Common origin for De Geer moraines of variable composition in Raudvassdalen,northern Norway

The study of De Geer moraines in Raudvassdalen shows that most De Geer moraines are likely to have a common origin at the grounding line of glaciers despite variability in composition of the ridges. Pebble fabric, grain‐size analysis and structures within exposures of De Geer moraines in the Raudvassdalen area, with compositions ranging from mostly till to mostly sorted sediment, indicate that the ridges all formed at the grounding line of a tidewater glacier by common processes: deposition of sorted sediments beyond the grounding line followed by deformation of pre‐existing sediments and deposition of till as the glacier overrode the ridges. The compositional variation of the ridges is probably related to the position of the section studied relative to the location of the outlet of subglacial streams. Ridges composed entirely of till form at locations remote from the outlet of subglacial streams, and ridges with a component of sorted sediments form in closer proximity to these streams. This unifying theory of De Geer moraine formation, along with theoretical and geological evidence showing that there are limited physical conditions where basal crevasses can form, suggests that the number of De Geer moraines interpreted to have formed in basal crevasses is probably unrealistic. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

Formation of De Geer moraines and implications for deglaciation dynamics

De Geer moraines are very common in the Møre area, western Norway. These moraines occur below the marine limit and outside the Younger Dryas ice limit and occupy tributaries that connect the main fjords through the mountain passes. During deglaciation, ice in these tributaries flowed to the major ice streams. Sections across three De Geer moraines show that the ridges are composed of diamictons and fine-grained sediment, partly in stacked sequences. The diamicton units are interpreted as being composed of water-lain tills, lodgements tills and subaqueous flow deposits. The fine-grained sediment is though to have formed in a proglacial marine environment. Clast fabric of diamictons and deformation structures in underlying sands show that depositional directions for diamicton units and the direction of deformation for the sands is perpendicular to the ridge crests. Mainly based on this evidence, the ridges are thought to have formed by push at the glacier grounding line. The formation of transverse ridges (relative to ice flow) do occur in basal crevasses on modern glaciers, as do swarms of ridges along the front of retreating glaciers. The first mechanism of deposition does not seem to explain the ridges studied in the present paper and hence the importance of this process in the formation of De Geer moraines is questioned. The De Geer moraines were deposited by ice lobes advancing from one main fjord into another; therefore by studying the drainage pattern of the tributary lobes and their sequence of deglaciation, many features of the style of deglaciation of the ice sheet across the area can be determined. The northwestern part of the area was deglaciated earliest. After that, deglaciation proceeded to the southwest parallel to the coast. Subsequently the outer and the central part of Romsdalsfjorden were deglaciated causing ice to drain towards this fjord from both the north and south. The last fjord to be deglaciated was Storfjorden in the south.     

Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines

The extent and behaviour of the southeast margin of the Laurentide Ice Sheet in Atlantic Canada is of significance in the study of Late Wisconsinan ice sheet-ocean interactions. Multibeam sonar imagery of subglacial, ice-marginal and glaciomarine landforms on German Bank, Scotian Shelf, provides evidence of the pattern of glacial-dynamic events in the eastern Gulf of Maine. Northwest-southeast trending drumlins and megaflutes dominate northern German Bank. On southern German Bank, megaflutes of thin glacial deposits create a distinct northwest-southeast grain. Lobate regional moraines (>10km long) are concave to the northwest, up-ice direction and strike southwest-northeast, normal to the direction of ice flow. Ubiquitous, overlying De Geer moraines (<10 km long) also strike southwest-northeast. The mapped pattern of moraines implies that, shortly after the last maximum glaciation, the tidewater ice sheet began to retreat north from German Bank, forming De Geer moraines at the grounding line with at least one glacial re-advance during the general retreat. The results indicate that the Laurentide Ice Sheet extended onto the continental shelf.     

Submarine sediment and landform record of a palaeo‐ice stream within the British−Irish Ice Sheet

This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice‐stream system that drained ～3% of the entire British?Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice‐stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid‐shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft‐bed (sediment) and hard‐bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast‐flowing ice‐sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice‐bed coupling in a zone of flow convergence associated with ice‐stream onset; and a downstream bedform evolution (short drumlins to km‐scale glacial lineations) suggesting an ice‐flow velocity transition associated with a bed substrate and roughness change in the ice‐stream trunk. Chronology is still lacking for the timing of ice‐stream demise; however, the seismic stratigraphy, absence of moraines or grounding‐line features, and presence of well‐preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long‐lived, ice‐sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice‐discharge values for the Minch ice stream (12–20 Gt a^?1) are comparable to high mass‐flux ice streams today, underlining it as an excellent palaeo‐analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.     

Seafloor evidence for palaeo-ice streaming and calving of the grounded Irish Sea Ice Stream: Implications for the interpretation of its final deglaciation phase

High resolution swath bathymetry data reveal a previously glaciated submarine terrain 20 km offshore Anglesey, north Wales, UK. The detailed documentation of remarkably well-preserved subglacial and ice-marginal bedforms provides evidence for a grounded part of the Irish Sea Ice Stream in a phase of deglaciation. The observed ribbed moraines, drumlins, flutes and eskers indicate a converging ice flow to the west, which then turns south into the deeper central Irish Sea Basin. Using the relative position of the bedforms, their spatial distribution and the morphological resemblance with bedforms described in the literature, this subglacial terrain is interpreted as representing a transition zone of frozen to thawed bed conditions during deglaciation, with an eastwards migrating thawing front that partly altered the edge of the surveyed ribbed moraine field by drumlinization. The abundant De Geer moraines and iceberg scour marks superimposed on drumlins and flutes reveal that the final retreat of the grounded ice margin in the surveyed area terminated into a water-mass with extensive iceberg calving. As the glacial terrain is well preserved, no significant burial has taken place, either by glacially or terrestrially derived sediment. The strong tidal currents at present keep the submarine terrain swept clean of contemporary sediment cover.     

Ribbed moraine formed by subglacial folding, thrust stacking and lee-side cavity infill

Transverse-to-iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine-dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee-side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre-existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity-flow deposition in lee-side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice-marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up-glacier compared to a less mobile bed down-glacier. It is considered that the lee-side cavities were formed as a result of ice-bed separation on the distal slope of the thrust/fold-created obstruction. The lee-side cavities formed an integral part of a subglacial linked-cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down-flow at the receding ice-sheet margin.     

Seafloor glacial features reveal the extent and decay of the last British Ice Sheet,east of Scotland

Three‐dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1–2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast‐flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd.     

Geophysical surveys of the sediments of Loch Ness,Scotland: implications for the deglaciation of the Moray Firth Ice Stream,British–Irish Ice Sheet

We present results from three geophysical campaigns using high‐resolution sub‐bottom profiling to image sediments deposited in Loch Ness, Scotland. Sonar profiles show distinct packages of sediment, providing insight into the loch's deglacial history. A recessional moraine complex in the north of the loch indicates initial punctuated retreat. Subsequent retreat was rapid before stabilisation at Foyers Rise formed a large stillstand moraine. Here, the calving margin produced significant volumes of laminated sediments in a proglacial fjord‐like environment. Subsequent to this, ice retreated rapidly to the southern end of the loch, where it again deposited a sequence of proglacial laminated sediments. Sediment sequences were then disturbed by the deposition of a thick gravel layer and a large turbidite deposit as a result of a jökulhlaup from the Spean/Roy ice‐dammed lake. These sediments are overlain by a Holocene sheet drape. Data indicate: (i) a former tributary of the Moray Firth Ice Stream migrated back into Loch Ness as a major outlet glacier with a calving margin in a fjord‐like setting; (ii) there was significant sediment supply to the terminus of this outlet glacier in Loch Ness; and (iii) that jökulhlaups are important for sediment supply into proglacial fjord/lake environments and may compose >20% of proglacial sedimentary sequences. Copyright © 2011 John Wiley & Sons, Ltd.     

Processes of annual moraine formation at a temperate alpine valley glacier: insights into glacier dynamics and climatic controls

This paper presents the first detailed sedimentological study of annual moraines formed by an alpine valley glacier. The moraines have been forming since at least AD 1980 by a subsidiary lobe of Gornergletscher, Switzerland that advances up a reverse bedrock slope. They reach heights of 0.5–1.5 m, widths of up to 6 m and lengths of up to several hundreds of metres. Sediments in these moraines are composed of proglacial outwash and debris flow units; subglacial traction till is absent entirely. Based on four representative sections, three genetic process combinations have been identified: (i) inefficient bulldozing of a gently sloping ice margin transfers proglacial sediments onto the ice, causing differential ablation and dead‐ice incorporation upon retreat; (ii) terrestrial ice‐contact fans are formed by the dumping of englacial and supraglacial material from point sources such as englacial conduit fills; debris flows and associated fluvial sediments are stacked against a temporarily stationary margin at the start, and deformed during glacier advance in the remainder, of the accumulation season; (iii) a steep ice margin without supraglacial input leads to efficient bulldozing and deformation of pre‐existing foreland sediments by wholesale folding. Ice‐surface slope appears to be a key control on the type of process responsible for moraine formation in any given place and year. The second and third modes result in stable and higher moraines that have a higher preservation potential than those containing dead ice. Analysis of the spacing and climatic records at Gornergletscher reveals that winter temperature controls marginal retreat and hence moraine formation. However, any climatic signal is complicated by other factors, most notably the presence of a reverse bedrock slope, so that the extraction of a clear climatic signal is not straightforward. This study highlights the complexity of annual moraine formation in high‐mountain environments and suggests avenues for further research.     

Contemporary terminal-moraine ridge formation at a temperate glacier: Styggedalsbreen, Jotunheimen, southern Norway

Terminal-moraine ridges up to 6 m high have been forming at the snout of Styggedalsbreen for two decades. Based on intermittent observations during this period, combined with a detailed study of ridge morphology, sedimentary structures and composition during the 1993 field season, a model of terminal-moraine formation that involves the interaction of glacial and glacio-fluvial processes at a seasonally oscillating ice margin is presented. In winter, subglacial debris is frozen-on to the glacier sole; in summer, ice-marginal and supraglacial streams deposit sediments on the wasting ice tongue. The ice tongue overrides an embryonic moraine ridge during a late-winter advance and a double layer of sediment (diamicton overlain by sorted sands and gravels) is added to the moraine ridge during the subsequent ablation season. Particular ridges grow incrementally over many years and exert positive feedback by enhancing snout up-arching during the winter advance and constraining the course of summer meltwater streams close to the ice margin. The double-layer annual-meltout model is related to moraine formation by the stacking of subglacial frozen-on sediment slabs (Krüger 1993). Moraine ridges of this type have a complex origin. are not push moraines, and may be characteristic of dynamic high-latitude and high-altitude temperate glaciers.     

Glacier thermal regime linked to processes of annual moraine formation at Midtdalsbreen,southern Norway

Glacier thermal regime is shown to have a significant influence on the formation of ice‐marginal moraines. Annual moraines at the margin of Midtdalsbreen are asymmetrical and contain sorted fine sediment and diamicton layers dipping gently up‐glacier. The sorted fine sediments include sands and gravels that were initially deposited fluvially directly in front of the glacier. Clast‐form data indicate that the diamictons have a mixed subglacial and fluvial origin. Winter cold is able to penetrate through the thin (<10 m) ice margin and freeze these sediments to the glacier sole. During winter, sediment becomes elevated along the wedge‐shaped advancing glacier snout before melting out and being deposited as asymmetrical ridges. These annual moraines have a limited preservation potential of ～40 years, and this is reflected in the evolution of landforms across the glacier foreland. Despite changing climatic conditions since the Little Ice Age and particularly within the last 10 years when frontal retreat has significantly speeded up, glacier dynamics have remained relatively constant with moraines deposited via basal freeze‐on, which requires stable glacier geometry. While the annual moraines on the eastern side of Midtdalsbreen indicate a slow steady retreat, the western foreland contains contrasting ice‐stagnation topography, highlighting the importance of local forcing factors such as shielding, aspect and debris cover in addition to changing climate. This study indicates that, even in temperate glacial environments, restricted or localised areas of cold‐based ice can have a significant impact on the geomorphic imprint of the glacier system and may actually be more widespread within both modern and ancient glacial environments than previously thought.     

Paleolimnology of two lakes in the Klutlan Glacier region,Yukon Territory,Canada

Lakes developed on progressively younger end moraines of the Klutlan Glacier were initially assumed to have originated shortly after moraine emplacement and to have persisted to the present. Limnological differences between lakes on old vs young moraines were thought to result from limnological maturation within the lakes and ponds themselves and in response to the development of soils and vegetation on moraine surfaces. This study represents a paleolimnological test of this hypothesis. If true, the first-formed sediments of lakes on old moraines should be comparable to sediments presently forming in lakes on young moraines. Geochemical and paleontological studies of surface sediment to a series of lakes on progressively older moraines provide baseline information for comparing successive levels of lake sediment cores from older moraines. Results indicate that the time of lake initiation seldom reflects moraine age. Even on the oldest moraine (Harris Creek), lake basins are presently forming. Their sediment character more closely relates to the rapidity of basin formation due to melting of buried ice than to age of the lake itself or of the moraine on which it is situated. Vegetation and soil development play an important but secondary role in determining the character of lake sediments; rapid subsidence can convert humic-water lakes surrounded by second-generation spruce forests into turbid-water lakes with unstable, slumping margins. A detailed paleolimnological study of two lakes, one on the unglaciated upland and another in an outwash channel penetrating the oldest moraine, revealed progressive limnologic changes through time, suggesting that their basins were stable for 1200 and 400 yr, respectively. The changes in diatom stratigraphy of these lakes appear to relate to natural limnological changes associated with lake maturation and accumulation of nutrients as well as to changes in the surrounding vegetation and soils.     

Holocene glacial evolution of Mohnbukta in eastern Spitsbergen

Submarine geomorphology is one of the main tools for understanding past fluctuations of tidewater glaciers. In this study we investigate the glacial history of Mohnbukta, on the east coast of Spitsbergen, Svalbard, by combining multibeam‐bathymetric data, marine sediment cores and remote sensing data. Presently, three tidewater glaciers, Heuglinbreen, Königsbergbreen and Hayesbreen calve into Mohnbukta. Hayesbreen surged at the end of the Little Ice Age, between 1901 and 1910. The submarine landform assemblage in Mohnbukta contains two large transverse ridges, interpreted as terminal moraines, with debrisflow lobes on their distal slopes and sets of well‐preserved geometric networks of ridges, interpreted as crevasse‐squeeze ridges inshore of the moraines. The arrangement of crevasse‐squeeze ridges suggests that both landform sets were produced during surge‐type advances. The terminus position of the 1901–1910 Hayesbreen surge correlates with the inner (R.2) terminal moraine ridge suggesting that the R.1 ridge formed prior to 1901. Marine sediment cores display ¹⁴C ages between 5700–7700 cal. a BP derived from benthic foraminifera, from a clast‐rich mud unit. This unit represents pre‐surge unconsolidated Holocene sediments pushed in front of the glacier terminus and mixed up during the 1901 surge. An absence of retreat moraines in the deeper part of the inner basin and the observation of tabular icebergs calving off the glacier front during retreat suggest that the front of Hayesbreen was close to flotation, at least in the deeper parts of the basin. As the MOH15‐01 core does not penetrate into a subglacial till and the foraminifera in the samples were well preserved, the R.1 ridge is suggested to have formed prior to the deposition of the foraminifera. Based on these data we propose that a surge‐type advance occurred in Mohnbukta in the early Holocene, prior to 7700 cal. a BP, which in turn indicates that glaciers can switch to and from surge mode.     

Stratigraphic architecture and sedimentology of a Late Pleistocene subaqueous moraine complex,southwest Ireland

Glacigenic sediments exposed in coastal cliffs cut through undulatory terrain fronting the Last Glacial Maximum laterofrontal moraine at Waterville on the Iveragh Peninsula, southwest Ireland, comprise three lithofacies. Lithofacies 1 and 2 consist of interdigitated, offlapping and superimposed ice‐proximal subaqueous outwash and stacked sequences of cohesionless and cohesive subaqueous debris flows, winnowed lag gravels and coarse‐grained suspension deposits. These are indicative of sedimentation in and around small grounding line fans that prograded from an oscillating glacier margin into a proglacial, interlobate lake. Lithofacies 3 comprises braided river deposits that have undergone significant syn‐sedimentary soft‐sediment deformation. Deposition was likely related to proglacial outwash activity and records the reduction of accommodation space for subaqueous sedimentation, either through the lowering of proglacial water levels or due to basin infilling. The stratigraphic architecture and sedimentology of the moraine at Waterville highlight the role of ice‐marginal depositional processes in the construction of morphostratigraphically significant ‘end moraine’ complexes in Great Britain and Ireland. Traditional ‘tills’ in these moraines are often crudely stratified diamictons and gravelly clinoforms deposited in ice‐proximal subaqueous and subaerial fans. Copyright © 2011 John Wiley & Sons, Ltd.     

The last deglaciation of the Norwegian Channel – geomorphology,stratigraphy and radiocarbon dating

Based on high‐resolution TOPAS acoustic data, bathymetric data sets and sediment cores from the Norwegian Channel, the last retreat of the Norwegian Channel Ice Stream has been investigated. Mapping of ice‐marginal features such as grounding‐zone wedges and terminal moraines off western Norway suggest that the retreat of the grounding line in this part of the channel was interrupted by frequent stillstands, whereas the channel south of the threshold at Jæren does not have crossing ice‐marginal landforms. Three main seismic units have been identified, and, based on their seismic characteristics, in addition to study of sediment cores, these units are interpreted as till (U1), glacial marine sediment (U2) and Holocene hemipelagic sediment (U3). Based on new and published radiocarbon dates of the lower part of U2, combined with dates from the adjacent areas, it is concluded that the grounding line started to retreat from the shelf edge at about 19 ka and that the inner part of Skagerrak was ice free at 17.6 ka. This gives an average retreat rate of 450 m a⁻¹, which is generally higher than mean retreat rates estimated for other palaeo‐ice streams (15–310 m a⁻¹).     

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.     

Push-moraines and glacier-contact fans in marine and terrestrial environments

ABSTRACT The local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts, which in turn determine the sediments and landforms produced at the glacier front. Many modern glaciers undergoing overall retreat in areas of'maritime'climate produce winter push moraines during a late winter readvance, followed by a summer retreat, whilst in more'continental'regions no significant winter readvance occurs and annual push-moraines are absent. The frontal dynamics which lead to these changes are analysed and the form, structure, sequence and field relations of both terrestrial and marine push-moraines are described from Iceland, Spitsbergen and Baffin Island.
Long-term changes in mass balance leading to major glacier advances or readvances also generate large push-moraines. In terrestrial environments push-moraine formation is accompanied by uplift, rejuvenation and down-cutting of outwash systems whose sediments become closely associated with glaciotectonic structures, which permit pre-, syn- and post-tectonic sequences to be identified.
The development of ice marginal fan/moraine complexes is modelled as a function of the relative magnitude of two parameters: the velocity of ice movement and the calving rate. A high ice velocity just exceeded by the calving rate gives closely spaced push-moraines and confluent ice marginal fans. A high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans. A low ice velocity in combination with a high calving rate results in well separated and feebly developed push-moraines, while a low ice velocity and a low calving rate produces feeble push-moraines and coalescent fans.     

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.     

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.