波堆藏布谷地冰碛丘陵形成机制及其环境意义

波堆藏布谷地中分布着大面积的冰碛丘陵, 通过考察发现其个体大小、外形、分布规模及内部砾石组成等方面都与高纬大冰盖外围形成的冰碛丘陵有很大的区别. 以冰川沉积学理论为基础, 从沉积动力学的角度讨论中低纬度波堆藏布谷地中冰碛丘陵的形成机制. 结果表明: 气候变化造成冰川的大面积死冰加之宽阔的河谷、海洋性冰川的特性促使波堆藏布谷中形成如此大面积的冰碛丘陵; 同时,大规模的冰碛丘陵表明气候转暖(抑或变干)的过程是突变的.     

Drumlin formation: a time transgressive model

Graphical and numerical reconstructions of the Rainy and Superior lobes of the Laurentide Ice Sheet suggest that drumlin formation was time transgressive. Suites of glacial landforms including drumlins, tunnel valleys, eskers, and ice-collapse features can be correlated with specific recessional ice margins and are used as boundary conditions in the modeling. A contour map of the ice surface is then drawn using a specified basal shear stress. The shear stress can be constant or allowed to vary with position on the bed and is chosen to be consistent with the subglacial regime indicated by field evidence. Assuming that ice flow is parallel to drumlin orientations and perpendicular to the ice surface contours and moraines, the trend of drumlin axes is best accommodated by time transgressive drumlin formation during minor stillstands in the overall ice recession. The alternative, that drumlins were formed while the ice was at the Late Wisconsin maximum limit, requires large spatial variations in the basal shear stress distribution and therefore implies large mass-balance gradients or large variations in basal sliding velocities over small distances, for which there is little evidence.     

Late Wisconsinan glacial landsystems on Atlantic Canadian shelves: New evidence from multibeam and single-beam sonar data

Multibeam sonar surveys in the past decade, augmented by single-beam data from the OLEX charting system, reveal landsystems on Atlantic Canadian shelves that are diagnostic of Late Wisconsinan ice-sheet dynamics. Four landsystems are described. (1) The Bay of Fundy landsystem comprises two contrasting sets of bedforms, and is interpreted as evidence of topographically controlled fast-flowing ice adjacent to slower-moving ice. (2) The German Bank landsystem off southwest Nova Scotia is comprised of glacially fluted terrain overprinted by De Geer moraines and arcuate recessional moraines. We infer that a flow of grounded glacial ice out of the Bay of Fundy was followed by steady retreat, punctuated by at least one major re-advance. (3) The Placentia Bay landsystem consists of a convergent field of streamlined landforms with superimposed De Geer moraines, overprinted in one area by flutings. We infer that this landsystem was formed in the onset zone of fast-flowing ice, and that overprinting was due to a re-advance of ice from offshore. (4) The south coast of Newfoundland landsystem, which includes arcuate, fjord-mouth moraines and a coast-parallel, fluted moraine, indicates strong topographic control on a retreating marine ice margin as it reached a fjord coastline. These submarine glacial landsystems are not inconsistent with a conceptual model showing Late Wisconsinan ice advance to shelf edges, rapid calving retreat along deepwater channels and slower retreat of ice margins grounded in shallow water. The re-advances documented two of the study areas have parallels in the Last British Ice Sheet, confirming that the reorganization of marine-based ice sheets, caused by calving in embayments, led to internally forced re-advances.     

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.     

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.     

Lithostratigraphy of the Mérida (Wisconsinan) glaciation and Pedregal interstade, Mérida Andes, northwestern Venezuela

At Pedregal, more than 40 m of sediments are exposed within a ‘fan complex’ formed between lateral moraines of the adjacent Mucuchache and El Caballo valleys. Early and late Mérida (Wisconsinan) glaciations are represented by till and till plus proglacial sediments, respectively. A middle Wisconsinan interstadial event, here termed the Pedregal interstade, began at the end of the Early Mérida glaciation at approximately 60 ka BP. Following the retreat of ice from the small Pedregal Basin, a lake formed when the local drainage was blocked due to movement of the Mesa de Caballo along the Boconó Fault. Shallow lake or no-lake phases lasted approximately a few hundred to, at most, 2000 years, and each lake phase was marked by peat accumulation. Four of seven peats identified formed during sufficiently long intervals for soil profiles (incipient to mature Spodosols) also to develop. The Spodosol with the strongest development (Eb/Bsb/Coxb/Cub horizons) is found adjacent to the lowest peat and reflects ongoing early Mérida stadial (MIS 3) conditions; the youngest peats, associated with weak podzolic soils (Eb/Bsb horizons), formed under slightly warmer interstadial conditions, presumably with less soil water. Cyclic lacustrine deposition is related to lake level and relative depth fluctuations, due in part to variable shoreline/delta progradation and shallowing as the lake deepened in general. Whereas final drainage of the lake is related to movement of the Boconó Fault and breach of the moraines that form the Mesa de Caballo, earlier lake level fluctuations appear related to climate change. Radiocarbon dating of the peats suggests they are related to warmer periods and may tentatively correlate with small ‘interstadials’ or ‘D-O events’ detected in the oxygen-isotope record of Greenland ice cores and North Atlantic marine sediments.     

Re-interpretation of ‘hummocky moraine’ in the Gaick,Scotland, as erosional remnants: Implications for palaeoglacier dynamics

Many glaciated valleys in Scotland contain distinctive, closely spaced ridges and mounds, which have been termed ‘hummocky moraine’. The ridges and mounds are widely interpreted as ice-marginal moraines, constructed during active retreat of mainly temperate glaciers. However, hummocky terrain can form by various processes in glacial environments, and it may relate to a range of contrasting glaciodynamic regimes. Thus, detailed geomorphological and sedimentological studies of hummocky surfaces in Scottish glaciated valleys are important for robust interpretations of former depositional environments and glacier dynamics. In this contribution, we examine irregularly shaped ridges and mounds that occur outside the limits of former Loch Lomond Readvance (≈ Younger Dryas; ~ 12.9–11.7 ka) glaciers in the Gaick, Central Scotland. These ridges and mounds are intimately associated with series of sinuous channels, and their planform shape mimics the form of the adjacent channels. Available exposures through ridges in one valley reveal that those particular ridges contain lacustrine, subglacial, and glaciofluvial sediments. The internal sedimentary architecture is not related to the surface morphology; thus, we interpret the irregularly shaped ridges and mounds as erosional remnants (or interfluves). Based on the forms and spatial arrangement of the associated channels, we suggest that the ridges and mounds were generated by a combination of ice-marginal and proglacial glaciofluvial incision of glaciogenic sediments. The evidence for glaciofluvial incision, rather than ice-marginal moraine formation, at pre-Loch Lomond Readvance glacier margins in the Gaick may reflect differences in glaciodynamic regimes and/or efficient debris delivery from the glacier margins to the glaciofluvial systems.     

Problems of moraine morphology: Rogen moraine and Blattnick moraine

In Västerbotten County, Sweden, both Rogen moraine and Blattnick moraine are common in the inner part of the county. Rogen moraines are found primarily in basins and upslope positions, whereas the Blattnick type (first found in the Blattnicksele area in the county) mostly occurs in broader, more plain-like areas. Moreover, both types show a great variety of individual forms, due to topography, basal conditions and ice tectonics. Most Rogen moraine ridges are characterized by features due to active ice. The Blattnick moraine type is characterized by broader and more drumlinized ridges. They are often proximally higher and laterally-distally extended, thus merging into streamlined mounds or hills. The material composition of Rogen and Blattnick moraines is similar. The authors have found the following sequence of transitional forms from the Scandinavian mountains in the west towards the east: (1) Rogen moraines, (2) crescent ridges, (3) Blattnick moraines, (4) drumlins.     

A study of fluted moraines in the Torridon area,NW Scotland

A study of certain small-scale fluted moraines, all less than 4m high but up to 400m long, in the Torridon area has shown that they are subglacial features composed of a clast-rich till, and formed in the Loch Lomond Stadial beneath ice with a maximum depth of 100-200m. The evidence from lithological analysis of till samples from the fluted moraines suggests that they were formed by subglacial deformation of a pre-existing till with little in the way of net down-glacier movement of the material.     

Subglacial and ice‐marginal landforms in south‐central Ontario: implications for ice‐sheet reconfiguration during deglaciation

Regional‐scale, high‐resolution terrain data permit the study of landforms across south‐central Ontario, where the bed of the former Laurentide Ice Sheet is well exposed and passes downflow from irregular topography on Precambrian Shield highlands to flat‐lying Palaeozoic carbonate bedrock, and thick (50 to >200 m) unconsolidated sediment substrates. Rock drumlins and megagrooves are eroded into bedrock and mega‐scale glacial lineations (MSGL) occur on patchy streamlined till residuals in the Algonquin Highlands. Downflow, MSGL pass into juxtaposed rock and drift drumlins on Palaeozoic bedrock and predominantly till‐cored drumlins in areas of thick drift. The Lake Simcoe Moraines, now traceable for more than 80 km across the Peterborough drumlin field (PDF), form a distinct morphological boundary: downflow of the moraine system, drumlins are larger, broader and show no indication of subsequent reworking by the ice, whereas upflow of the moraines, a higher degree of complexity in bedform pattern and morphology is distinguished. Discrete radial and/or cross‐cutting flowset terminate at subtle till‐cored moraine ridges downflow of local topographic lows, indicating multiple phases of late‐stage ice flow with strong local topographic steering. More regional‐scale flow switching is evident as NW‐orientated bedforms modify drumlins south of the Oak Ridges Moraine, and radial flowset emanate from areas within the St. Lawrence and Ottawa River valleys. Most of the drumlins in the PDF formed during an early, regional drumlinization phase of NE–SW flow that followed the deposition of a thick regional till sheet. These were subsequently modified by local‐scale, topographically controlled flows that terminate at till‐cored moraines, providing evidence that the superimposed bedforms record dynamic ice (re)advances throughout the deglaciation of south‐central Ontario. The patterns and relationships of glacial landform distribution and characteristics in south‐central Ontario hold significance for many modern and palaeo‐ice sheets, where similar downflow changes in bed topography and substrate lithology are observed.     

Glacial geomorphology of Teesdale,northern Pennines,England: Implications for upland styles of ice stream operation and deglaciation in the British-Irish Ice Sheet

The glacial geomorphology of Teesdale and the North Pennines uplands is analysed in order to decipher: a) the operation of easterly flowing palaeo-ice streams in the British-Irish Ice Sheet; and b) the style of regional deglaciation. Six landform categories are: i) bedrock controlled features, including glacitectonic bedrock megablocks or ‘rubble moraine’; ii) discrete mounds and hills, often of unknown composition, interpreted as weakly streamlined moraines and potential ‘rubble moraine’; iii) non-streamlined drift mounds and ridges, representing lateral, frontal and inter-ice stream/interlobate moraines; iv) streamlined landforms, including drumlins of various elongation ratios and bedrock controlled lineations; v) glacifluvial outwash and depositional ridges; and vi) relict channels and valleys, related to glacial meltwater incision or meltwater re-occupation of preglacial fluvial features. Multiple tills in valley-floor drumlin exposures indicate that the subglacial bedform record is a blend of flow directions typical of areas of discontinuous till cover and extensive bedrock erosional landforms. Arcuate assemblages of partially streamlined drift mounds are likely to be glacially overridden latero-frontal moraines related to phases of “average glacial conditions” (palimpsests). Deglacial oscillations of a glacier lobe in mid-Teesdale are marked by five inset assemblages of moraines and associated drift and meltwater channels, named the Glacial Lake Eggleshope, Mill Hill, Gueswick, Hayberries and Lonton stages. The Lonton stage moraines are thought to be coeval with bedrock-cored moraines in the central Stainmore Gap and likely record the temporary development of cold-based or polythermal ice conditions around the margins of a plateau-based icefield during the Scottish Readvance.     

Minor end moraines of the Wisconsin Valley Lobe, north-central Wisconsin, USA

Approximately 35 parallel, discontinuous glacial ridges occur in an area of about 100 km² in north‐central Wisconsin. The ridges are located between about 6 and 15 km north (formerly up‐ice) of the maximum extent of the Wisconsin Valley Lobe of the Laurentide Ice Sheet. The ridges are between 1 and 4 m high, up to 1 km long, and spaced between 30 and 80 m apart. They are typically asymmetrical with a steep proximal (ice‐contact) slope and gentle distal slope. The ridges are composed primarily of subglacial till on their proximal sides and glacial debris‐flow sediment on the distal sides. In some ridges the till and debris‐flow sediment are underlain by sorted sediment that was deformed in the former direction of ice flow. We interpret the ridges to be recessional moraines that formed as the Wisconsin Valley Lobe wasted back from its maximum extent, with each ridge having formed by a sequence of (1) pushing of sorted ice‐marginal sediment, (2) partial overriding by the glacier and deposition of subglacial till on the proximal side of the ridge, and (3) deposition of debris‐flow sediment on the distal side of the ridge after the frozen till at the crest of the ridge melted. The moraines are similar to annual recessional moraines described at several modern glaciers, especially the northern margin of Myrdalsjokull, Iceland. Thus, we believe the ridges probably formed as a result of minor winter advances of the ice margin during deglaciation. Based on this assumption, we calculate the net rate of ice‐surface lowering of the Wisconsin Valley Lobe during the period when the moraines formed. Various estimates of ice‐surface slope and rates of ice‐margin retreat yield a wide range of values for ice‐surface lowering (1.7–14.5 m/yr). Given that ablation rates must exceed those of ice‐surface lowering, this range of values suggests relatively high summer temperatures along the margin of the Wisconsin Valley Lobe when it began retreating from its maximum extent. In addition, the formation of annual moraines indicates that the glacier toe was thin, the ice surface was clean, and the ice margin experienced relatively cold winters.     

Controlled moraines: origins,characteristics and palaeoglaciological implications

Controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de-icing and possess clear linearity due to the inheritance of the former pattern of debris-rich folia in the parent ice. Linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt-out. As a result, moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris-rich ice was concentrated in frozen toe zones. Recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that: (a) controlled moraine development can be ascribed to a specific process (e.g. englacial thrusting or supercooling); and (b) controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts (e.g. ancient polythermal or supercooled snouts). These assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris-rich ice thickening mechanisms are seen to produce the same geomorphic features. Polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier–permafrost interface. End moraines lie on a process–form continuum constrained by basal thermal regime. The morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores, but the final deposits/landforms tend to consist of discontinuous transverse ridges with intervening hummocks, preserving only a weak impression of the former englacial structure. These are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice-walled lake plains and lying down flow of streamlined landforms produced by warm-based ice. A variety of debris entrainment mechanisms can produce the same geomorphic signature. Spatial and temporal variability in process–form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin.     

Exploring controls of the early and stepped deglaciation on the western margin of the British Irish Ice Sheet

New optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE-CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice-streams (MSIS). Shelf-edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf-wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a^–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still-stands. By ~23–22 ka the outer shelf (~25 000 km²) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a^–1 vs. ~2–6 m a^–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine-terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice-stream retreat from mid-shelf to the coast.     

Litho- and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnel-valley infill in relation to a receding ice margin

Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23–21 kyr BP, Vendsyssel was part of an ice‐dammed lake where the Ribjerg Formation was deposited c. 27–23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice‐marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45–50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re‐advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20–19 kyr BP in a large ice‐dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice‐contact fans, which make up the Jyske Ås and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north–south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re‐advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice‐dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19–18 kyr BP. Related to this stage of deglaciation, eight ice‐marginal positions have been identified based on the distribution of large tunnel‐valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice‐contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel‐valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine‐grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice‐proximal environment, and the infill consists of alternating layers of fine‐ to coarse‐grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.     

Hummocky moraine: sedimentary record of stagnant Laurentide Ice Sheet lobes resting on soft beds

Over large areas of the western interior plains of North America, hummocky moraine (HM) formed at the margins of Laurentide Ice Sheet (LIS) lobes that flowed upslope against topographic highs. Current depositional models argue that HM was deposited supraglacially from stagnant debris-rich ice (`disintegration moraine'). Across southern Alberta, Canada, map and outcrop data show that HM is composed of fine-grained till as much as 25 m thick containing rafts of soft, glaciotectonized bedrock and sediment. Chaotic, non-oriented HM commonly passes downslope into weakly-oriented hummocks (`washboard moraine') that are transitional to drumlins in topographic lows; the same subsurface stratigraphy and till facies is present throughout. These landforms, and others such as doughnut-like `rim ridges', flat-topped `moraine plateaux' and linear disintegration ridges, are identified as belonging to subglacially-deposited soft-bed terrain. This terrain is the record of ice lobes moving over deformation till derived from weakly-lithified, bentonite-rich shale. Drumlins record continued active ice flow in topographic lows during deglaciation whereas HM was produced below the outer stagnant margins of ice lobes by gravitational loading (`pressing') of remnant dead ice blocks into wet, plastic till. Intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show `hybrid' drumlins whose streamlined form has been altered by subglacial pressing (`humdrums') below dead ice. The presence of hummocky moraine over a very large area of interior North America provides additional support for glaciological models of a soft-bedded Laurentide Ice Sheet.     

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.     

Late glacial and late Holocene moraines in the Cerros Cuchpanga, central Peru

Small ice fields on the western cordillera northeast of Lima were expanded to three times their present size in the recent past, and the regional snow line was probably about 100 m lower than it is today. Outwash from the expanded glaciers formed deltas of silt in valley-bottom lakes. When the ice lobes retreated, the reduced outwash was trapped behind recessional moraines, and the clear meltwater infiltrated into the limestone bedrock and emerged at the heads of the deltas in spring pools. The delta surfaces then became covered with peat, and radiocarbon dates for the base of the peat (1100 ± 70 and 430 ± 70 yr B.P. for two different deltas) indicate that the maximum ice advance was older than those dates and, thus, older than the Little Ice Age of many north-temperate regions. Much older moraines date from expansion of the same local summit glaciers to even lower levels in the main valleys, which had previously been inundated by the cordilleran ice field. The cordilleran deglaciation and this expansion of local glaciers probably occurred between 12,000 and 10,000 yr ago, on the basis of slightly contradictory radiocarbon dates.     

Refining the pattern and style of Cordilleran Ice Sheet retreat: palaeogeography,evolution and implications of lateglacial ice‐dammed lake systems on the southern Fraser Plateau,British Columbia,Canada

Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conceptualized as being dominated by passive downwasting (stagnation), in part because of the lack of large recessional moraines. Yet, multiple lines of evidence, including reconstructions of glacio‐isostatic rebound from palaeoglacial lake shoreline deformation suggest a sloping ice surface and a more systematic pattern of ice‐margin retreat. Here we reconstructed ice‐marginal lake evolution across the subdued topography of the southern Fraser Plateau in order to elucidate the pattern and style of lateglacial CIS decay. Lake stage extent was reconstructed using primary and secondary palaeo‐water‐plane indicators: deltas, spillways, ice‐marginal channels, subaqueous fans and lake‐bottom sediments identified from aerial photograph and digital elevation model interpretation combined with field observations of geomorphology and sedimentology, and ground‐penetrating radar surveys. Ice‐contact indicators, such as ice‐marginal channels, and grounding‐line moraines were used to refine and constrain ice‐margin positions. The results show that ice‐dammed lakes were extensive (average 27 km²; max. 116 km²) and relatively shallow (average 18 m). Within basins successive lake stages appear to have evolved by expansion, decanting or drainage (glacial lake outburst flood, outburst flood or lake maintenance) from southeast to northwest, implicating a systematic northwestward retreating ice margin (rather than chaotic stagnation) back toward the Coast Mountains, similar in style and pattern to that proposed for the Fennoscandian Ice Sheet. This pattern is confirmed by cross‐cutting drainage networks between lake basins and is in agreement with numerical models of North American ice‐sheet retreat and recent hypotheses on lateglacial CIS reorganization during decay. Reconstructed lake systems are dynamic and transitory and probably had significant effects on the dynamics of ice‐marginal retreat, the importance of which is currently being recognized in the modern context of the Greenland Ice Sheet, where >35% of meltwater streams from land‐terminating portions of the ice sheet end in ice‐contact lakes.