hills provides evidence of a readvance of locally nourished glaciers during the Loch Lomond (Younger Dryas) Stade, in the form of an icefield 67.7 km² in area drained by outlet glaciers. The icefield limits accord broadly with those proposed by Sissons (1980) but all geomorphic, stratigraphic and sedimentological evidence conflicts with a recent proposal that the landforms in the area reflect southwestwards retreat of the last ice sheet. Up‐valley continuity of recessional moraines indicates that the ice remained active and close to climatic equilibrium during the earlier stages of glacier retreat, consistent with slow warming following the coldest part of the stade. The pattern of equilibrium line altitudes (ELAs) across the icefield is consistent with transfer of snow by westerly and southerly winds. The ELA of the reconstructed icefield as a whole is 622–629 m, although this figure is likely to be lower than the regional (climatic) ELA because the icefield probably received additional snow blown from adjacent plateau surfaces and slopes. Inclusion of potential snow‐blow areas in the ELA calculation yields a value of 648–656 m; the climatic ELA is therefore likely to have lain between 622 and 656 m. Mean June to August temperature at the ELA, based on chironomid assemblages at two sites, falls within the range 4.0 ± 0.7°C. Empirical relationships between temperature and precipitation at modern glacier ELAs indicate that mean annual precipitation (MAP) at the ELA was 1977 ± 464 mm, statistically indistinguishable from modern values. Comparison with precipitation values calculated for the Isle of Mull on the west coast suggest that the precipitation gradient across the Central Highlands of Scotland was steeper during the Loch Lomond Stade than at present, probably as the result of efficient scavenging of precipitation from westerly airflows by the West Highland Icefield. Copyright © 2005 John Wiley & Sons, Ltd.     

The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review

This paper systematically reviews the glacial geomorphological evidence of the Loch Lomond Stadial (LLS; Younger Dryas) glaciation in Britain (12.9–11.7 ka). The geomorphology of sub‐regions within Scotland, England and Wales is assessed, providing the most comprehensive synthesis of this evidence to date. The contrasting nature of the evidence at the local scale is reviewed and conceptual themes common to multiple sub‐regions are examined. Advancements in glaciological theory, mapping technologies, numerical modelling and dating have been applied unevenly to localities across Britain, inhibiting a holistic understanding of the extent and dynamics of the LLS glaciation at a regional scale. The quantity and quality of evidence is highly uneven, leading to uncertainties regarding the extent of glaciation and inhibiting detailed analysis of ice dynamics and chronology. Robust dates are relatively scarce, making it difficult to confidently identify the limits of LLS glaciers and assess their synchroneity. Numerical models have allowed the glacier–climate relationships of the LLS to be assessed but have, thus far, been unable to incorporate local conditions which influenced glaciation. Recommendations for future research are made that will allow refined reconstructions of the LLS in Britain and contribute to a more comprehensive understanding of glacier–climate interactions during the Younger Dryas.     

Loch Lomond Stadial glaciers in North Harris, Outer Hebrides, North-West Scotland: glacier reconstruction and palaeoclimatic implications

Geomorphological mapping of North Harris provides evidence for the former existence of 10 glaciers with a total area of ca 35 km². A Loch Lomond (Younger Dryas) Stadial age (ca 12.9–11.5 kyr BP) for this glacial episode is inferred from glacier configuration, landsystems dominated by hummocky recessional moraines, and relationships with Lateglacial periglacial phenomena. Equilibrium line altitudes (ELAs) of 150–289 m were calculated for individual glaciers. ELA variability mainly reflects differences in snow-contributing area. The area-weighted mean ELA (204 m) is consistent with a northwards decline in ELAs along the western seaboard of the British Isles of 69.5 m (100 km)⁻¹, equivalent to a northwards ablation-season temperature decrease of 0.42 °C (100 km)⁻¹. This latitudinal temperature gradient implies a mean July sea-level temperature of ca 7.2 °C for the coldest part of the stade, roughly 6 °C lower than at present. Sea-level precipitation at the time of the LLS glacial maximum is inferred to have been between ca 1970±200 and 2350±200 mm yr⁻¹, implying that LLS precipitation was up to 25% greater than now. Patterns of recessional moraines indicate that the glaciers remained close to climatic equilibrium as they retreated to their sources, though moraine belts implying near-stationary or readvancing ice margins on flat valley floors are separated by moraine-free zones indicating uninterrupted retreat. Calculation of ELAs for ‘residual’ glaciers in former source areas suggests that summer warming of 1.0 °C would have resulted in shrinkage of the glaciers to their sources.     

Characteristics and implications of some Loch Lomond Stadial moraine ridges and later landforms,eastern Lake District,northern England

Moraine ridges and mounds of inferred Loch Lomond Stadial (LLS) age have been mapped at three sites (Fordingdale, Swindale and Wet Sleddale) in part of the eastern Lake District, northern England, and indicate glaciers were more widespread than envisaged by Sissons (1980, Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 71, pp. 13–27). The moraines delimit closely the downslope/downvalley limits of the former glaciers but there is no geomorphological evidence with which to define their upslope/upvalley margins. The former glaciers are considered to have been nourished within the confines of their individual valley, cirque and hillside embayment respectively, rather than being outlet glaciers of plateau icefields. Estimated equilibrium line altitudes (ELAs) are within the range of values determined previously for LLS glaciers in the Lake District and do not necessitate revision of established palaeoclimatic parameters. Individual ELAs were probably influenced by local factors; all three former glaciers had accumulation-area aspects between north and east, limiting the impact of direct solar radiation during the ablation season, and were adjacent to extensive areas of high ground to the west and/or south that would have facilitated transfer of snow to their surfaces by winds from those directions. In Fordingdale, three essentially contemporaneous depositional landforms occur upslope of the moraines and are considered to represent hillslope adjustments following wastage of glacier ice from the site. © 1998 John Wiley & Sons, Ltd.     

Glacial landsystems,retreat dynamics and controls on Loch Lomond Stadial (Younger Dryas) glaciation in Britain

Glacial geomorphology relating to the Loch Lomond Stadial (Younger Dryas) in Britain is used to construct five glacial landsystem models. These landsystems lie on a continuum of increasing ice thickness and decreasing topographic control and typify the principal styles of glaciation during the stadial. The landsystems comprise: the cirque/niche glacier landsystem, the alpine icefield landsystem, the lowland piedmont lobe landsystem, the plateau icefield landsystem and the icecap landsystem. Geomorphological features representing the icecap landsystem are present only at the centre of the West Highland Glacier Complex, which was flanked primarily by satellite alpine and plateau icefields. The cirque/niche glacier landsystem was present predominantly in areas that experienced conditions only marginally favourable for glacier development at peripheral sites. Three styles of glacier retreat are recorded by the geomorphology: active, two‐phase and uninterrupted retreat. Of these, active retreat appears to be most widespread within the Loch Lomond Stadial limits. These retreat styles reflect a combination of climatic and topographic conditions, coupled with local factors influencing the preservation of landforms from which retreat dynamics can be inferred. Likewise, the distribution of landsystems was influenced by an interplay between topography and climate, with glacier formation being facilitated in locations where topographical conditions aided in the accumulation of snow. The pattern also supports the existence of previously recognized northward and eastward precipitation gradients across Britain during the stadial.     

Devensian glacigenic sedimentation and landscape evolution in the Cardigan area of southwest Wales

The depositional processes associated with late Devensian ice in areas bordering the Irish Sea basin have been the subject of considerable debate. Among the key areas around the Irish Sea, southwest Wales occupies a particularly crucial position because it is here that ice flowing from the north impinged upon the coast orthogonally and encroached inland. Two main hypotheses have emerged concerning deglaciation of the Irish Sea basin. The traditional hypothesis holds that sedimentation was ice‐marginal or subglacial, whereas an alternative hypothesis that emerged in the 1980s argued that sedimentation was glaciomarine. Southwest Wales is well‐placed to contribute to this debate. However, few detailed sedimentological studies, linked to topography, have been made previously in order to reconstruct glacial environments in this area. In this paper, evidence is presented from four boreholes drilled recently in the Cardigan area, combined with data from coastal and inland exposures in the lower Teifi valley and adjacent areas. A complex history of glaciation has emerged: (i) subglacial drainage channel formation in pre‐Devensian time, (ii) deposition of iron‐cemented breccias and conglomerates possibly during the last interglacial (or in the early/mid‐Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a glacial lake known as Llyn Teifi, (iv) a second high‐level glaciolacustrine succession formed near Llandudoch, (v) outside the Teifi valley, ice‐marginal, subglacial and glaciofluvial sediments were also laid down, providing a near‐continuous cover of drift throughout the area. Glacial advance was characterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wales. Copyright © 2001 John Wiley & Sons, Ltd.     

Loch Lomond Stadial and Flandrian shorelines in the inner Moray Firth area,Scotland

Morphological mapping and stratigraphical investigations have identified surface and buried relict marine features in the inner Moray Firth area. The features consist of a buried gravel layer formed during the Loch Lomond Stadial, a buried beach of early Flandrian age, and surface beaches and estuarine flats of mid-late Flandrian age. Analysis of the altitudes of morphological features has identified two buried and five (possibly six) surface glacio-isostatically tilted raised shorelines. The steepest shoreline is associated with the buried gravel layer and slopes down towards N20°E at a gradient of 0.20m/km. Younger shorelines have lower gradients between 0.16–0.03m/km. The shoreline sequence combined with published data defines relative sea-level movements in the area during the last 11000 years. The inner Moray Firth shorelines are correlated with similar features in other areas of Scotland which include the Main Lateglacial, Main Buried and Main Postglacial Shorelines.     

Late Devensian and Holocene relative sea‐level change in North Wales,UK

Deglacial sea‐level index points defining relative sea‐level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British‐Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS ¹⁴C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay‐silts to silty sands from the NE Menai Strait, North Wales. Ten new sea‐level index points and nine new limiting dates from the Devensian Late‐glacial and early Holocene are integrated with twelve pre‐existing Holocene sea‐level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid‐Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood‐dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre‐existing Late‐glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea‐level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.     

Deglaciation of the younger dryas or Loch Lomond Stadial ice-field in the northern Highlands,Scotland

When viewed from the air, Scottish ‘hummocky moraine’ can be resolved into a series of linear ridges that resemble those found at the margins of actively retreating glaciers today. Recent work has supported the interpretation of these linear ridges as ice-marginal landforms and the authors believe that the majority of ‘hummocky moraine’ deposits can be interpreted in this way. Consequently the pattern of deglaciation can be established fairly precisely from the pattern of linear ridges. This approach is applied to the landforms of the northern part of the Loch Lomond Stadial ice-field in order to reconstruct the regional pattern of deglaciation. This leads to important inferences about the significance of topographic control during deglaciation and more importantly it provides fresh insight into the environment of the British Isles during the Loch Lomond Stadial.     

The Loch Lomond Readvance on north Arran,Scotland: glacier reconstruction and palaeoclimatic implications

Geomorphological mapping of northern Arran provides evidence for two advances of locally nourished glaciers, the younger being attributable to the Loch Lomond Stade (LLS) of ca. 12.9–11.5 k yr BP, primarily through the mutually exclusive relationship between glacial limits and Lateglacial periglacial features. The age of the earlier advance is unknown. Inferred LLS glacier cover comprised two small icefields and eight small corrie or valley glaciers and totalled 11.1 km². ELAs reconstructed using area–altitude balance ratio methods range from 268 m to 631 m for individual glaciers, with an area‐weighted mean ELA of 371 m. ELAs of individual glaciers are strongly related to snow‐contributing areas. The area‐weighted mean ELA is consistent with a north–south decline in LLS ELAs along the west coast of Great Britain. This decline has an average latitudinal gradient of 70 m 100 km^?1, equivalent to a mean southwards ablation‐season temperature increase of ca. 0.42°C 100 km^?1. Mean June–August temperatures at the regional climatic ELA, estimated from chironomid assemblages in SE Scotland, lay between 5.7 ± 0.1°C and 4.1 ± 0.2°C. Empirical relationships between temperature and precipitation at modern glacier ELAs indicate equivalent mean annual precipitation at the ELA lay between 2002 ± 490 mm and 2615 ± 449 mm. These figures suggest that stadial precipitation on Arran fell within a range between +8% and ?33% of present mean annual precipitation. Copyright © 2006 John Wiley & Sons, Ltd.     

The Loch Lomond Readvance on the Isle of Mull,Scotland: glacier reconstruction and palaeoclimatic implications

Geomorphological mapping of Mull provides evidence for an icefield 143 km² in area flanked by six corrie glaciers with a total area of ca. 13 km². The absence of Lateglacial periglacial features, shorelines and pollen sites from the area occupied by this readvance, together with radiocarbon dating of shell fragments, confirm that it occurred during the Loch Lomond (Younger Dryas) Stade. The thickness of glacigenic deposits within the area of the readvance is attributed to reworking of paraglacial sediments. Up‐valley continuity of recessional moraines indicates that the ice remained active and near to equilibrium during retreat, consistent with slow warming following the coldest part of the stade. Reconstructed equilibrium line altitudes (ELAs) imply vigorous snow‐blowing by westerly winds, and are consistent with a general southwestwards decline in ELAs across the Scottish Highlands. An ELA of 250 m was calculated for the Mull Icefield using an ablation:accumulation balance ratio (ABR) approach. Palaeotemperature and palaeoprecipitation estimates were derived by calculating a theoretical regional ELA from meteorological data and assuming that the combination of temperature and precipitation implied by the theoretical ELA approximates conditions at 250 m on Mull during the Loch Lomond Stade. The result indicates a mean July sea‐level temperature of 5.7 ± 0.5°C and a mean annual precipitation at 250 m of ca. 2700–3800 mm (best estimate 3200 mm), indicating higher precipitation totals than at present owing to more vigorous atmospheric circulation. Copyright © 2002 John Wiley & Sons, Ltd.     

Graptolite dating of Ordovician vulcanicity in the Arenig area,North Wales

Graptolite assemblages of multidens Biozone age have been recovered from mudstones immediately above and below the Llyn Conwy Formation, the local representative of the regionally extensive acid tuffs that mark the end of vulcanicity in the south and east of the Harlech Dome. They include several taxa new to Wales and are markedly different to contemporaneous faunas from the Welsh Borders. One new species, Pseudoclimacograptus isknos, is described.     

The geomorphological impact of Loch Lomond (Younger Dryas) Stadial plateau icefields in the central Lake District,northwest England

Detailed geomorphological mapping has revealed evidence for the development of plateau icefields in the central fells of the English Lake District during the Loch Lomond (Younger Dryas) Stadial (ca. 12.9–11.5 ka). The largest plateau icefield system, which covered an area of approximately 55 km² (including outlet glaciers), was centred on High Raise. To the west, smaller plateau icefields developed on Grey Knotts/Brandreth and Dale Head, covering areas of 7 km² and 3 km² respectively. The geomorphological impact of these plateau icefields appears to have been minimal on the summits, where the survival of blockfields and other frost‐weathered debris (mostly peat‐covered) implies the existence of at least patches of protective, cold‐based ice. Ice‐moulded bedrock at some plateau edges, however, documents a transition to wet‐based, erosive conditions. Prominent moraine systems were produced by outlet glaciers, which descended into the surrounding valleys where their margins became sediment traps for supraglacial debris and inwash. In some valleys, ice‐marginal moraines record successive positions of outlet glaciers, which actively backwasted towards their plateau source. This interpretation differs from that of previous workers, who assumed an alpine style of glaciation, with reconstructed glaciers emanating from corries and valley heads. It is likely that plateau icefields were more common at this time in upland Britain than hitherto has been appreciated. Copyright © 2001 John Wiley & Sons, Ltd.     

Palaeomagnetism of the Tan y Grisiau granite,North Wales: Evidence for a subvolcanic origin in Late Ordovician times

Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a₉₅ = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.     

青藏高原东北缘玛雅雪山晚第四纪冰川发育的气候和构造耦合

青藏高原东北缘的玛雅雪山(海拔4 447 m)保存着确切的第四纪冰川遗迹. 野外地貌调查与光释光测年方法相结合, 确认玛雅雪山晚第四纪主要经历3次冰川作用: 第Ⅰ组冰碛时代为新冰期; 第Ⅱ组冰碛物年龄为(23.2±1.0)ka, 其上覆泥石流年龄为(2.9±0.3)~(2.3±0.1)ka, 上层土壤年龄为(3.6±0.2)ka, 对应于深海氧同位素2阶段(MIS 2)的末次冰盛期(LGM); 第Ⅲ组冰碛年龄为(42.6±1.9)~(45.7±3.0) ka, 属于末次冰期中冰阶, 对应MIS 3中期. 采用最新综合因子法计算玛雅雪山现代冰川物质平衡线为海拔4 605 m. 依据冰川地貌形态, 计算末次冰期平衡线为海拔3 800 m. 通过庄浪河阶地的拔河高度及各级阶地的年代, 以河流的下切速率代表玛雅雪山的抬升速率, 计算得到末次冰期中期以来玛雅雪山抬升了50~60 m. 利用玛雅雪山周边的达里加山和太白山冰川漂砾的¹⁰Be 数据近似代表流域侵蚀速率, 推算出玛雅雪山剥蚀速率大约为29 mm·ka^-1, 推断MIS 3以来流域的剥蚀量为1~2 m. 综合末次冰期中期以来的构造抬升量和剥蚀量, 恢复末次冰期中期时的流域高度为海拔4 200 m, 平衡线高度为海拔3 750 m. 研究结果显示: 研究区在MIS 3时, 流域平均高度已经在平衡线之上, 在流域平均高度到主峰之间冰川开始积累, 发育冰川. 结合其他环境指标综合推断, 玛雅雪山晚第四纪冰川的发育是气候和构造耦合的产物.