Late and Middle Pleistocene deposits at Somersham,Cambridgeshire, U.K.: a model for reconstructing fluvial/estuarine depositional environments

Gravel quarries at Somersham, Cambridgeshire, have yielded evidence for a Pleistocene channel of the R. Great Ouse, containing temperate stage sediments between cold stage sediments. In the earlier cold stage, fluviatile gravels and floodplain loessic sediment accumulated. In the later cold stage a further series of gravel units and floodplain sediments were deposited, together with lake sediments. The lake sediments are associated with Lake Sparks, dammed by Late Devensian ice in the Wash at ca. 18.5 ka BP. The lake sediments overlie gravels with a radiocarbon date from an organic horizon indicating a Middle Devensian age. Clast lithological analyses from the earlier and later gravels suggest that reworking of gravels has occurred within a relatively stable catchment. The petrography of the earlier cold stage loessic sediment and temperate stage fine sediment indicates an Anglian affinity, which conflicts with the biostratigraphic interpretation. Pollen and macroscopic plant remains from sediments of both cold stages and from the temperate stage indicate, respectively, assemblages with a typical full-glacial aspect with a rich flora of shrubs and open ground herbs(including an assemblage at ca. 18 ka), and temperate freshwater and marine-influenced organic sediments. On the basis of pollen analysis these are ascribed to substages Ip II and III of the Ipswichian Stage(O.I.S. 5e), with a Pinus-Quercus-Corylus biozone in the former and a biozone with Carpinus in the latter. Marine-influenced sediments, at −3.7 to −0.3 m OD, indicate transgression in Ip II and regression in Ip III.Molluscan assemblages from the temperate stage and the later cold stage are described; two are from the Late Devensian, at a time near the maximum extension of ice into the Wash. Foraminifer and ostracod faunas are described from post-Ipswichian sediments and may be reworked. Radiocarbon dates confirm the age of the later gravel suite as Devensian and a calibration of the measurements is given. Amino acid ratios from Corbicula fluminalis valves from temperate stage sediments are reported, with measurements from different parts of the valve; the results tend to support an Ipswichian age. TL measurements of the earlier cold stage loessic sediment and associated sand indicate a pre-Ipswichian age for the sediments. The earlier cold stage is correlated with the pre-Ipswichian cold stage, the Wolstonian of Mitchell et al.(1973); problems with this correlation are discussed.Various periglacial phenomena, including thermal contraction networks and cracks, diapirs, involutions and coversand are associated with the Devensian sequence. The complex environmental history, based on stratigraphy and palaeontology, is described, and related to other nearby sites in southern Fenland.     

The Quaternary history of the subtidal central Wash,eastern England

The history of Quaternary sedimentation in the subtidal Wash is described using high-resolution seismic profiles. The Pleistocene sequence is divided into three depositional units, comprising Anglian till overlain by possible Late Devensian subglacial scour fill and lacustrine sediments. These latter sediments may provide further evidence for a lake in the Wash impounded by ice along the Lincolnshire–Norfolk coast. The Holocene sequence is divided into six depositional units, each truncated by the one above. Estuarine sediment resting on a marine flooding surface forms the earliest unit. This sediment was partially eroded by migration of the shoreface as the marine flooding progressed landward. The following four units comprise sand and gravel banks deposited on the erosion surface. Bank deposition was followed by an episode of tidal scour caused either by increased tidal current velocities following reclamation of the Fenland or by breakdown of postulated former offshore barriers. The youngest and most extensive Holocene unit rests on the scoured surface and comprises several types of deposit. These are: large sand banks around the periphery of the subtidal area with sediment extending seawards into two NE–SW aligned troughs; low sand banks on a central ridge dividing the troughs and partially covering the sediments in the troughs; thick gravels towards the mouth of the Wash; muddy sediments forming drapes over the sand in the centre of the Wash. The data provide information on the variety of processes related to the advance and retreat of Pleistocene ice sheets in eastern England and the subsequent Holocene marine flooding of the Wash–Fenland embayment. The Holocene sequence reveals periods of widespread sedimentation separated by periods of both local and regional erosion, with possible implications for climatic and hydrodynamic change. © 1997 John Wiley & Sons, Ltd.     

Sedimentary evidence for a major glacial oscillation and proglacial lake formation in the Solway Lowlands (Cumbria,UK) during Late Devensian deglaciation

Livingstone, S. J., Ó Cofaigh, C., Evans, D. J. A. & Palmer, A. 2010: Sedimentary evidence for a major glacial oscillation and proglacial lake formation in the Solway Lowlands (Cumbria, UK) during Late Devensian deglaciation. Boreas, Vol. 39, pp. 505–527. 10.1111/j.1502‐3885.2010.00149.x. ISSN 0300‐9483. This paper is a sedimentological investigation of Late Devensian glacial deposits from the Solway Lowlands, northwest England, in the central sector of the last British–Irish Ice Sheet. In this region, laminated glaciolacustrine sediments occur, sandwiched between diamictons interpreted as subglacial tills. At one location the laminated sediments are interpreted as varves, and indicate the former presence of a proglacial lake. Correlation of these varves with other laminated sediments indicates that the glacial lake was at least 140 km² in area and probably much larger. Extensive beds of sand, silt and gravel throughout the Solway Basin associated with the lake demonstrate ice‐free conditions over a large area. Based on the number of varves, the lake was in existence for at least 261 years. The stratigraphic sequence of varves bracketed by tills implies a major glacial oscillation prior to the Scottish Re‐advance (16.8 cal. ka BP). This oscillation is tentatively correlated with the Gosforth oscillation at c.19.5 cal. ka BP. Subsequent overriding of these glaciolacustrine sediments during a westward‐moving re‐advance demonstrates rapid ice loss and then gain within the Solway Lowlands from ice‐dispersal centres in the Lake District, Pennines and Southern Uplands. It is speculated that the existence of this and other lakes along the northeastern edge of the Irish Sea Basin would have influenced ice‐sheet dynamics.     

The glacial and interstadial sediments at the Burn of Benholm,Kincardineshire: evidence for onshore pre‐Devensian ice movement in northeast Scotland

Pebbly clays and diamictons containing marine shell fragments and peat lenses exposed beneath subglacially deposited Late Devensian till at the Burn of Benholm provide new insights into the glacial history of Quaternary sequences in eastern Scotland. The peat yielded pollen of interstadial affinity (including Bruckenthalia spiculifolia) and non‐finite radiocarbon dates. Comparisons with other pre‐Late Devensian pollen records in northern Scotland suggest that the peat lenses are remnants of an Early Devensian interstadial deposit, of Oxygen Isotope Substage 5c or 5a age. Reworked faunal assemblages in the shelly sediments include Quaternary marine molluscs of low boreal aspect, as well as Mesozoic and Palaeozoic microfossils. Amino acid ratios from fragments of Arctica islandica suggest that the shells are of Oxygen Isotope Stage 9 age or older. The fabric and composition of the shelly sediments are consistent with their emplacement as deformation till during the onshore movement of glacially transported rafts of marine sediment. Folded and sheared contacts between the shelly deposits, peat lenses and the overlying Late Devensian till indicate that the fossiliferous sediments were glacitectonised during the main Late Devensian glaciation, when ice moved from Strathmore and overrode the site from the southwest. British Geological Survey. © NERC 2000.     

Evolution of a periglacial landscape in the Late Devensian: environments and palaeobotany of the Mepal area,Cambridgeshire, England

The landscape evolution of the Mepal area from Late Devensian Block Fen Terrace times to the beginning of the Flandrian, a period of ca. 8000 radiocarbon years, is reconstructed. Stratigraphy is based on borehole transects and single boreholes, centred on a depression between the Block Fen Terrace and the Isle of Ely. Within the depression is a Devensian late‐glacial sequence, with the Windermere Interstadial represented by radiocarbon‐dated organic sediments. Pollen and plant macroscopic remains of the late‐glacial sediments are analysed. Plant communities with Betula developed in the interstadial. Before and after the interstadial there is much reworked pollen in the inorganic sediments, derived from local pre‐Devensian Pleistocene sediments, including temperate Ipswichian Stage sediments, and from mass‐wasting of the local Jurassic bedrock. Periods of such mass‐movement occur before and after the deposition of the late‐glacial lake sediments. Deposition of aeolian sediment occurs later than the main period of mass movement, but before the Windermere Interstadial. The relationship of the aeolian sediments in time and space to permafrost, indicated by local contraction polygons and cracks, is discussed. Solifluction diverted the flow of the River Great Ouse from a northeast direction in Block Fen Terrace times to a southwest direction as a channel developed to the west of the Chatteris–March ‘island’. This led to a drainage divide in Flandrian times. Copyright © 2002 John Wiley & Sons, Ltd.     

Fluvial system response to Late Devensian (Weichselian) aridity,Baston, Lincolnshire,England

Little is known about the impact of Late Devensian (Weichselian) aridity on lowland British landscapes, largely because they lack the widespread coversand deposits of the adjacent continent. The concentration of large interformational ice‐wedge casts in the upper part of many Devensian fluvial sequences suggests that fluvial activity may have decreased considerably during this time. The development of optically stimulated luminescence (OSL) dating enables this period of ice‐wedge cast formation to be constrained for the first time in eastern England, where a marked horizon of ice‐wedge casts is found between two distinctive dateable facies associations. Contrasts between this horizon and adjacent sediments show clear changes in environment and fluvial system behaviour in response to changing water supply, in line with palaeontological evidence. In addition to providing chronological control on the period of ice‐wedge formation, the study shows good agreement of the radiocarbon and OSL dating techniques during the Middle and Late Devensian, with direct comparison of these techniques beyond 15 000 yr for the first time in Britain. It is suggested that aridity during the Late Devensian forced a significant decrease in fluvial activity compared with preceding and following periods, initiating a system with low peak flows and widespread permafrost development. Copyright © 2004 John Wiley & Sons, Ltd.     

Late Pleistocene deposits at Block Fen,Cambridgeshire, England

Organic sediments in a gravel quarry at Block Fen, Cambridgeshire, form a sheet dividing lower from upper gravels. Analyses of pollen, macroscopic plant remains and molluscs from these organic sediments are presented. They indicate the presence of temperate freshwater and slightly brackish fine floodplain sediments, which, on the basis of the palaeobotany, are correlated with the temperate Ipswichian Stage. The freshwater sediments, ascribed to Ipswichian substage IIb, occur at ca. ?3 m OD. Marine-influenced tidal sediments, ascribed to Ipswichian substage III, occur at ca. ?6 m OD. No evidence was found for the presence of more than one temperate stage in the sequence. The lower gravels are then correlated with the cold Wolstonian Stage and the upper gravels with the cold Devensian Stage. In contrast to the woodland environments indicated by the palaeobotany of the Ipswichian organic sediments, post-Ipswichian pollen diagrams and macroscopic plant remains in the upper suite of sands and gravels indicate open tree-less vegetation typical of the cold Devensian Stage. They also contain a typical cold-stage mollusc fauna. The sediments containing these floras and faunas are associated with thermal contraction cracks, indicating the presence of permafrost. The final sand and gravel aggradation in the Devensian forms the Block Fen Terrace, near 0 m OD. The evidence indicates that it is younger than the lacustrine sediments resulting from the blocking of the Fenland at the Wash by Late Devensian ice at ca. 18.5 ka BP. The sequence at Block Fen is related to nearby Ipswichian and Devensian sediments at Chatteris, March, Wimblington and Mepal, and to deposits at Wretton on the east margin of Fenland. The correlation permits an outline reconstruction of the history of the valley carrying the River Great Ouse between the Isle of Ely and the Chatteris and March ‘islands’ from the time of a gravel aggradation before the Ipswichian to the Flandrian. The reconstruction shows the time and level of the Ipswichian marine incursion into the Middle Level of Fenland and the extent of aggradation and erosion in the Devensian.     

Reinterpreting Rotherslade,Gower Peninsula: implications for Last Glacial ice limits and Quaternary stratigraphy of the British Isles

Rotherslade on the Gower Peninsula in south Wales has been viewed as a key site for the reconstruction of Quaternary depositional environments in the British Isles. Since the early 20th century, and certainly since the 1980s, the accepted view has been that Rotherslade is the most westerly location on the south Gower coast where there is in situ basal till exposed and that, logically, this location marks the position of the LGM ice limit. However, reinvestigation of the sediments and their architecture, and analysis of clast fabrics and thin sections of critical sedimentary units, show that none of the exposed sediments has properties diagnostic of subglacial deposition or deformation. We postulate here that LGM ice terminated at the western side of Swansea Bay, a few kilometres to the north‐east of Rotherslade, and propose that the sedimentary sequence comprises Early to Middle Devensian periglacial sediments, overlain by a complex of Late Devensian, ice‐proximal outwash fan deposits, an assemblage of paraglacial debris and, finally, periglacial mass movement deposits. The proposed repositioning of the Late Devensian ice limit and the associated new subaerial interpretation of the sediments suggest that a reassessment of sedimentary sequences (Hunts Bay, Western Slade) and landforms (Paviland Moraine) farther west on Gower, which have attained similar stratigraphical status, is now warranted. Copyright © 2008 John Wiley & Sons, Ltd.     

The Pre-Windermere Interstadial (Late Devensian) raised marine strata of eastern Scotland and their macrofauna: a review

Late Devensian raised marine deposits predating the Windermere Interstadial (c. 13–11 ka BP) are found between the Moray Firth and Berwick. The widely distributed, sparsely fossiliferous Errol Clay Formation of the firths of Forth and Tay was laid down in a high-arctic environment immediately following the retreat of the Late Devensian (Dimlington Stadial) ice. In the Tay Estuary, sedimentation took place under distal glaciomarine to marine conditions at a time when there was a fully marine connection between the Scottish east coast and the then high-arctic Norwegian Sea. On the south shore of the Moray Firth, the similar, but undated Spynie Clay Formation seems to have been laid down in a wholly glaciomarine environment. Part of the macrofauna attributed to the St. Fergus Silt Formation of the NE Scottish coast may have been either misidentified, or is not in situ. The preservation of the fauna and of delicate sedimentary structures indicate that the arctic clays as a whole were laid down rapidly. It is suggested that tidal currents were minimal, and that waves were dampened by sea ice for much of the year. Bones of the ringed seal, Phoca hispida, have been recorded from 12 sites in eastern Scotland. About 40 macrofaunal taxa are present in the Errol Clay Formation, a number similar to that recorded in the Danish Younger Yoldia Clay, which is of comparable age. The faunal nomenclature is updated, and three species (Cylichna occulta, Retusa obtusa and Lyonsia arenosa) are added to the macrofaunal list for the Errol Clay Formation. Reports of in situ boreal molluscs and of one possibly North American species in the otherwise high-arctic assemblage are not supported by specimens in extant collections. Differential decay of the fauna below the zone of weathering in the Errol Clay Formation may have resulted from early diagenesis. Deposition of the Late Devensian, pre-Windermere Interstadial marine sediments as a whole was probably diachronous, beginning after 15–14 ka BP on the outer coast, but was confined to a short interval (c. 13.5–13 ka BP) at the type site in the Tay Estuary. In the Forth Estuary, the high-arctic marine fauna adjacent to the retreating ice-front may have survived the rapid climatic amelioration (c. 13 ka BP) at the beginning of the Windermere Interstadial (marked by the Main Perth Shoreline) for perhaps a few decades.     

Thermokarst-lake-basin sediments, Tuktoyaktuk Coastlands, western arctic Canada

Three stages of deposition are distinguished in thermokarst-lake-basin sequences in ice-rich permafrost of the Tuktoyaktuk Coastlands, western arctic Canada: (1) widespread retrogressive thaw slumping around lake margins that rapidly transports upland sediments into thermokarst lakes, forming a distinctive basal unit of impure sand and/or diamicton; (2) a reduction or cessation of slumping-because of the pinching out of adjacent ground ice, slump stabilization or climatic cooling, that reduces the input of clastic sediment, permitting reworking of sediment around lake margins and suspension settling, principally in basin centres; (3) lakes drain and deposition may continue by gelifluction and accumulation of in situ peat or aeolian sand. Radiocarbon dating of detrital peat and wood from a progradational sequence (basal unit) defines a lateral younging trend in the direction of progradation. A progradation rate is calculated to be ～ 4 cm yr^?1, consistent with rapid deposition during stage (1) above. The nonuniform nature of the trend is attributed to episodic influxes of old organic material by slumping and reworking by waves and currents. In comparison with thermokarst-lake-basin sequences previously described in Alaska, Canada and Siberia, the middle unit of those in the Tuktoyaktuk Coastlands is similar, whereas the basal unit is generally thicker and, by contrast, often contains diamicton. These differences are attributed, respectively, to larger-scale resedimentation of upland sediments by retrogressive thaw slumping and debris-flow deposition in thermokarst lakes in the Tuktoyaktuk Coastlands. Compared with the sediments within supraglacial lakes in areas of moderate to high relief, the middle unit of thermokarst-lake-basin sequences in the Tuktoyaktuk Coastlands lacks clastic varves and the basal unit is much thinner and texturally less variable. These differences are attributed to higher relief and larger volumes of meltwater and glacigenic sediment in supraglacial lakes, which promote more suspension settling and resedimentation of glacigenic sediment than in thermokarst lakes in the Tuktoyaktuk Coastlands. It may be impossible to distinguish glacial and periglacial thermokarst-lake-basin sediments in permafrost areas of incomplete deglaciation. Not only is it often difficult to distinguish intrasedimental and buried glacier ice, but the depositional processes associated with thaw of both ice types are presumably the same and the host sediments very similar.     

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.     

Glacigenic rafting at Castle Hill,Gardenstown, and its significance for the glacial history of northern Banffshire,Scotland

Reinvestigation of the lower part of the key Quaternary section at Castle Hill, Gardenstown, has shown that the sediments are not in stratigraphical order, but consist chiefly of glaciotectonites, including rafts of soft sediments, which were deposited by ice moving southeastwards from the Moray Firth. Sedimentary structures are preserved in some of the rafts, which are separated by subhorizontal shears. The rafts accreted subglacially under conditions of moderate to high strain, the final glacial event being the deposition of a thin, discontinuous sheet of till, probably derived from a more westerly direction. It is proposed that interbedded dark grey shelly clay, till and sand elsewhere in northern Banffshire were emplaced, at least in part, by a similar mechanism during either the Middle Devensian, or more likely, the Late Devensian. Sand and clay with paired bivalve shells, which were formerly exposed within the Quaternary successions at Castle Hill and inland at King Edward, some 12 km to the south, are interpreted to be within glacigenic rafts, and are not in situ deposits formed during a widespread marine transgression. It is suggested that the alternation of phases of constructional and excavational deformation within a single glacial event rather than discrete glaciations provides a useful model for glacial deposition in northern Banffshire, and more generally in northeast Scotland. © 1997 John Wiley & Sons, Ltd.     

A late Devensian glaciolacustrine fan-delta at Rhosesmor,Clwyd, North Wales

Glacigenic sediments exposed at Rhosesmor, Clwyd, North Wales, were deposited in a small proglacial lake ponded by the stagnating margin of the Late Devensian Irish Sea icesheet. Three major fades assemblages are identified and show a prograding sequence of topset, foreset, and bottomset deposits associated with two Gilbert-style deltas stacked one above the other. The lower delta is associated with a lake level at c. 180 m OD. A subsequent ice-marginal readvance raised the lake to c. 192 m OD, drowned the lower delta and caused the growth of a second delta by an iceward shift of facies. It is estimated that the life of the lake was of the order of 90 years during which average sediment in-fill amounted to some 4±10⁴ m³ per annum.     

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

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

A revised seismic stratigraphy for Quaternary deposits on the inner continental shelf west of Scotland between 55°30'N and 57°30'N

A stratigraphy for Quaternary deposits on the western Scottish shelf has been erected using seismic and borehole data. Eight new formations are defined and described with evidence presented for the environ-ment of deposition of each formation. Most of the Quaternary sediments preserved on the shelf arc shown to have accumulated under stadial or glacial conditions. The possible age of each formation is discussed within the context of evidence provided from the mainland, shelf and deep-sea cores. Two are thought to be pre-Devensian, one is possibly pre-Devcnsian. one is possibly Early and/or Middle Devensian, two are probably Late Devensian, one is Late Devensian to Holocenc and one Present day in age. It is suggested that the Late Devensian ice reached the shelf margin south of the Outer Hebridcan Platform.     

Palaeontology and taphonomy of Late Quaternary fossil assemblages at Somersham,Cambridgeshire, England,and the problem of reworking

Assemblages of foraminifers, ostracods and molluscs from temperate Ipswichian Stage (last temperate stage) sediments and overlying cold Devensian Stage (last cold stage) sediments at Somersham in the southern Fenland of Cambridgeshire have been analysed. The Ipswichian sediments contain faunas consistent with temperate brackish water conditions under tidal influence. The Devensian assemblages were recovered from a series of sands and gravels laterally accreting in a channel cutting into Ipswichian sediments. In contrast to the Ipswichian faunas, the faunas of particular Devensian samples show a complex mixture of temperate freshwater, brackish and marine taxa. The molluscs are mainly freshwater, with few land snails; they occur together with foraminifers and ostracods. Freshwater, brackish water and marine ostracods are present with foraminifers. A sample of Devensian fine laminated sediment in the channel was analysed for pollen; only abundant pre-Quaternary spores were present, with abundant foraminifers in the same sample. The taphonomy of the assemblages and the difficulties of their interpretation in environmental terms are discussed. The importance of taphonomy in assessing environments, climate, range of taxa and dating is stressed.     

Glacial advance,occupation and retreat sediments associated with multi‐stage ice‐dammed lakes: north‐central Alberta,Canada

Ice sheets that advance upvalley, against the regional gradient, commonly block drainage and result in ice‐dammed proglacial lakes along their margins during advance and retreat phases. Ice‐dammed glacial lakes described in regional depositional models, in which ice blocks a major lake outlet, are often confined to basins in which the glacial lake palaeogeographical position generally remains semi‐stable (e.g. Great Lakes basins). However, in places where ice retreats downvalley, blocking regional drainage, the palaeogeographical position and lake level of glacial lakes evolve temporally in response to the position of the ice margin (referred to here as ‘multi‐stage’ lakes). In order to understand the sedimentary record of multi‐stage lakes, sediments were examined in 14 cored boreholes in the Peace and Wabasca valleys in north‐central Alberta, Canada. Three facies associations (FAI–III) were identified from core, and record Middle Wisconsinan ice‐distal to ice‐proximal glaciolacustrine (FAI) sediments deposited during ice advance, Late Wisconsinan subglacial and ice‐marginal sediments (FAII) deposited during ice‐occupation, and glaciolacustrine sediments (FAIII) that record ice retreat from the study area. Modelling of the lateral extent of FAs using water wells and gamma‐ray logs, combined with interpreted outlets and mapped moraines based on LiDAR imagery, facilitated palaeogeographical reconstruction of lakes and the identification of four major retreat‐phase lake stages. These lake reconstructions, together with the vertical succession of FAs, are used to develop a depositional model for ice‐dammed lakes during a cycle of glacial advance and retreat. This depositional model may be applied in other areas where meltwater was impounded by glacial ice advancing up the regional gradient, in order to understand the complex interaction between depositional processes, ice‐marginal position, and supply of meltwater and sediment in the lake basin. In particular, this model could be applied to decipher the genetic origin of diamicts previously interpreted to record strictly subglacial deposition or multiple re‐advances.     

Late-glacial varved clays in the Bornholm Basin and Hanö Bay

Distribution, thickness, lithology and stratigraphy of varved clays are discussed. The sediments were deposited in the Baltic Ice Lake during the Late Weichselian.