Meltwater Channels Research Articles

Depositional Environments and Archeological Site Formation in the Cypress Hills, Southeastern Alberta

Previous research on the Cypress Hills of southeastern Alberta and southwestern Saskatchewan suggests that they have experienced a dynamic history of Holocene landscape change. In particular, the meltwater channels flanking the Cypress Hills appear to have experienced intermittent deposition of sediment transported from upslope alternating with episodes of landscape stability and soil formation. This pattern favours the formation of archeological sites containing multiple stratigraphically discrete occupations, an important characteristic because the geomorphic processes necessary to create such interpretively valuable sites are rare across much of the Northern Plains. Furthermore, the rich resources offered by the Cypress Hills suggest that they were attractive to past human groups, a feature that enhances the prospects for such sites. This paper discusses a study designed to investigate the geomorphic history of the meltwater channels and to identify buried archeological sites in these channels. The preliminary results of this study confirm that the channels have experienced a cyclical pattern of stability and deposition favourable to the stratified burial of cultural remains. Moreover, this study has recovered archeological material from numerous locations, strongly suggesting that the meltwater channels represent an excellent environment for the formation of high-quality sites with multiple discrete cultural layers.

Glacial erosional landforms: origins and significance for palaeoglaciology

Glacial inversion modelling of continental-scale palaeo-ice sheets is now recognized as an important tool in palaeoglaciology. Existing palaeoglaciological reconstructions of the dimensions, geometry and dynamics of former ice sheets are based mainly on glacial depositional, as opposed to glacial erosional, landforms. Part of the reason for this is a lack of detailed understanding of the origin and significance of glacial erosional landforms. Here we review recent developments in our understanding of the processes and landforms of glacial erosion and consider their value in palaeoglaciology. Glacial erosion involves the removal and transport of bedrock and/or sediment by glacial quarrying, glacial abrasion and glacial meltwater. These processes combine to create a suite of landforms that are frequently observed in areas formerly occupied by ice sheets and glaciers, and which can be used in palaeoglaciological reconstructions. For example, all landforms of glacial erosion provide evidence for the release of subglacial meltwater and the existence of warm-based ice. Landforms of glacial quarrying such as roches moutonnées, rock basins and zones of areal scouring are created when cavities form between an ice sheet and its bed and therefore are indicative of low effective basal pressures (0.1-1 MPa) and high sliding velocities that are necessary for ice-bed separation. Fluctuations in basal water pressure also play an important role in the formation of glacially quarried landforms. Landforms of glacial abrasion include streamlined bedrock features (‘whalebacks’), some ‘p-forms’, striae, grooves, micro-crag and tails, bedrock gouges and cracks. Abrasion can be achieved by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. Although abrasion models depend critically on whether clasts are treated as dependent or independent of subglacial water pressure, it appears that abrasion is favoured in situations where effective basal pressures are greater than 1 MPa and where there are low sliding velocities. Consequently, landforms dominated by glacial abrasion are created when there is no ice-bed separation. Landforms of glacial meltwater erosion include both subglacial and ice-marginal meltwater channels. Investigations of the relationship between glacial meltwater channels and other aspects of the subglacial drainage system, such as areas of ice-bed contact, areas of ice-bed separation and precipitate-filled depressions, enable inferences to be made concerning former subglacial water pressure-drainage relationships, effective pressures and glacier velocities. Meltwater palaeovelocity and palaeodischarge can also be calculated from measurements of channel shape, channel width and the size of material transported within former glacial meltwater channels. We surmize that glacial erosional landforms offer insight into former glacio-logical conditions at both the landform- and landscape-scale within palaeoglaciology. Exposure-age dating techniques, including cosmogenic isotope dating of bedrock surfaces, will be important in increasing our understanding of the age and chronological significance of landforms of glacial erosion. We conclude that landforms of glacial erosion are of great value in ice mass reconstruction and speculate that these landforms will achieve greater recognition within palaeoglaciology in line with improvements in exposure-age dating techniques.

Regional unconformities and the sedimentary architecture of the Oak Ridges Moraine area, southern Ontario

Seismic stratigraphy, geometry, and sediment facies within the Oak Ridges Moraine (ORM) area of Ontario record major structural elements and surfaces of the Quaternary sedimentary sequence. The derived stratigraphic architecture can be used to identify the key elements of a regional erosional surface, represented by an unconformity in the subsurface, and associated overlying channel sediments. The erosional surface – unconformity forms a distinct time datum in the Quaternary sequence, which provides an important aid to lithostratigraphic correlation. The architecture also gives improved understanding of the effects of erosion on the late-glacial landscape. The surfaces of erosional drumlins and intervening troughs, and the beds and banks of meltwater channels in the ORM area, define the regional unconformity, highlighted by seismic profiles linked to continuously cored boreholes. These features are attributed to regional-scale, subglacial meltwater flow events. The sculpted surfaces, which are analogous to water-eroded forms, the presence of boulder lags and coarse-grained deposits on the regional erosional surface, and the channels with undulating profiles provide the vital supporting evidence for a meltwater interpretation. The inter-regional extent of the unconformity is inferred from the coherence of regional paleoflows and the extent of drumlinized uplands, tunnel channels, and scoured bedrock terrain across ~75% of the landscape from the ORM area east and south to the Finger Lakes, New York. The implied magnitude of erosion suggests a pressing need for directed sedimentological study in those ocean basins that were probable depositional sites for flood deposits.

A multiproxy study of a basal till: a time‐transgressive accretion and deformation hypothesis

AbstractA two‐part basal till at Knud Strand, Denmark reveals a uniform fabric pattern and strength, petrographical composition and clay mineralogy. The nature of the contact with the underlying sediments, ductile deformation structures, partly intact soft sediment clasts, small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water. A time‐transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds. It is suggested that, although strain indicators occur throughout the entire till thickness, the deformation at any point of time encompassed the uppermost part of the till only, allowing preservation of fragile clasts below. The substantial thickness of the till (up to 6 m) coupled with a much smaller (by more than one order of magnitude) inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition. The lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path. Copyright © 2004 John Wiley & Sons, Ltd.

Early Holocene glacial lakes and ice marginal retreat pattern in Labrador/Ungava, Canada

The existence of glacial lakes Naskaupi, McLean, Minto, Mélèzes, and Wapussakatoo during the deglaciation of Labrador/Ungava, Canada, was recognized at the end of 1950s. However, glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in Labrador/Ungava. Reconstruction of the ice flow succession in Labrador/Ungava has often primarily focused on till lineation systems. These reconstructions often depict a late glacial ice dispersal center situated over central Labrador/Ungava. More recent studies on glacial geomorphology including meltwater features, however, suggest that north-central Labrador/Ungava exhibited cold-based conditions at least during the latest deglaciation. Cold-based conditions inhibit basal sliding and formation of landforms, except for meltwater traces such as meltwater channels, glacial lake shorelines and deltas. This situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold-based deglaciation. Evidence presented in this study, such as glacial lake shorelines, fossil deltas, and spillway and drainage channels in north-central Labrador/Ungava, indicates the existence of numerous previously unmapped glacial lakes. The mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which, in turn, are used to constrain the late glacial ice margin retreat pattern over the inferred cold-based areas of north-central Labrador/Ungava. A total of 26 glacial lakes (65 sub-stages) existed during the deglaciation of Labrador/Ungava. These lakes were impounded along the southern margin of the shrinking ice sheet. The required damming ice margins indicate that the last ice remnant of the Laurentide Ice Sheet in Labrador/Ungava was situated over the southern Ungava Bay and the adjacent southern shore.

Late Wisconsinan glaciation of Amund and Ellef Ringnes islands, Nunavut: evidence for the configuration, dynamics, and deglacial chronology of the northwest sector of the Innuitian Ice Sheet

Geomorphic and chronologic evidence from Amund and Ellef Ringnes islands documents the configuration, dynamics, and collapse of the northwest sector of the Innuitian Ice Sheet. These data record the inundation of the Ringnes Islands by northwestward-flowing ice from divides spanning the alpine and lowland sectors of the Innuitian Ice Sheet. Ice-flow indicators and granite dispersal along eastern Amund Ringnes Island suggest Massey Sound was filled by an ice stream discharging coalescent alpine and lowland ice from Norwegian Bay. In contrast, the interior of Amund Ringnes Island was overridden by predominantly non-erosive, granite-free ice from a divide in the lowland sector of the ice sheet. Glacial landforms on Ellef Ringnes Island record coverage by largely non-erosive ice, but it remains uncertain whether these features relate to northward-flowing lowland ice or a cold-based local ice cap. Deglaciation of the Ringnes Islands commenced ~10 000 14C years ago. Deglacial dates between 9.7 and 9.2 ka BP record the sequential entry of marine fauna along Massey and Hassel sounds, concomitant with the southward retreat of trunk ice towards Norwegian Bay. These data suggest marine-based trunk glaciers were vulnerable to calving during pre-Holocene eustatic sea-level rise. However, deglacial dates from inner embayments indicate that residual ice caps persisted on Amund and Ellef Ringnes islands for 800 to 1400 14C years after retreat of trunk ice from the adjacent marine channels. Lateral meltwater channels record the subsequent retreat of these ice caps, which became increasingly confined within upland valleys after 8.6 ka BP.

The glacial and sea-level history of Darling Peninsula, eastern Ellesmere Island

The glacial history of Darling Peninsula is recorded by meltwater channels and lateral moraines deposited by local ice that extended seaward of the present coast during the last glacial maximum. Above these moraines, shelly till and erratics of both Greenland and Ellesmere Island prove nance record more extensive ice of unknown age. At the time of this more extensive ice cover, Ellesmere Island ice displaced Greenland ice from many parts of this coastline, as shown by the widespread absence of Greenland erratics and shelly tills above Holocene marine limit. The chronology of deglaciation is based on 14 C dates obtained on marine shells collected from either ice-contact deltas or raised beaches close to marine limit (79-88 m asl). Deglaciation began at least 7.5 ka BP and the distribution of ice on the peninsula was similar to present conditions by 6.0 ka BP. The reconstruction of the sea level history of Darling Peninsula contributes to the reconstruction of regional isobases drawn on 7.5 ka BP shorelines which locally reach 80-90 m asl.

A middle Pleistocene paleosol sequence from Dawson Range, Central Yukon Territory

Four paleosols were intersected in a core drilled into the colluvial fill of a largely buried meltwater channel that was last active during the youngest of the pre-Reid glaciation (0.99-0.78 Ma) in the Dawson Range, Yukon Territory. The paleosols are classified as Podzols. The sedimentary sequence and paleosols indicate that at least two middle Pleistocene glacial and interglacial periods are represented in the core. The mean annual temperature exceeded 0° C for thousands of years in the upland environment of the Dawson Range at 61° N during these interglacial periods.

Quaternary lithology and shallow gas from high resolution gravity and seismic data in the central North Sea

High resolution marine gravity data and 3D post-stack seismic data from the central North Sea have been jointly interpreted. The accuracy of the gravity data allowed the detection of density contrasts related to a Quaternary sub-glacial melt-water channel, a shallow gas accumulation and a Tertiary gas chimney. The combination of gravity and seismic data is shown to particularly improve the detection of the shallow gas accumulation. The interpretation included visual correlation of gravity images and seismic data performed in a seismic workstation environment, as well as 2.5D gravity modelling along selected seismic profiles. The successful application of this method on shallow targets requires a limited complexity of the shallow strata as well as targets defined by a distinct density contrast and a reasonable size. Data requirements include high-resolution bathymetric and free air gravity data. Bouguer gravity data, which are commonly used in exploration, cannot be used here as densities may vary within the uppermost layer below the sea bottom.

Formation and disintegration of a high‐arctic ice‐cored moraine complex, Scott Turnerbreen, Svalbard

Englacial debris structures, morphology and sediment distribution at the frontal part and at the proglacial area of the Scott Turnerbreen glacier have been studied through fieldwork and aerial photograph interpretation. The main emphasis has been on processes controlling the morphological development of the proglacial area. Three types of supraglacial ridges have been related to different types of englacial debris bands. We suggest that the sediments were transported in thrusts, along flow lines and in englacial meltwater channels prior to, and during a surge in, the 1930s, before the glacier turned cold. Melting‐out of englacial debris and debris that flows down the glacier front has formed an isolating debris cover on the glacier surface, preventing further melting. As the glacier wasted, the stagnant, debris‐covered front became separated from the glacier and formed ice‐cored moraine ridges. Three moraine ridges were formed outside the present ice‐front. The further glacier wastage formed a low‐relief proglacial area with debris‐flow deposits resting directly on glacier ice. Melting of this buried ice initiated a second phase of slides and debris flows with a flow direction independent of the present glacier surface. The rapid disintegration of the proglacial morphology is mainly caused by slides and stream erosion that uncover buried ice and often cause sediments to be transported into the main river and out of the proglacial area. Inactive stream channels are probably one of the morphological elements that have the best potential for preservation in a wasting ice‐cored moraine complex and may indicate former ice‐front positions.

A relict landscape in the centre of Fennoscandian glaciation: cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles

The presence of well-developed tors, boulder fields, and weathering mantles in the Parkajoki area of northeastern Sweden, near the centre of Fennoscandian glaciation, has been used to suggest that these landscapes were preserved during all glacial cycles since ice-sheet initiation in the late Cainozoic. This implies that all successive large-scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation. Cosmogenic 10Be and 26Al data from three tors and a meltwater channel in the Parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles. Apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at ∼11 ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion. Single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles. This estimate is based on (i) the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the DSDP 607 marine benthic foraminifer oxygen isotope record, in conjunction with (ii) a record of Fennoscandian ice sheet flow traces (and hence, ice sheet extent), and (iii) noting that cosmogenic nuclides are accumulated only during ice free periods. In addition, mean cosmogenic 26Al/ 10Be concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1.6 m Ma −1. Thus, the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles.

A relict landscape in the centre of Fennoscandian glaciation: Geomorphological evidence of minimal Quaternary glacial erosion

The Parkajoki area in northeastern Sweden is situated near the central area of Fennoscandian glaciation. Despite its location, the area is dominated by landforms induced by subaerial weathering and erosion processes, such as well-developed tors and associated saprolites, boulder fields, and boulder depressions. The glacial geomorphology is dominated by lateral and proglacial meltwater channels. Subglacial imprints indicative of thawed-bed conditions and reshaping by glacier sliding (e.g., fluting, drumlins, striae) are lacking. Hence, most of the landscape still exhibits a preglacial appearance. Because of its location, near the central area of glaciation, we attribute preservation to frozen-bed conditions of overriding ice sheets. The widespread distribution of well-developed tors and boulder fields, and the degree of chemical weathering of the bedrock, indicate that the area has been protected from glacial erosion during all glacial cycles since ice sheet initiation in the late Cenozoic. Unlike most other areas with tors in glaciated regions, the Parkajoki area is uniquely situated in the lowlands at 150–400 m.a.s.l. Moreover, this relict landscape is surrounded by glacial landscapes (including drumlins, ribbed moraine, and eskers) of varying age and at similar elevation. Hence, topographical reasons for this area being persistently cold-based cannot be invoked. By inference, we conclude that strain heat release never managed to cancel the initial subglacial permafrost conditions. We attribute this to divergent ice flow towards the convex outline of the ice sheet margin during deglaciations and to the relative roughness of the area compared to its surroundings. The implication is that to explain preservation throughout the Quaternary, all large-scale glaciations must have had a similar evolution concerning ice sheet configuration and internal dynamics as the last glaciation for which good constraints on evolution and outline are available.

Geometry of englacial water conduits, Austre Brøggerbreen, Svalbard

The internal geometry of three englacial conduits located on Austre Brøggerbreen, Svalbard, was investigated by tracer experiments and by direct exploration. The tracer tests produced short-lived and highly peaked tracer return curves with high through-flow velocities. The direct inspections revealed the entrances to be of two types. Type I consisted of a moulin with an initial 39 m vertical shaft drained by a semicircular and near-horizontal englacial channel trending diagonally across the direction of ice flow, parallel to tectonic structures observed on the ice surface. The type II entrances sloped gently into the ice in the direction of the ice flow, and became englacial by gradual down-cutting of supraglacial meltwater channels into the glacier surface. No traces of tectonic structures were observed in the ice surface that could have initiated the type II entrances. Englacial conduits with gentle slopes were meandering. With increasing slope, a winding waterfall?plunge-pool geometry was developed, the channels being high (>3 m) compared to their width (30?150 cm).

Subglacial meltwater channels at Lymm Dam, Cheshire

Small-scale erosion features in the lowlands of Britain have only previously been described from the Wirral, Cheshire. In this paper a suite of similar erosion forms located within a subglacial meltwater channel is described and compared with those previously reported. The forms consist of almost linear channels incised into soft Triassic sandstone bedrock to a depth of up to 1.76 m and a group of scallops occupying a vertical rock face. The channels show two preferred orientations, one parallel to the last (Late Devensian) ice flow and another transverse to the ice flow. Two possible types of erosion processes are identified: (i) erosion by phreatic (below the water table) subglacial water flow, which produced the subglacial gorge, channels orientated parallel to the ice flow, and scallops; and (ii) erosion by vadose (above the water table) subglacial water flow, which produced the channels orientated parallel to the local bedrock slope (transverse to the ice flow). The implications for Late Devensian glaciation of the area are: (i) deglaciation was accompanied by active ice; (ii) deglaciation was achieved under warm base conditions; (iii) vertical ice thickness was greater than 40 m; and (iv) the hydraulic potential gradient was possibly orientated N14°E at the formation of the features.

NEW OR POORLY KNOWN DIATOMS FROM HIMALAYAN STREAMS

Diatoms were sampled from over 200 first to third order Himalayan streams, at altitudes of 350–4700 m asl, between 1992 and 1996. This paper describes two new species, one new variety and Cymbella stuxbergii, providing extensive water chemistry and habitat data for these taxa. Gomphonema chubichuensis occurred at high altitudes (2300–4320 m) in meltwater channels or other unstable streams. Navicula obtecta was common in base-poor and nutrient-poor streams in undisturbed catchments of Nepal's Middle Hills. These species have the potential to indicate climatic change and catchment disturbance, respectively. The geographical distribution of the species was restricted: Synedra inaequalis var. jumlensis was only found in Western Nepal and north-western India, G. chubichuensis occurred from northwestern India to Central Nepal, and N. obtecta was mainly found in Eastern Nepal. C. stuxbergii has previously been recorded from more northerly latitudes, apart from one other record from the Eastern Himalaya.

Submarine and onshore end moraines in southern Newfoundland: implications for the history of late Wisconsinan ice retreat

Recessional positions of the Newfoundland ice sheet 14–9 ka BP are represented by fjord‐mouth submarine moraines, fjord‐head emerged ice‐contact marine deltas, and inland moraine belts. The arcuate submarine moraines have steep frontal ramparts and comprise up to 80 m of acoustically incoherent ice‐contact sediment (or till) interfingered distally with glaciomarine sediment that began to be deposited c. 14.2 ka BP. The moraines formed by stabilization of ice that calved rapidly back along troughs on the continental shelf. The ice front retreated to fjord‐heads and stabilized to form ice‐contact delta terraces declining in elevation westward from +26 m to just below present sea level. Stratified glaciomarine sediments accumulated in fjords, while currents outside fjords eroded the upper part of the glaciomarine deposits, forming an unconformity bracketed by dates of 12.8 and 8.5 ka BP. The delta terraces are broadly correlated with the 12.7 ka BP Robinson's Head readvance west of the area. The ice front retreated inland, pausing three or four times to form lines of small bouldery stillstand moraines, heads of outwash, sidehill meltwater channels, and beaded eskers. Lake‐sediment cores across this belt yield dated pollen evidence of three climatic reversals to which the moraines are equated: the Killarney Oscillation c. 11.2 ka BP, the Younger Dryas chronozone 11.0–10.4 ka BP, and an unnamed cold event c. 9.7 ka BP. Relative sea level fell in the early Holocene because of crustal rebound, so that out‐wash and other alluvium accumulated in deltas now submerged due to relative sea‐level rise.

Evidence for restricted ice extent during the last glacial maximum in the Koryak Mountains of Chukotka, far eastern Russia

Field evidence in the Koryak Mountains‐ Lake Mainitz region of far eastern Russia supports three Pleistocene glacial advances. The early Wisconsinan and pre-Wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the Koryak Mountains. Field evidence demonstrates that the terminal, lateral, and medial moraines, as well as meltwater channels, dead ice topography, kettles, and outwash plains mark the extent of ice during the last glacial maximum (LGM), during which glaciers reached no more than 20 km beyond their present limits. Those emanating from the southern Koryak Mountains may have reached the Bering Sea. Numerical and relative dating techniques support these results and test the theoretical models of the LGM in western Beringia. Erratics on moraines and glaciofluvial terraces, common to all valleys at 13‐13.8 m above river level, yield 36 Cl exposure ages ranging from 10.08 to 25.78 ka. The Koryak Mountains‐Lake Mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central Beringia found in other terrestrial and marine records. Glacier growth in the Koryak Mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest Pacific region. Evidence from the northern Koryak Mountains (lat 64°N, long 177°E) indicates that the extent of ice in western Beringia was limited to mountain and valley glaciers during the LGM. This fieldbased research contradicts M.G. Grosswald’s theoretical Beringian ice sheet hypothesis.

Glacial Sediment–Landform Associations and Paleoclimate during the Last Glaciation, Strait of Magellan, Chile

Glacial sediments and landforms preserved beside the Strait of Magellan record repeated advances of an outlet of the Patagonian Icefield during and following the last glacial maximum (LGM; ∼25,000–14,000 14C yr B.P.). Ice-marginal landform assemblages consist of thrust moraine complexes, kame and kettle topography, and lateral meltwater channels, very similar to those found at the margins of modern subpolar glaciers. Taken together with other forms of paleoenvironmental evidence, the landform assemblages show that, during the LGM and late-glacial time, permafrost occurred near sea level in southernmost South America. This finding implies that mean annual temperatures were ∼7–8°C lower than at present, somewhat lower than those reconstructed by current glacier–climatic models. Comparison with precipitation–temperature relationships for modern glaciers suggests, in addition, that precipitation levels were lower than today. Reduced glacial-age precipitation may have resulted from a precipitation shadow induced by the Patagonian Icefield, an equatorward migration of the average position of westerly cyclonic storm tracks in the southern midlatitudes, or both these factors.

Overdeepened Quaternary valleys in the eastern Danish North Sea: morphology and origin

A system of 100–350 m deep, overdeepened and buried valleys has been mapped in the eastern Danish North Sea. Individual valleys can be traced for several tens of kilometres, and valleys generally begin and terminate abruptly. A correlation to buried valleys onshore southwest Denmark is possible. The valleys post date glaciotectonic structures, and are interpreted to be Elsterian and possibly Saalian in age. The origin of valleys in the eastern Danish North Sea is explained by repeated cycles of sub-glacial down-cutting of meltwater channels and channel closure due to ice creep, eventually producing deeply incised valleys. Glacial erosion may have been significant in cases where valleys are broad and shallow. The origin of overdeepened Quaternary valleys in general can be attributed to a combination of three end-member processes: (i) steady-state drainage of meltwater; (ii) catastrophic outbursts of meltwater; and (iii) glacial erosion. The main requirement for sub-glacial valley formation is the presence of a melting ice sheet overlying a poorly consolidated substrate. Permeability variations of the substrate may produce subregional variations in the pattern of valley incision, while pre-glacial rivers, faults or salt structures are of local importance only.

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

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