The meltwater hypothesis for subglacial bedforms

This paper presents an historical and in places informal account of the meltwater hypothesis, which invokes enormous outburst floods for the formation of subglacial bedforms. It begins with a brief discussion of the difficulties of determining processes of formation for landforms, which are not seen in formation. Analogy provides a solution to these difficulties. Analogy between erosional marks at the bases of turbidites and drumlins, which were the starting point for this hypothesis, rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins. Field tests on the sedimentology, architecture, and landform associations of drumlins in the Livingstone Lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced. Bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial. Their form and genesis are discussed mainly with reference to sites at French River and Wilton Creek, Ontario, though some remarkable bedrock erosional forms in Antarctica support their regional extent. Initially in the meltwater hypothesis, drumlins were thought to be cavity fills and erosional drumlins were recognized later. This development is shown to be central to the realization that drumlin composition may be inferred from drumlin form. The scale of drumlin fields, measured at about 10³ km², and the magnitude of the inferred floods require that the flood events were regional. Regional-scale flood tracts in Ontario, Quebec, Alberta and the Northwest Territories extending over 1000 km in length and several hundred kilometers in width, support this suggestion. Floods, had they occurred, would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures. The meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marks—fluting, Rogen moraine, hummocky terrain, and transverse ridges. Recent work shows how these forms are best explained by the meltwater hypothesis. The roles of water storage and release, which underpin the theory of the meltwater hypothesis, remain poorly understood.     

Is most hummocky cross‐stratification formed by large‐scale ripples?

Although normal isotropic hummocky cross‐stratification is commonly interpreted to be the deposit of large‐scale ripples, there are many reasons why this may not usually be the case. These reasons include: (i) that the stratification produced by large‐scale ripples does not particularly look like isotropic hummocky cross‐stratification; (ii) that it is difficult reconciling the abundance of HCS with the restricted hydraulic stability of large‐scale ripples in silt to fine sand (i.e. the grain sizes in which hummocky cross‐stratification is usually found); (iii) that the distribution of hummocky cross‐stratification within ancient storm beds is not the distribution that would be expected from large‐scale ripples; (iv) that the flows calculated to have formed ancient examples of hummocky cross‐stratification would be expected to generate an upper stage plane bed rather than ripples; and (v) that it is difficult to explain why large‐scale ripples would predominate in the proximal parts of storm beds when modern storm flows commonly exceed the threshold for entrainment. In contrast to the various hypotheses which propose that isotropic hummocky cross‐stratification is generated by ripples, an alternative hypothesis which suggests that it is generated by instabilities, does seem to adequately explain the origin of hummocky cross‐stratification. However, it is difficult to accept this hypothesis given that the origin of the proposed instabilities is unproven. These conclusions highlight the continued uncertainty regarding the process, which generates hummocky cross‐stratification.     

Lateglacial ice dynamics of the Cordilleran Ice Sheet in northern British Columbia and southern Yukon Territory: retreat pattern of the Liard Lobe reconstructed from the glacial landform record

The Liard Lobe formed a part of the north‐eastern sector of the Cordilleran Ice Sheet and drained ice from accumulation areas in the Selwyn, Pelly, Cassiar and Skeena mountains. This study reconstructs the ice retreat pattern of the Liard Lobe during the last deglaciation from the glacial landform record that comprises glacial lineations and landforms of the meltwater system such as eskers, meltwater channels, perched deltas and outwash fans. The spatial distribution of these landforms defines the successive configurations of the ice sheet during the deglaciation. The Liard Lobe retreated to the west and south‐west across the Hyland Highland from its local Last Glacial Maximum position in the south‐eastern Mackenzie Mountains where it coalesced with the Laurentide Ice Sheet. Retreat across the Liard Lowland is evidenced by large esker complexes that stretch across the Liard Lowland cutting across the contemporary drainage network. Ice margin positions from the late stage of deglaciation are reconstructed locally at the foot of the Cassiar Mountains and further up‐valley in an eastern‐facing valley of the Cassiar Mountains. The presented landform record indicates that the deglaciation of the Liard Lobe was accomplished mainly by active ice retreat and that ice stagnation played a minor role in the deglaciation of this region. Copyright © 2013 John Wiley & Sons, Ltd.     

Regional subglacial quarrying and abrasion below hard-bedded palaeo-ice streams crossing the Shield–Palaeozoic boundary of central Canada: the importance of substrate control

Three-dimensional surface visualization models derived from high-resolution LiDAR data provide new information about the type and scale of erosional processes below Late Wisconsin palaeo-ice streams traversing the boundary between Canadian Shield crystalline rocks with offlapping Palaeozoic limestones in central Ontario. The hard bed is directly analogous to that found below ice streams in East Antarctica and East Greenland and provides insight into the effects of abrupt changes in substrate type on subglacial processes. Erosion of hard crystalline Canadian Shield rock was largely ineffectual consisting of areal abrasion of rounded whalebacks and local lee side plucking. In contrast, fast flow over the strike of gently dipping well-bedded and jointed Palaeozoic limestones cut large flow-parallel grooves and ridges akin to mega-scale glacial lineations reflecting intense abrasion below narrow streams of subglacial debris dominated by hard crystalline Shield clasts (erodents). Regionally extensive plucking of structurally weak, well-jointed and bedded limestone produced large volumes of rubbly carbonate debris leaving a 25-km-wide belt of uncontrolled hummocky rubble terrain (long known as the Dummer Moraine in Southern Ontario) some 350 km long and locally as much as 10 m thick. Subglacial plucking and abrasion under fast flowing ice were highly effective in stripping limestone cover rocks from Precambrian basement, and over many glacial cycles, may have played a role in the location and excavation of numerous large and deep lake basins around the Shield–Palaeozoic boundary zone in North America.     

Boulder-gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south-central Michigan, USA

Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.     

Relative sea‐level changes,glacial isostatic modelling and ice‐sheet reconstructions from the British Isles since the Last Glacial Maximum

While contributing <1 m equivalent eustatic sea‐level rise the British Isles ice sheet produced glacio‐isostatic rebound in northern Britain of similar magnitude to eustatic sea‐level change, or global meltwater influx, over the last 18 000 years. The resulting spatially variable relative sea‐level changes combine with observations from far‐field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment, local ice‐sheet history and global meltwater influx. After a review of the attributes of relative sea‐level observations significant for constraining large‐scale models of the isostatic adjustment process we summarise long records of relative sea‐level change from the British Isles and far‐field locations. We give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the British Isles and far‐field for a range of Earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the British Isles. For the first time we show a good degree of fit between relative sea‐level observations and predictions that are based upon global Earth and ice model parameters, independently derived from analysis of far‐field data, with a terrain‐corrected model of the British Isles ice sheet that includes extensive glaciation of the North Sea and western continental shelf, that does not assume isostatic equilibrium at the Last Glacial Maximum and keeps to trimline constraints of ice surface elevation. We do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local‐scale detail of the ice model in order to provide a fit for all sites, but argue that the next stage should be to incorporate an ice‐sheet model that is based on quantitative, glaciological model simulations. We hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology, glaciology, sea‐level change, Earth rheology and quantitative modelling. Copyright © 2006 John Wiley & Sons, Ltd.     

Subglacial erosion forms in northwest Ireland

Subglacial erosional forms are commonly found on bedrock substrates inside the Late Weichselian ice margin in County Donegal, northwest Ireland, and can be used to provide detailed information on subglacial processes and environments. The erosional forms occur on spatial scales from whalebacks (tens of metres in scale), to asymmetric and channelized bedrock-cut scours (tens of cm in scale) and striations (mm scale). Processes responsible for development of subglacial erosional forms occur along a continuum, from free meltwater existing as a laterally extensive sheet at the ice-bed interface, to abrasion by basal ice. Channelized bedrock-cut scours are particularly common in County Donegal, and show asymmetric and meandering thalwegs, U-shaped cross-profiles and steep lateral margins. Innermost parts of the scours are highly polished and have striations that follow thalweg direction. In places, bedrock surfaces are overlain by a delicate polish and thin calcite cement, and are buried beneath glacial till. Based on their morphology, the bedrock scours are interpreted as s-forms caused by high-pressure subglacial meltwater erosion. Striations within the scoured channels reflect periods of ice-bed coupling and subglacial abrasion. The range of features observed here was used to consider relationships between subglacial topography, hydraulic processes and ice-bed coupling. Precipitation of calcite cement took place in depressions on the bedrock surface by CO₂ degassing. Infilling of depressions by glacial till formed a new type of 'sticky spot' related to spatial variations in subglacial water pressure. The temporal evolution of sticky spots reflects interactions within the subglacial environment between subglacial relief, hydraulic regime and ice-bed coupling.     

Genesis of hummocky moraine in the Bolmen area, southwestern Sweden

During the late Weichselian deglaciation of southern Scandinavia vast areas of hummocky moraine were formed. The genesis of this landform was studied by geomorphological and sedimentological methods in an area in southwestern Sweden. Four exposures in moraine hummocks were investigated using sedimentological methods. They were found to be composed of stratified diamictons with frost-shattered boulders and deformed intrabeds of sorted sediments. The diamictons were interpreted as sediment-flow deposits, with the hummocks formed in stagnant ice by flowage of supraglacial glacial debris into depressions and subsequent inversion of the landscape due to ice melt. With the exception of hummocky moraine, the study area contains low relief moraine, which like the hummocky moraine was supraglacially formed. A new model is presented where the distribution of hummocky and low relief moraine is dependent on the vertical distribution of glacial debris in the ice sheet, which in turn is related to the flow regime of the ice sheet prior to stagnation. A compressive flow before stagnation favoured development of the hummocky moraine, while low relief moraine formation occurred where the ice flow was extending or at steady state.     

Landform and sediment imprints of fast glacier flow in the southwest Laurentide Ice Sheet

Evidence for former fast glacier flow (ice streaming) in the southwest Laurentide Ice Sheet is identified on the basis of regional glacial geomorphology and sedimentology, highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream. Preliminary mapping from a digital elevation model of Alberta identifies corridors of smoothed topography and corridor‐parallel streamlined landforms (megaflutes to mega‐lineations) that display high levels of spatial coherency. Ridges that lie transverse to the dominant streamlining patterns are interpreted as: (a) series of minor recessional push moraines; (b) thrust block moraines or composite ridges/hill–hole pairs constructed during readvances/surges; and (c) overridden moraines (cupola hills), apparently of thrust origin. Together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms. Localised cross‐cutting and/or misalignment of flow sets indicates temporal separation and the overprinting of ice streams/lobes. The fast‐flow tracks are separated by areas of interlobate or inter‐stream terrain in which moraines have been constructed at the margins of neighbouring (competing) ice streams/outlet glaciers; this inter‐stream terrain was covered by more sluggish, non‐streaming ice during full glacial conditions. Thin tills at the centres of the fast‐flow corridors, in many places unconformably overlying stratified sediments, suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams/outlet glaciers is consistent with subglacial deformation theory. In this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory, sometimes with fast‐flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally. The occurrence of minor push moraines and flutings and associated landforms, because of their similarity to modern active temperate glacial landsystems, are interpreted as indicative of ice lobe marginal oscillations, possibly in response to seasonal climatic forcing, in locations where meltwater was more effectively drained from the glacier bed. Further north, the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging, possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice‐bed decoupling and drawdown initiated by the development of ice‐dammed lakes. Copyright © 2008 John Wiley & Sons, Ltd.     

Palaeoglaciation of Bayan Har Shan,northeastern Tibetan Plateau: glacial geology indicates maximum extents limited to ice cap and ice field scales

Key locations within an extensive area of the northeastern Tibetan Plateau, centred on Bayan Har Shan, have been mapped to distinguish glacial from non‐glacial deposits. Prior work suggests palaeo‐glaciers ranging from valley glaciers and local ice caps in the highest mountains to a regional or even plateau‐scale ice sheet. New field data show that glacial deposits are abundant in high mountain areas in association with large‐scale glacial landforms. In addition, glacial deposits are present in several locations outside areas with distinct glacial erosional landforms, indicating that the most extensive palaeo‐glaciers had little geomorphological impact on the landscape towards their margins. The glacial geological record does indicate extensive maximum glaciation, with local ice caps covering entire elevated mountain areas. However, absence of glacial traces in intervening lower‐lying plateau areas suggests that local ice caps did not merge to form a regional ice sheet on the northeastern Tibetan Plateau around Bayan Har Shan. No evidence exists for past ice sheet glaciation. Copyright © 2009 John Wiley & Sons, Ltd.     

Permian glaciated bedrock surfaces and associated sediments on Kangaroo Island,South Australia: Implications for local Gondwanan ice‐mass dynamics

Late Palaeozoic glaciated rock surfaces and associated sediments occur along the northeastern coast of Kangaroo Island. The erosional forms include glacially polished rock surfaces, striae, grooves, chatter marks, friction cracks, crescentic gouges, p‐forms, sichelwannen, miniature rock crag‐and‐tails and roches moutonnées. The distribution and orientation of these along with till fabrics indicate a general northwesterly ice flow in this part of the Troubridge Basin. The glacial erosional forms and the presence of thick lodgement till imply that the local basal ice was at pressure‐melting point during their formation. Temperate to subpolar glacial ice conditions, similar to those currently prevailing in glaciers in Spitsbergen, are inferred.     

Sediment‐rich meltwater plumes and ice‐proximal fans at the margins of modern and ancient tidewater glaciers: Observations and modelling

Turbid meltwater plumes and ice‐proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers. However, the spacing and temporal stability of these subglacial channels is poorly understood. This has significant implications for understanding the geometry and distribution of Quaternary and ancient ice‐proximal fans that can form important aquifers and hydrocarbon reservoirs. Remote‐sensing and numerical‐modelling techniques are applied to the 200 km long marine margin of a Svalbard ice cap, Austfonna, to quantify turbid meltwater‐plume distribution and predict its temporal stability. Results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice‐proximal fans. Plumes are spaced ca 3 km apart and their distribution along the ice front is stable over decades. Numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap; modelled water‐routing patterns are in reasonable agreement with satellite‐mapped plume locations. However, glacial retreat of several kilometres over the past 40 years has limited build‐up of significant ice‐proximal fans. A single fan and moraine ridge is noted from marine‐geophysical surveys. Closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans. Schematic models of ice‐proximal deposits represent varying glacier‐terminus stability: (i) stable terminus where meltwater sedimentation produces an ice‐proximal fan; (ii) quasi‐stable terminus, where glacier readvance pushes or thrusts up ice‐proximal deposits into a morainal bank; and (iii) retreating terminus, with short still‐stands, allowing only small sediment lobes to build up at melt‐stream portals. These modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient, Ordovician glacial system. Thick turbidite successions and large fans in the Late Ordovician suggest either high‐magnitude events or sustained high discharge, consistent with a relatively mild palaeo‐glacial setting for the former North African ice sheet.     

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.     

Configuration,history and impact of the Norwegian Channel Ice Stream

The Norwegian Channel between Skagerrak, in the southeast, and the continental margin of the northern North Sea, in the northwest, is the result of processes related to repeated ice stream activity through the last 1.1 m yr. In such periods the Skagerrak Trough (700 m deep) has acted as a confluence area for glacial ice from southeastern Norway, southern Sweden and parts of the Baltic. Possibly related to the threshold in the Norwegian Channel off Jæren (250 m deep), the ice stream, on a number of occasions over the last 400 ka, inundated the coastal lowlands and left an imprint of NW‐oriented ice directional features (drumlins, stone orientations in tills and striations). Marine interstadial sediments found up to 200 m a.s.l. on Jæren have been suggested to reflect glacial isostasy related to the Norwegian Channel Ice Stream (NCIS). In the channel itself, the ice stream activity is evidenced by mega‐scale glacial lineations on till surfaces. As a result of subsidence, the most complete sedimentary records of early phases of the NCIS are preserved close to the continental margin in the North Sea Fan region. The strongest evidence for ice stream erosion during the last glacial phase is found in the Skagerrak. On the continental slope the ice stream activity is evidenced by the large North Sea Fan, which is mainly a result of deposition of glacial‐fed debris flows. Northwards of the North Sea Fan, rapid deposition of meltwater plume deposits, possibly related to the NCIS, is detected as far north as the Vøring Plateau. The NCIS system offers a unique possibility to study ice stream related processes and the impact the ice stream development had on open ocean sedimentation and circulation.     

The role of shelf morphology on storm‐bed variability and stratigraphic architecture,Lower Cretaceous,Svalbard

The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.     

Basal processes beneath an Arctic glacier and their geomorphic imprint after a surge, Elisebreen, Svalbard

The foreground of Elisebreen, a retreating valley glacier in West Svalbard, exhibits a well-preserved assemblage of subglacial landforms including ice-flow parallel ridges (flutings), ice-flow oblique ridges (crevasse-fill features), and meandering ridges (infill of basal meltwater conduits). Other landforms are thrust-block moraine, hummocky terrain, and drumlinoid hills. We argue in agreement with geomorphological models that this landform assemblage was generated by ice-flow instability, possibly a surge, which took place in the past when the ice was thicker and the bed warmer. The surge likely occurred due to elevated pore-water pressure in a thin layer of thawed and water-saturated till that separated glacier ice from a frozen substratum. Termination may have been caused by a combination of water drainage and loss of lubricating sediment. Sedimentological investigations indicate that key landforms may be formed by weak till oozing into basal cavities and crevasses, opening in response to accelerated ice flow, and into water conduits abandoned during rearrangement of the basal water system. Today, Elisebreen may no longer have surge potential due to its diminished size. The ability to identify ice-flow instability from geomorphological criteria is important in deglaciated terrain as well as in regions where ice dynamics are adapting to climate change.     

The role of meltwater in glacial processes

Water plays a dominant role in many glacial processes and the erosional, depositional and climatic significance of meltwaters and associated fluvioglacial processes cannot be overemphasized. At its maximum extent c. 20,000 years ago, the volume of the Laurentide ice sheet was 33 × 10⁶ km³ (about the same as the volume of all ice present today on planet Earth). The bulk of this was released as water in little more than 10,000 years. Pulses of meltwater flowing to the Atlantic Ocean from large ice dammed lakes altered thermohaline circulation of the world's oceans and global climate. One such discharge event via Hudson Bay at 8200 years BP released 160,000 km³ of water in 12 months. Global sea levels recovered from glacial maximum low stands reached at about 20,000 years ago at an average rate of 15 m per thousand years but estimates of shorter term rates suggest as much as 20 m sea level rise in 1000 years and for short periods, rates as high as 4 m per hundred years. Meltwaters played a key role in lubricating ice sheet motion (and thus areal abrasion) across the inner portions of the ice sheet where it slid over rigid crystalline bedrock of the Canadian Shield. The recharge of meltwater into the ice sheets bed was instrumental in generating poorly sorted diamict sediments (till) by sliding-induced shearing and deformation of overpressured sediment and soft rock. The transformation of overpressured till into hyperconcentrated slurries in subglacial channels may have generated a highly effective erosional tool for selective overdeepening and sculpting of bedrock substrates. Some workers credit catastrophic subglacial ‘megafloods’ with the formation of drumlins and flutes on till surfaces. Subglacial melt river systems were instrumental in reworking large volumes of glaciclastic sediment to marine basins; it has been estimated that less than 6% of the total volume of glaciclastic sediment produced during the Pleistocene remains on land. Fluvioglacial and glaciolacustrine sediments and landforms dominate large tracts of the ‘glacial’ landscape in North America. The recharge of subglacial meltwater into underlying bedrock and sediment aquifers created transient reversals in the long-term equilibrium flow directions of basinal fluids. With regard to pre-Pleistocene glacial record, meltwaters moved enormous volumes of terrestrial ‘glaciclastic’ sediment to marine basins and thus played a key role in preserving a record of glaciation, a record otherwise almost entirely lost on land.     

The Bothnian Sea ice stream: early Holocene retreat dynamics of the south‐central Fennoscandian Ice Sheet

The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.     

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.     

Tunnel channels in southeast Alberta, Canada: : evidence for catastrophic channelized drainage

Tunnel channels in southeast Alberta are attributed to erosion by channelized, subglacial meltwater flows. An anabranching tunnel channel network dissects the preglacial drainage divide of the ancestral Milk River. Channel morphology and landform associations are used to evaluate competing hypotheses of tunnel channel formation. Mechanisms that invoke subaerial channel incision, direct glacial erosion or steady state, time-transgressive erosion at the ice margin cannot explain convex-up longitudinal channel profiles, anabranching channel networks or confinement to the preglacial drainage divide. Results conclude that the tunnel channel network in southeast Alberta represents late-stage erosion by a channelised subglacial flow of catastrophic dimensions. Interpretations for this tunnel channel network are in agreement with conclusions obtained for the regional subglacial landform assemblage.