CONTEXT

A perspective on D. R. Bridgland's paper 'The Middle and UpperPleistocene sequence in the Lower Thames: a record of Milankovitchclimatic fluctuation and early human occupation of southern Britain'

James Rose

Key words: Quaternary, fluvial, Thames, Palaeolithic, Milankovitch forcing

Department of Geography, Royal Holloway. University of London, Egham, Surrey TW20 OEX.UK (e-mail: j.rosetiorhul.ac. uk )

In his Henry Stopes Memorial Lecture, published inthis issue of the Proceedings of the Geologists' Associ­ation, David Bridgland outlines in detail the evidencefor the development of the lower part of the RiverThames over the last c. 440 ka (Bridgland, 2006). Thisis a regional narrative outlining the evidence used topiece together the changing river patterns and theassociation of these river patterns with human occu­pance of the region. It is built upon work by the authorover a period of more than 26 years, much of the workbeing published in earlier issues of PGA (Bridgland,1980, 1988a, b, 2003; Bridgland & Harding, 1993;Bridgland et al., 2003). The narrative outlines theimportant contribution made by previous dis­tinguished scientists such as, for example, FrederickZeuner and John Wymer, and the recent conflictsbetween himself and Philip Gibbard, bringing to thereaders the status and controversy of this research.

However, this work has more than a regional signifi­cance. Since the publication of his classic Quaternaryof the Thames (Bridgland, 1994), David Bridgland hasdeveloped a model in which he is able to explain riveractivity in terms of Milankovitch-scale climate forcing.The principles of this model are now used widelythroughout many parts of the globe (Bridgland et al.,2004; Lee et al., 2004; Boenigk & Frechen, 2006;Bridgland & Westaway, in press) and, amongst many,it is known as the 'Bridgland Model'.

This climate forcing model is based on the principlethat during periods of cold climate, when vegetationcover is limited, or absent, and powerful physicalprocesses such as frost shattering, gelifluction andseasonally high river discharges control the dynamicsof the Earth's surface, material is removed from theslopes to the river channels and transported through­out the river catchment. The net effect of this process,within the well-integrated parts of large river systems,is that more sediment is removed from the slopes thancan be transported from the catchment - net aggrada­tion takes place. In contrast, during periods of tem-

Proceedings of the Geologists' Association, 117, 277-279.

perate climate, when physical processes are rela­tively ineffective and peak discharges are muted byvegetation cover and thick, chemically weathered soils,sediment is locked-up on hillside slopes and riveractivity is confined to the small, vegetation-free zoneswhich are, in effect, the river channels. In consequence,river energy is concentrated on eroding the river chan­nel, transporting the small amounts of sedimentthrough most of the system and causing river incision.Furthermore, the limited discharge variations andthe relatively high supplies of fine-grained sedimentsuitable for the formation of cohesive banks mean thatsingle-thread channels are dominant throughout mostof the fluvial system and such aggradation as doestake place takes the form of fine-grained overbank"alluvium'.

The net result is that, over long periods of time, inthe well-integrated parts of river catchments (usuallythe lower parts), aggradation takes place during thecold climate periods, while, in these same areas, inci­sion takes place over the temperate climate periods,Aggradation during cold climates is mainly coarse­grained, deposited by multiple-thread, braided rivers.Any aggradation during temperate climates is mainlyfine-grained deposited beyond the banks of single­thread meandering channels. In the non-integratedparts of the catchments where river activity is deter­mined by other, independent processes, such as varia­tions in water surface slope in the upper parts ofcatchments, or where water surface slope is determinedby changes of sea-level, then the climate signal can beobscured or confused and the process of complexresponse operates (Schumm, 1979).

The Bridgland Model, as developed for the HenryStopes Lecture, is based on the two-stage climateforcing model, but refined with extra stages to takeinto account elements of complex response. Forexample, temporary aggradation caused by the short­term transfer of sediment through the catchment isrepresented by Phase 2 of Bridgland's figure 3, while

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short-term storage of fine-grained material during tem­perate climate conditions is represented by Phase 3 offigure 3. Likewise the elaboration shown in figure 3also takes into account the short-term responses toclimate change (Phases 2 and 5) which are ephemeralepisodes (see previous sentence). Consideration is alsogiven to a minimal-effect episode in which it is consid­ered that moisture is locked-up during cold climateconditions (Phase 6).

It is very difficult to unravel sequences of eventswhere these processes operate within a constant height­range along a river valley (West, 1991; Rose, 1995a).However, Earth surface systems have been very kindand a mechanism exists by which the different climate­forced episodes can be separated, at least at the scale ofthe two-stage model. This separation is achieved by theprocess of erosional isostasy, a concept introduced toBritain by Darrel Maddy (1997) and further elaboratedby Westaway et al. (2002). This process is based onthe fact that land areas are isostatically uplifted inresponse to the removal of mass by subaerial erosion,and the replacement of this mass by crustal materialfrom offshore regions loaded with sea water andsediment. The result of this uplift is that over periodsof time, which appear to be tuned to Milankovitcheccentricity 100 ODD-year cycles, individual net aggra­dations and net incisions are separated by altitude andthe classical staircase of river terraces is developed. It isessential to note that the climate-forcing model is notdependent upon erosion-driven uplift; it is simply thatthe interpretation of the climate forcing model isfacilitated by this uplift.

In the lower Thames region, studied by Bridgland,the altitudinal separation is limited because of theproximity to the North Sea depositional centre and, inmany ways, the fluvial geomorphology is not as simpleas it is in other areas such as the Middle Thames(Gibbard, 1985). However. another line of evidence iswell developed within the lower Thames region andthis has permitted, to some degree. testing of theBridgland Model and, in so doing, has reinforced theBridgland scheme. This additional evidence is mammalassemblage biostratigraphy, which is the basis ofMammal Assemblage Zones (MAZ) developed bySchreve (2001, see fig. I and table 1). This biostra­tigraphic scheme is based on distinctive assemblages ofmammals that are associated with different periods ofriver activity. For example, overbank sedimentsformed during temperate Marine Isotope Stage (MIS)5e are characterized by the presence of Hippopotamusamphibius (Hippopotamus), Crocuta crocuta (SpottedHyaena), Dama duma dama (Fallow Deer), Ursusarctos (Brown bear), Stephanorhinus hemitoechus(Narrow-nosed Rhinoceros) and Palaeoloxodon an­tiquus (Straight-tusked Elephant), and the absence(relative to fluvial sediments of different age in theregion) of Coelodonta antiquitatis (Woolly Rhino),Mammuthus primigenius (Woolly Mammoth), Homosp. (early humans) and Equus ferus (Eurasian wildhorse). Like all assemblage biostratigraphy, the assem-

blages are empirically, rather than conceptually based,and consequently their use for stratigraphic purposes issubject to self-reinforcement and may be proven falsewith any new discovery (as has been the case withpollen assemblage biostratigraphy (Seppa & Bennett,2003». Hitherto there is no evidence to suggest that thescheme is flawed and each of the temperate stageoverbank sediments contains a distinctive MAZ. In­deed, all the replicable geochronometry from the re­gion is in accord with the interpretation of the terracesequence and MAZ proposed, respectively, by Bridg­land and Schreve.

As a result of this work, we now understand that theriver Thames in Essex formed a series of terraces inwhich substantial bodies of sediment were aggradedduring the cold climates of MIS 12 (although this iscomplicated in places by an input of glaciofluvialsediment from the Anglian ice sheet), 10, 8, 6 and 2.Upon and within these coarse-grained deposits arediscontinuous bodies of fine-grained sediment formedduring the intervening temperate stages (MIS II, 9, 7and 5e). The current temperate episode (MIS I) cannotbe considered within this scheme because of the effectsof human activity on the supply of sediment and theriver regime. Temperate stage deposits are at a varietyof levels reflecting the chance survival of highly erod­ible fine-grained material that formed at the end of theperiod of cold climate aggradation and throughout theperiod of temperate climate incision. There is localevidence that some of the temperate climate depositsare associated with adjacent high sea-levels, but there isno evidence that this is a persistent factor forcingsedimentation during the temperate episodes. Indeed,during some of the temperate stages, such as MIS 7,global sea-level did not reach a level close to thepresent (Bintanja et al., 2005) and the contemporarycoastline would have been far distant in what is nowthe North Sea. Thus, David Bridgland has recon­structed the palaeogeography of southern and easternEssex since the Anglian Glaciation (Bridgland, 2006,fig. 9) and we see a complex pattern of change from aninitial route toward Clacton and the northeast, to analignment that is the present estuary. This pattern ofchange can be explained in terms of factors such asrock control in relation to erodible London Clay andless erodible Chalk, but the progressive southerly shiftmay, perhaps, be explained best in terms of crustalsubsidence over the last 450 000 years towards the lineof the present estuary.

The conceptual basis of the Bridgland Model meansthat it may be used to predict. As the concept proposesthat net aggradation, net incision and net uplift caus­ing terrace separation are tuned to 100 ODD-year(eccentricity) Milankovitch climate forcing, we are ableto date river aggradations, incisions and terrace sys­tems that do not have independent dating control. Thishas been done recently in relation to major events inthe Quaternary of Britain and northern Europe. First,the direct correlation of glacial deposits with an aggra­dation of the Bytham river, which formed in the

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second aggradation prior to the Anglian Glaciation­(MIS 12), means that we now have demonstrableevidence that lowland glaciation reached Britain priorto the Anglian and, by the application of the BridglandModel (two aggradationlincision cycles prior to theaggradation of MIS 12) to the lower reaches of theBytham river (which was the largest river in Britain atthe time (Rose, 1994)), we can date this glaciation toMIS 16 which occurred around 640 000 years ago (Leeet al., 2004). Furthermore, the sediments of theBytham river system have proved to be the majorrepository of evidence for human occupance of Britain(Wymer, 1985; Rose, 1995b; Parfitt et al., 2005; Lee

et al., 2006) and this concept has provided a potentiallysound method of determining the age of this humanoccupance, when other dating methods are not avail­able or have the inherent weaknesses of being empiri­cally derived.

The Quaternary river terraces of the lower Thames,the source of the Bridgland Model, therefore, notonly provide the basis for a complex and challengingnarrative of landscape development, but also providethe basis of a concept that helps us explain the changesand timing of changes in river landscapes throughoutmany parts of the Earth.

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