Late Devensian river deposits beneath the Floodplain Terrace ofthe River Thames at Abingdon, Berkshire, England

M. M. Aalto, G. R. Coope and P. L. Gibbardwith an appendix by D. T. Holyoak

AALTO, M. M., G. R. COOPE & P. L. GIBBARD. 1984. Late Devensian river depositsbeneath the Floodplain Terrace of the River Thames at Abingdon, Berkshire, England. Proc.Geol. Ass. 95 (1), 65-79. An organic channel-fill deposit in gravel and sand exposed at ThruppHouse Farm quarry is described. The gravels and sands were laid down in a bed-load dominatedriver under a cold climate regime. Two radiocarbon dates of 13,260 ± 180 BP and 13,580 ± 120 BPwere obtained and indicate that the deposit accumulated during the Late Devensian substage.However, the abundance of certain members of the insect fauna, particularly Pycnoglyptalurida and Olophrum assimile suggest that the deposits may date from the latter half of theWindermere Interstadial, i.e. the C14 dates may be 1000 yr. too old. The discrepancy isattributed to inwashed, older carbonaceous material. This is supported by the find of Pinus

bark, radiocarbon-dated at 47,700 ±~~~~BP. The organic deposit accumulated in a floodplain

channel with a variable water level. Initially, shallow water supported a rich aquatic and marshvegetation with a diverse beetle fauna. Later, fluctuating water levelled to a decline in speciesdiversity, meadow vegetation invaded the pool and was occasionally damaged by fire. Finally,slightly higher water levels returned and semi-aquatic vegetation colonised the pool area with arange of beetle species present. Little can be judged of the regional vegetation. New Devensianplant records include Sagittaria natans Pall., Apium inundatum (L) Reichb. F., Naumburgia(Lysimachia) thyrsifiora (L) and Ranunculus platanifolius L. The climate is thought to havedeteriorated during deposition of the organic bed. The relation of the sediments to thosepreviously described from the Floodplain gravels of the Upper Thames Valley is discussed andtheir significance in dating the terrace aggradation within the Thames Valley as a whole isconsidered.

M. M. Aalto, Department of Botany, Helsinki University, Unioninkatu 44, 00170 Helsinki 17,Finland.G. R. Coope, Department of Geological Sciences, University of Birmingham, P.O. Box 363,Birmingham B15 2TT, England.P. L. Gibbard, Subdepartment of Quaternary Research, Botany School, Downing Street,Cambridge CB23EA, England.

1. INTRODUCTION

Gravel and sand excavations in the Floodplain Terracesurface of the River Thames at Thrupp House Farm,near Abingdon, Berkshire (SU 522971) exposedfossiliferous deposits in 1973 (Fig. 1). The discovery oforganic deposits in a sequence of river gravels andsands is of great importance for two reasons. First,detailed identification of plant and animal fossilspresent can provide invaluable palaeoecologicalinformation which, when combined with thesedimentary evidence, gives insight into the contem­porary environment. Secondly, by direct radiocarbondating of the deposits where possible, an age for thesediment and, therefore, the age of the containinggravels and sands may also be obtained.

The succession of terraces and their underlyingdeposits in the Upper Thames Valley, i.e. north of theGoring Gap, has been described by a number of

65

workers. Sandford (1924, 1926, 1954) originallydescribed and mapped the terrace succession whichwas more recently reviewed by Goudie & Hart (1975).Because of the abundance of deposits and availabilityof exposure, the Floodplain Terrace (NorthmoorTerrace of Arkell, 1947) deposits have receivedparticular attention, notably by Briggs (1976),Gilbertson (1976) and Briggs & Gilbertson (1980).Organic deposits within the gravels beneath theFloodplain Terrace have been radiocarbon dated from40,000-1O,600BP and indicate that they, and thegravels in which they occur, accumulated duringvarious parts of the Devensian stage (Shotton, 1973;Briggs, 1976; Gilbertson, 1976).

The results of analyses from the organic deposits atThrupp House Farm described below complimentthose already available from other localities and addmuch new detail to the palaeoenvironmental evolutionof the Upper Thames Valley.

66 M. M. AALTO, G. R. COOPE AND P L. GIBBARD

FP

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Farm

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Fig. 1. Location map showing the setting of the sample site and its relation to local river terrace surfaces.

2. GEOLOGY OF THE SITE

The sections exposed at Thrupp House Farm quarry in1973 are summarised in Fig. 2, The sequencecomprised a maximum thickness of 4.5 m of current­bedded gravel and sand resting on an irregularlychannelled surface of Jurassic Kimmeridge Clay. Thesediments consisted predominantly of horizontallybedded, subrounded to subangular fine to mediumlimestone gravels (for pebble lithology see Corser,1979). Sand occurred both in the gravel matrix and innarrow, laterally impersistent bands 1O-20cm thick,interstratified with the gravel. In places these bandswere tabular cross-bedded. Within the gravels andsands several cross-cutting channel-fills of fine tomedium gravel and current-bedded pebbly sand werevisible at several horizons. The lateral extent ofindividual gravel bodies was extremely variable sincethey were generally lenticular in form. The materialcomprising the deposits was almost exclusively oftraction load size and the gravel was predominantlymatrix-supported. Although minor fining-upwardsequences were present in some channel-fills, thesequence was dominated by a complex interbedding ofbar-like gravel, bar-top gravel and channel-fill sandbed facies. The deposits must have accumulated underhighly variable flow and unstable channel conditions ina bed-load dominated fluvial regime. Such conditions

typify the braided river environment described byMiall (1977), Williams & Rust (1969) and Rust (1972)which is characteristic of regions where high volumesof sediment and highly variable discharges occur.These conditions may be expected to have occurredunder a cold, probably periglacial climate in lowlandBritain.

In the eastern part of the quarry, the sequenceshowed slight variations from that described above.Firstly, there was a rise in the bedrock surface of 1.5 mabove the general level found in the rest of the pit.Secondly, above the bedrock high an elongate darkgrey organic bed occurred within the gravels (Fig. 2).The bed lay on the gravel with a sharply-defined butconformable base. However, the base of the overlyinggravel was irregular and channelled into the top of theorganic bed. The organic bed was of considerableextent. It reached a maximum thickness of 1.3 m at itswestern end but gradually thinned eastwards. Wherethe bed was less than 40 em thick (point D), it becamegreatly disturbed by structures of 'ball-and-pillow' typethat resembled involutions. Whilst it is possible thatthese structures may have originated by periglacialfreeze-thaw action before emplacement of theoverlying gravel, it is equally possible that they couldhave resulted from loading during gravel deposition.They cannot therefore be regarded as significantclimatically. The same may be said of involutions of a

D E VENSIAN RI VER D E POSITS IN B E R KSHIRE 67

Thrupp House Farm 1973

50mL--...J

8

N

/

Fig. 2. Verti cal section at Thrupp House Farm in 1973.

-53maJ

Gravel and

sandOrganic deposit

Grovelandsand

-49

sand band overlying the organic sediments close topoint A (Fig. 2). At the extreme eastern end of thesection the organic bed was absent (point E) .

The microstratigraphy of the organic bed showedconsiderable internal facies variation. At the westernend , the fill comprised predominantly grey sand with afinely divided organic matrix. The sediments becamefiner toward s the east. The samples studied here weretaken at point A (Fig. 2) where the organi c contentwas highest, since further to the east and south thedeposit became more clayey and proportionally lessorganic. The sequence at point A is described belowusing the notation system of Tro els-Smith (1955):

em77-92

69-77

62- 69

61-6257-61

Gravelly sandLight bro wn sandy silt with few organic fragment sbecoming inte rmixed with gravel in the upper­most 3 em (sand band present between 78- 79 em)(Agl , As2, Gs+ , Dgl)Brown sand grading upwards into grey silty mudwith occasional mollusc shells and small chalkpebbles (sand band present at 76.5-77 em) (As!,Ag2, Ga+ , Gs+ , Dgl!)Light grey silty clay becomin g silty mud (As ] ,Ag2, Dg! , Ld+ )Brown coar se siltBrown to grey sandy silt

51.5-57 Poorly laminated light grey clayey silt (Asl , Ag2,Gat Dg! , Ld+ )

50.5-51.5 Sand29- 50.5 Dark grey silty clay mud (As l , Agl ! , Gat DgI) ;

coarse silt band 41-43 cm14-29 Light grey clay mud with some narrow clal

lamina e and isolated clasts (As~ , Agl] , DgI:!,Ld!)

0-14 Very dark grey clay mud with some isolatedpebbles (AgI, Gal , Ldl , DgI , As+)Sand

The sediments are most organic in the lower half ofthe sequ ence . Above, they are richer in inorganic finesand the bed is subdivided by several narrow sand orcoarse silt bands. The occurrence of such bands withinan oth erwise fine or organic sediment fill of this typeindicates periodic influx of flowing water into therelatively still water of the depression or channel. Thehigh organic content of the lower sediments mayrepresent a period of little inwash and channel stabilitywhich led to the colonisation of the water by aquaticplants (see below). Later , per iodic flooding seems tohave taken place (above 50 ern) causing the accumula­tion of silt and inwashed plant detritus , including woodfragments. At least four cycles of inwash could be seenin the upper sediments, recorded by fining-upward

68 M . M . AALTO . G . R . COOPE AND P . L. G IBBAR D

TABLE I . Macroscopic plant remains , and some animal remains recovered during palaeobotanical analyses , from point A atThrupp House Farm. For the position of the sample point , see Fig . 2. Abbreviat ions used are as follows: a , achene ; b, bark ; bs ,bud scale; c, charred seed ; e , endocarp; eg, egg cocoon; f, frui t ; fr , fragment s of fruit or see d; I , leaf; n, nu tlet ; 0 , oospor e ofCha raceae ; s, seed ; sh , shoot; st , ste m; w, wood y tissue . Frequencies when not counted : + som e , + + frequent , + + + copious .

• Radiocarbon dated materi al

14 C dates

Local macrofossil assem blage biozone A

13580 ± 120 13260 ± 180

samples (em)

Trees and shrubs:Betula sp .Jun iperus communis L.Salix cf. reticulata L.Pinus sylvestris L.

Plants of aquatic habitat:Alisma plantago-aquatica L.Apium inundatum (L.) Rchb.Chara spp,flippuris vulgaris L.Nuphar lutea (L.) Sm.Ranunculus sec . BatrachiumPotam ogeton perfoliatus L.Sagittaria natans PallasSchoenopl ectus lacustris (L. ) PallaS. tabernaemontani (C. C. Gmel. ) PallaSparganium erectum L.

Plants of marsh and fen habitat:Carex nigra (type)C. rostrata StrokesC. pan icea L.Cicuta virosa L.Filipendula ulmaria (L.) MaximEleocharis palu stris coli .Naumburgia (L ysimachia)

thyrsiflora (L. ) Rchb.Mentha cr. aquatiea L.Menyanth es trifoliata L.Polygonu m hydr opiper L.Potentilla erecta (type )P. palustris (L.) ScopRanuneulus lingua L.R. platanifolius L.R. sceleratus L.Rorippa islandica (Oeder) BorbasStachys palustris L.Thali ctrum fiavum L.Valeriana cf. dioica L.

Weeds and ruderals:Potentilla anserina L.Ranunculus aeris L.Rumex acetosella L.Urtica dioica L.

Indetermined plant remains:herbaceous (mostly Carex) tissuestwig (smaller than 1 em)charred tissue s (exc\. seeds)MusciCarex sppCarpo lithus sp.

A nima l rem ains:Spiders (Agroeca sp .)Bones (Fishes? )

type ofremain

fs

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81

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312

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211

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DEVENSIAN RIVE R D EP O SI T S IN BERKSHIRE

B c

69

24-29

2bs

29-34 34-39 39- 45 45-50.5 51.5-57 57-61

11

61-65 65-69 69-76.5 77- 83 S3-S9

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70 M . M . AALTO , G . R . COOPE A ND P. L. G IBBA RD

3. RADIOCARBON DATES

Three radiocarbon dates have been determined fromsamples of organic material from the site . Thefollowing two dates were obtained from plant remainswashed out of the grey clay mud :

Th is date indicates that the pine bark was certainlynot contemporary with the organic aggradation atThrupp House Farm , but was washed from olderdeposits up-river. Dr. V. R . Switsur (Subdepartmentof Quatern ary Research , Cambr idge) estimated thatonly about 2 per cent of contemporary carboncont amination at the time of accumulation would beenough to give such a date to infinitely old barkderived from interglacial or early Devensian intersta­dial dep osits.

Thes e dates suggest that the Thrupp Hou se Farmdeposits accumulated during the Late Devensiansubstage (Mitchell, Penny, Shott on & West , 1973).The dates show a close correspondence and arestratigraphically consistent. They place the organicdep osits within the period preceding the Late-glacialWindermere Interstadial of Coope & Pennington(1977) or the Lateglacial Interstadial of Lowe & Gray(1980).

A third date was obtained from Pinus bark washedfrom the level between 40-50 em. The bark was datedbecause the occurrence of Pinus at this level was notsuspected from the palaeobotanical analyses (section4) . The radiocarbon date obtained was:

The deposit was investigated by cutting a verticalsediment column from point A (Fig. 2) into 19 blocks.These blocks were of variable thickness and weighedfrom 200-500 gm, the breaks between being deter­mined by natur al sedimentary bound aries, variationsof colour or texture. Pollen samples were taken fromthe centres of the blocks. The remaining sediment wasanalysed for macroscopic plant remains and insects(section 5).

(a) Macroscopic plant remains

Plant macrofossils were extr acted using mtnc acid(HN03) following the method of Backman (1965). Acomplete list of plant macrofossil s and the remains ofspiders (A groeca sp.) and possible fish bonesrecovered is given in Table 1. The nomenclature forthe plant remains of British species follows Clapham,Tutin & Warburg (1962). Non-British plant nomencla­ture is taken from Flora Europaea (Tutin et al. ,1964-1980).

Th e macrofossils of herbaceous plant species aregrouped in Table 1 according to their optimal habitat ,following Godwin (1975; Tables 39-41). The assemb­lages can be divided into thre e local macrofossilassemblage biozones (l.m.a. b.) . The differencesbetween the biozones almost cert ainly indicatevariation in local environmental conditions, such aswater level fluctuation , rather than any regionalchange. To aid comparison of the plant macrofossiland pollen assemblages, the I.m.a.b .'s are also shownon the pollen diagram (Fig. 3) .

(i) l.m.a.b . A: 0-14 em (4 samples)Species-rich aquatic and marsh-pioneer vegetation

characterise this zone. Both submerged and floating­leaved aquatic plants were growing in the channel.The assemblage suggests that acid-dystrophic ratherthan euthrophic water was present. In shallow water,stands of Schoenoplectus lacus tris (the most abundantplant remains in this zone), Hippuris vulgaris, Apiuminundatum , Alisma plantago-aquatica and Sparga niumerectum flourished ; whilst in deeper places Pota­mogeton perfoliatus, Nuphar lutea, Sagittaria natansand water crowfoots (Ranunculus subgenus Bat­rachium) among others were established.

The record of Sagittaria natan s Pall, a northeasternEuropean and Asian species, is the first from BritishPleistocene deposits and its presence here impliesboreo-continental conditions (cf. Hult en , 1971, fig.125). The species can be distinguished from S.sagittifo lia by the far smaller size of its fruits (abouthalf the size of S. sagittif olia) and by the differentsurface cell pattern (cf. Katz . Katz & Kipiani , 1965;table 14:15-19 and 21-31).

Plants of drier habitats are also well represented inthis biozone. The most notable are the tall herbspecies that grew on riverside meadows and include

4. PALAEOBOTANY

+ 400 BP- 2600

13.260 ± 180 BP

13,580 ± 120 BP

47,700

HeI-I092

Q-2017

Su-932

19-24cm

7,5-14cm

sequences from sand or coarse silt to silt, organic siltor clay mud (e.g. at 50,5-57 em and 61-65 em). Suchcycles appear to repre sent per iodic energetic inflowthat deposited the coarse material , followed by settlingof predominantly suspended load sediment in still orslowly flowing water. The brown silt band (57- 61 em)record s a marked inwash of inorganic sediment. Theband contains few plant remains, except for somecarb onised fragment s (see below), the presence ofwhich suggests that the sediment may representinwash of regolith following fire damage of the localvegetation cover.

The coarseness of the sedimentary fill at the westernend of the channel (Fig, 2, point B) may reflect theproximity of a contemporary channel carrying flowingwate r.

Overlying the gravel sequence in faces preserved atthe margins of the pit were 1-2 m of grey to brownmottled silty clay floodplain depo sits thought to be ofpostglacial age.

DEVENSIAN RIVER DEPOSITS IN BERKSHIRE 71

Thalictrum flavum (the plant that characterises thewhole profile), Filipendula ulmaria, Ranunculusplatanifolius, R. lingua, Mentha d. aquatica, Cicutavirosa and the tall Carex species C. rostrata and C.panicea. A slightly more marshy habitat may beindicated by species such as Potentilla palustris,Menyanthes trifoliata and Potentilla erecta. Somefragments of mosses were also found but notidentified. Weed and ruderal species such as Potentillaanserina, Rumex acetosella, Urtica dioica and Ranun­culus acris probably grew on nearby open ground.

The discovery of an endocarp of Ranunculusplatanifolius is interesting. This plant is a northernmontane and alpine species typically found inbrookside and meadows. If its modern distribution isclimatically determined then its presence here suggeststhat seasonally harsh conditions were prevalent at thetime.

Woody species represented include Betula spp. andJuniperus communis in level 5-9 ern. Small twigs ofdeciduous trees are found at all levels within this zone,but not later.

It is evident that during this zone the site was largelysubmerged as indicated by the aquatic plants and alsoby the relatively good preservation of the plantremains, including abundant delicate tissue fragments(Table 1). The sediments in this part of the profilecontain a large silt component indicating that inwashof both inorganic and probably organic material tookplace.

(ii) l.m.a.b. B: 14-65 cm (10 samples)In contrast to the preceding biozone, the major part

of the profile (51 em) contained very few macroscopicplant remains. A similar phenomenon is recorded bythe pollen and insect assemblages from this part of thesequence. Delicate tissue fragments, so numerous inthe previous biozone, were not present. Insteadabundant carbonised tissue fragments and seeds werefound in every sample. The charred seeds representspecies also present in the lower zone such as Carexrotratat, C. nigra, C. panicea and Potentilla palustris,all of which inhabit marsh habitats. Charred tissue,much of which was thin, hollow stem fragmentsresembling Gramineae straws, was also recordedboth in this and the other zones. On occasions themarsh vegetation suffered burning. There is no reasonto suppose that this was due to anything other thannatural causes, although the activities of man cannotbe entirely excluded. Uncharred seeds present in thisbiozone were the resistant, nutlike types includingMenyanthes trifoliata, Schoenoplectus lacustris and thedominant species Thalictrum flavum. The latter hasrelatively large, thick-walled fruits.

In summary, it appears that this biozone representsa relatively drier period during which the channel wasonly temporarily or periodically submerged or flooded.

The paucity of plant remains, except the mostresistant types, together with the poor pollen

preservation (section 4b) may have resulted fromdestruction of material by oxidation and processes ofdecay brought about by periodic drying of thesediment surface.

The discovery of Pinus bark (5-10 em thick) at40-50 em was unexpected because no other macrosco­pic remains of pine were found and there was littlepine pollen. The radiocarbon date obtained from thisbark (Su-932 section 3) demonstrated that it must havebeen derived.

(iii) l.m.a.b. C: 65-89 cm (4 samples)In the upper part of the profile both the number of

plant taxa and the preservation of delicate plant tissueincreases. However, the diversity of taxa andabundance of fossils found in biozone A is notachieved. The species represented are, in general, thesame as those in biozone A, marsh plants again beingthe dominant group. The water plants present are of asemi-aquatic habit with true hydrophytes absent.Three plants that first appear in this biozone areSchoenoplectus tabernaemontani, Naumburgia (Lysi­machia) thyrsiflora and Ranunculus sceleratus. Incomparison with the previous biozone, the site seemsto have become wetter and with more open-water (seealso Insects) but water levels continued to fluctuateduring the period represented. Fluctuating waterlevels are also indicated by the Mollusca from thissediment (see Appendix). The presence of shells,together with chalk pebbles in the sediment, suggestthat the substrate became more basic or neutral at thetime.

(b) Pollen analysesA series of twelve samples were analysed for pollen.The samples were prepared using the standardchemical technique used in the Subdepartment ofQuaternary Research modified to include the use ofsodium pyrophosphate (Bates, Coxon & Gibbard,1978). Pollen and spores were identified and referredto the nomenclature of Andrew (1970) with additionsby Birks (1973). The results are plotted in Fig. 3 usingthe pollen sum of total land pollen and sporesexcluding aquatic taxa.

Pollen preservation throughout the sequence wasvery poor with all samples containing high proportionsof broken, crumpled and degraded unidentifiablegrains. For practical reasons these grains were notcounted. The highest frequency of such grains waspresent in the central part of the profile (30-65 ern).All samples also contained a high proportion ofreworked, presumably inwashed, pre-Pleistocenepalynomorphs derived largely from the Jurassicbedrock of the district.

The contemporary pollen spectra show a ratheruniform composition throughout the deposit. Theassemblages are dominated by high percentages ofGramineae and Cyperaceae pollen accompanied by arange of herb pollen taxa. The pollen of trees and

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DEVENSIAN RIVER DEPOSITS IN BERKSHIRE 73

shrubs is sparsely represented by low frequencies ofBetula, Pinus and Salix throughout, but particularly inthe lower part of the diagram. At least some of theBetula pollen may be derived from B. nana (dwarfbirch) but the poor pollen preservation preventedseparation of the types.

Of the herb pollen taxa represented, the pollen ofmarsh and fen plants is more frequent particularly inthe lowermost 20 em of the profile. The abundance ofwell-preserved Cyperaceae pollen at all levels suggeststhat sedge was growing at or immediately adjacent tothe site, and supports the plant macrofossil and beetleevidence. Other marsh or fen plants are representedby the pollen of Filipendula, Mentha Type, Ranuncu­laceae, Thalictrum, Valeriana dioica, Alisma andMenyanthes. Aquatic pollen taxa include SparganiumType, Potamogeton, Nuphar and Ranunculus (Bat­rachium Type). Plants of drier, herb-dominatedmeadowland are indicated by the pollen of Composi­tae (Tubuliflorae), Matricaria Type, Compositae(Liguliflorae), Artemisia, Caryophyllaceae, Chenopo­diaceae, Helianthemum, Plantago major/media Type,Rosaceae, Rumex acetosa and Umbelliferae. Thepresence of Salix pollen suggests that members of thisgenus were growing nearby throughout the period.

Although the pollen spectra are generally uniformin composition during the time represented, it isapparent that there are some slight changes. The lowerpart of the profile (0-20 cm: equivalent to l.m.a. b. Aand part of B) has a much more diverse flora thanhigher levels. The local growth of Betula and Salixbushes is indicated by both pollen and macrofossilevidence from this part of the deposit. Above,between 20-65 em (equivalent to the remaining part ofl.m.a.b. B) pollen spectra are almost totallydominated by Cyperaceae and Thalictrum togetherwith grains of some dry land plants. As mentionedabove, this decrease in pollen types is paralleled by adecrease in preserved plant macrofossil and insect taxaand a rise in indeterminate pollen. Such a change mayresult either from seasonal increase in sedimentinwash from the banks or a partial drying-out of thechannel causing destruction of pollen. A combinationof both these factors may have occurred. The upperpart of the profile (65-92 em; equivalent to l.m.a.b. C)shows a slight increase in species abundance and areturn to conditions more reminiscent of those at thebase. However, Betula and Salix do not return to theirprevious frequencies. The overall rise in pollen taxa isaccompanied by a similar increase in numbers of plantmacrofossils and insects.

Little may be concluded regarding the climate fromthe floral assemblages described. However, the recordof the northern montane and alpine meadow plantRanunculus plantanifolius together with that of theboreo-continental Sagittaria natans suggest that acooler climate than present day south central Britainmay have prevailed.

S. INSECTS

Fragments of insects were obtained from the sedimentcolumn after samples had been removed for pollen,macroscopic plant remains and radiocarbon analyses.The intervals between samples processed for insectfossils were selected to be exactly comparable withthose of the plant macrofossils, but the quantities ofsediment available varied greatly from sample tosample so that some yielded as little as 50 gm whilstothers were as much as 500 gm. In the faunal list thefigures opposite each species indicate the minimumnumber of individuals that must have been present inthe sample (Table 2).

(a) Faunal succession

It is clear from the faunal list that the samples with thegreatest variety are grouped in the lowest anduppermost parts of the profile. Purely for descriptiveconvenience and without implying any correlation withother sites, the faunal succession is divided into three;the lower four samples (equivalent to l.m.a.b. A), therather unproductive middle section (equivalent tol.m.a.b. B and the lower 11.5 cm of C) and the richeruppermost three samples (the remaining part ofl.m.a.b. C).

(i) 0-20 emThe local environment in which these sediments

accumulated was a well vegetated marsh. There canhave been little open water because of the paucity ofdytiscids (carnivorous water beetles), but there musthave been shallow pools, largely choked with plantsfor the hydrophilid and hydraenid beetles. Mud besidesuch pools would have provided habitats for Dryops.Evidence that the marsh was fed by a trickling brook isprovided by Limnius volkmari and Riolus cupreus.Damp vegetable debris would have provided thehabitat for the staphylinid species that prey upon otherarthropods or worms. Some information on the floralcomposition of the marsh may be obtained from thephytophagous species present. Thus Olibrus affinispasses through most of its life history in the flowerheads of Matricaria recucita. The weevil Hyperasuspiciosa feeds upon Lotus uliginosus or Lathyruspratensis, whilst Bruchidius debilis lives and developsin the seeds of Onobrychis. Donacia impressa feedslargely on Schoenoplectus lacustris and the weevilLimnobaris pilistriata attacks a variety of Cyperaceae ,plants known to be abundant at this time (section 4).Byrrhus and Syncalypta are exclusively attached tomosses. Some species of beetle occupy habitatspeculiar to themselves. Such a species is Lomechu­soides strumosa that lives as a parasite in the nests ofthe ant Formica, especially F. sanguinia.

(ii) 20-76.5 emThe faunal diversity in this series of samples was

very low. The one constituent species is the weevilNotaris bimaculatus that feeds upon the stems of

74 M . M . AALTO . G . R. COOPE A ND P. L. G IBBARD

TABLE 2. Insect remains from the orga nic deposit, profile A at Thrupp House Farm. Individual sample analyses correspondto those analysed for macroscopic plant remains (Table 1) . The position of the sample point is shown in Fig. 2.

Thrupp House Farm

Ca rabidae

113580 ±\ 1

13260I120 BP ± lS0 BP

EE E E E E E E E o 6 § E E E E E E

E E u u u u u u u if) u u u u u uu u u

~ :0: -s- a- ..,. a- if) 0 r-. "" s r- ,... a- N,.., if) a- N N ,.., ,.., ..,. if) vr-, -c -o r- cc 00 a-I I I I I I I I I I I I I I I I I I I

0 ,.., if) a- ~ a- ..,. a- ..,. a- if) v-, -c if) s r- ,.., a-N N ,.., ,.., -e r- <r. -o -o e- oc 00

if)

Elaphrus cupreus Du fts.Dyschirius globosus (Hbst.)Patrobus assimilis Chaud.Patrobus cf. septentrionis (Dej .)Bembidion do ris (Pz.)Bembidion guttula (F.)Agonum sp .

DytiscidaeHy drop orus sp .Rha ntus sp .Agab us sp .Colym betes sp.

HydrophilidseHelophorus cf. brevipa lpis BedelCercyon Iris/is (Ill.)Hydrobius fascipes (L.)

Hydr aen idaeOchthebius ditatatus Steph.Ochthebius minimus (F.)Hy draena ct. riparia Kugel.

Sta phylinidaePycnoglypta lurida Gyll.Oloph rum assimile (Payk.)Oloph rum [usc um (Grav.)A rpedium brachypterum (Grav.)Omalium sp .Bledius sp.A notylus rugosus (F.)Stenus sp.Staphylinus [uscatus Grav.Quedius sp .Tachyporus sp .Tachinus elongatus Gyll.Gy mnusa sp.Lom echus oides strum osa (F.)Aleocharinae gen. et sp. indet.

Scarabaei daeAphodius sp.

Byrrh idaeSimplocaria semis/ria/a (F.)Byrrhus sp.Syn calypta sp.

HeteroceridaeHeterocerus sp.

Dryop idaeDryop s sp,

Elmid aeLim nius volkmari (Pz.)Riolus cupreus (M ull .)

221

22

14

22

14

2

41I

2

17961

2

6 4

2

DEVENSIAN R IVER DEPOSITS IN BERKSHIRE 75

TABLE 2. (Cont.)

113580 ±I113260

IThrupp House Farm 120BP ± 180 BPE

E E E E E E E u E E E E E E E EE E E u u u u u u u V") u u u u u u u u

Carabidae u u u~ ::: v "" -e- "" V") 6 t-- :0 V") "" r-. '" a- N.... V") a- N N .... .... v V") V") '" '" r- cc co a-

d, I I 1 I 1 I 1 I I 1 I 1 1 I 1 1 I 1.... V") "" ~ ::: v g; v a- V") V") '-C ;;; V") "" r- '" a-N .... '" -e- r-. V") "" '-C r-. co co

V")

PtinidaePtinus sp.

CryptophagidaeAtomaria sp.

PhalacridaeOlibrus a/finis (Sturm)

ChrysomelidaeDonacia impressa Payk.BruchidaeBruchidius debilis (Gyll.)

ApionidaeApion sp.

CurculionidaeOtiorhyn chus nodosus (Mull.)Otiorhynchus o vatus (L.)Sitona sp.Hyp era suspiciosa (Hbst.)Notaris aethiops (F .)Notaris bimacu latus (F .)Limnobaris pilistriata (Steph.)Gymn etron sp.

2

?

2

Typha latifolia and the grass Phalaris arundinacea.Fragments of this beetle were relatively common andit is likely that the method of estimating minimumnumbers of individuals underrepresented this species.Otiorhyhchus ovatus is a polyphagous weevil thatprovides little information on the flora. Patrobusassimilis and sep tentrionis are species of meadow typevegetation , a suitable habitat for Olophrum fuscu m.

(iii) 77-92 cmThere is an increase in the number of species

particular ly in the uppermost two samples. There aremore hydrophilids ad hydraenids implying a return ofsmall puddles but not obvious develo pment of themarsh community. The most striking increase is that ofArpedium brachypterum which lives under moss andleaf litte r. Simplocaria semistriata feeds upon moss.The survival of L imn obaris pilistriata and the singleoccurrence of Notaris aethiops indicate the continuedprese nce of Cyperaceae at least in the lowest sampleof this division.

(b) Climatic implications of the fauna

There is a general tendency for relatively southernspecies to occur at the bottom of the succession andfor more north ern species to occur nearer the top.There are , however, a few definitive species . Olibrusaffinis and Brachidius debilis are found in southernBritain and only the extreme south of Fennoscan dia.Though both are phytophages, it is inconceivable thatthey are restricted by their specific host plants becausethese have geogra phical ranges that extend muchfurther north than those of their beetle parasi tes. Incontrast , Patrobus septentrionis, Pycnoglypta lurida,Arpedium brachypterium, Otiorhychus 1l0dOSllS andNoarisaeth iops are boreo-montane species today andthough not necessarily indicative of arctic climatesthey do collectively suggest colder conditions thanthose of central Englan d at the present time. Of thesefive species only Pycnoglypta lurida occurs near thebase of the sequence. It is quite widespread in

76 M . M . AALTO. G. R . C OOPE AN D P. L. G IB B A R D

northern Europe today and is thus not at allincompatible with the occurrence at this level of themor e thermophilous species. In summary, the climatewould seem to have deterior ated from warmtemper at e , equivalent to that of the present day, at thecommencement of organic sedimenta tion, to cooltemperate conditions at least by the close ofdeposition .

It is interesting to note that B. J . Wilkinson hasidentified larval fragments of the Trichopteran (Caddisfly) A synarchus cf. lapponicus Zett. from level83-89 ern. This is a species of stagnant or very slowlymoving water. Its range is across boreal Europe andAsia . It is found in the British Isles at the present day .

6. COMPARISON OF THEPALAEONTOLOGICAL EVIDENCE AND AGE

OF THE DEPOSIT

Athough the insect assemblage is small and largelymade up of widespread species that have an equallyvaried stratigraphical occurrence, there are a fewspecies that suggest that the Thrupp House Farmdeposits are of late -glacial age . Only one coleopteranspecies, Pycnoglypta lurida, is now absent from theBritish Isles but it was common during the late-glacial.It is , however , a rare species in British full-glacialdeposits. Similarly, Olophrum assimile is common inLate-glacial faunas but almo st unknown from MiddleDevesian assemblages. L om echusoides strum osa is afrequent member of Late-glacial faunas usually fromthe early temperate episode , as is Bruchidius debiliswhich must have been very abunda nt at the time . Thewhole faunal assemblage would best fit into theWindermere Interstadial from the thermal maximumto the beginning of Loch Lom ond Stradial (YoungerDryas) times but there seems no evidence of the fullyarctic faunas that characterise the insect assemblagesthat immediately pre-date the Interstadial (e.g.Colnbrook: 13,450 ± 170 BP) or immedia tely post­date it (Coope , 1982).

If this interpretation is correct then the two LateDevensian radiocarbon dates would be as much as1,000 years too old. To obtain this discrepancyapproximately 12.5 per cent contamin ation with oldercarbon would be necessary (O lsson, 1972). This couldhave arisen from the inclusion of older organicmaterial throughout the sequence . Some support forthis interpretat ion is provided by the reworked pinebar k in the deposit. Howe ver , it would seem equ allylikely that hard water error resulting fro m the remainsof aqua tic plants that incorpor at ed carb on from car­bon ate-r ich water might be the reason (Sutherland,1980). The correlation of the Thrupp Hou se Farmsequence with the pre-Windermere Interstadial LateDevensian period as suggested by the dates obtained

appea rs quit e acceptable on the basis of the palae o­bot anical evidence. The latter is compa rable to thatfrom herb-dominated assembl ages at othe r south­eastern British sites of Late De vensian age, particular­ly Colney He ath (13,560 ± 210 BP: Godwin , 1964) andColn ey Street (14,320 ± 210 BP: Gibbard, 1977) inHertford shir e. Th e pollen spectra from these sitesshow similar high herb frequencies accompanied bylow numbers of Betula, Pinus and Salix pollen . Equal­ly, the macroscopic plant fossil assemblage comparesclosel y with these sites, and does not differ significant­ly from those of full-glacial type at several localitieslisted in Godwin (1975) , espe cially Ea rith (Bell, 1970),but also Co lnbrook and West Drayton (Gi bbard &Hall , 1982) . The vegetation indicated both by thepollen and the plant macrofossils is of extremely localcharacter and may not represent the regiona l vegeta­tion on interfluve areas. The channel was about sao mfrom the contempor ary valley and this may explain thepaucit y of the regional vegetational component.

According to Godwin (1975) there was closed birchfore st across most of eastern and central Britain duringthe later part of the Windermere Interstad ial. Pineformed an clement in the woodland cover, but app earsto have been restricted to the south-cas t of En gland(Kerney, Brown & Chandler , 1964). Juniperus andSalix were represented in the shrub vegeta tion . AtThrupp House Farm, these taxa were recorded asma croscopic rem ains and or pollen , but are concen­tr ated in the lower part of the depo sit and theirfrequencies are very low. However , any changes intheir proportions are overwhelmed by the diver semarsh and aquatic communities as the site . Neverthe­less, the assemblages provide little evide nce ofregional woodland.

7. GEOLOGICAL SIGNIFICANCE OF THEDEPOSITS

Th e Floodpl ain Terrace gravels of the Upp er Thamesvalley have been placed into the Devensian stage onthe basis of several radiocarbon dates (Shotton, 1973).Two groups of dates are found : a Middle Devensiangroup of 40,000-29 ,000 BP and a Late Devensiangroup of 11,250- 10,600 BP (Briggs , 1976; Gilbertson ,1976). G ilberts on (1976) and Coope (1976) havedescribed mollusc and insect faun as respectively fromseveral rad iocarbon-dated organic beds within thegravels. Both these fossil groups ind icate a predomi­nantly treeless lands cap e du ring the Lat e Devensian ;the deposits themselves having accumul ated in sha llowpools in the gravel river floodpl ain , under generallycool or cold continental climates.

Briggs & Gilbertson (1980) have recentl y discussedthe sedimentary en vironment of the FloodplainTerrace deposits and concluded that the grave ls and

DEVENSIAN RIVER DEPOSITS IN BERKSHIRE 77

sands aggraded in a braided river, the materials havingbeen derived mostly from the valley sides bysolifluction. Ice wedge casts and cryoturbated horizonspresent in the gravels indicate that a periglacial climateprevailed at least during part of the time represented.It is not yet known whether the deposition of theFloodplain gravel which began in the MiddleDevensian continued into the Late Devensian orwhether the two periods were separated by anerosional episode (Briggs, 1976). The beddingstructures in the gravels make differentiation of thetwo gravel aggradations very difficult except wherestratigraphically-significant organic deposits are pre­sent.

The Late Devensian age for the Thrupp House Farmorganic deposit implies that at least part of the gravelbeneath and that above accumulated during thissubstage. This together with the biological andsedimentological evidence indicating aggradation ofthe organic bed in a shallow depression in the gravelriver floodplain are in complete accord with previousdescriptions of Floodplain gravel deposits.

Recent studies downstream in the Middle Thamesand tributary valleys have shown that two distinctgravel aggradations are present. The Kempton ParkGravel, on which the 'Upper Floodplain' terrace is inpart developed, contains organic deposits that havebeen consistently dated to the Middle Devensiansubstage (Coope & Angus, 1975; Gibbard, Coope,Hall, Preece & Robinson, 1982; Kerney, Gibbard.Hall & Robinson, 1982). The gravels underlying the'Lower Floodplain' terrace form a distinct stage at alower level in the West London area and arecontinuous with the Floodplain gravels in the tributaryvalleys such as the Colne, the Lea and Kennet(Gibbard, 1981). Late Devensian dates ranging from14,500-11 ,200 BP have been obtained, for example,from organic deposits in the Colne floodplain gravels:Colney Street Gravel (Godwin, 1964; Gibbard, 1977;Gibbard & Hall, 1982). It is to be expected thatorganic deposits when they are discovered from the'Lower Floodplain' gravel, will have dates of the sameorder. These dates are completely consistent withthose from the Upper Thames Valley.

In the Lower and Middle Thames there is a markederosional event between the two floodplain gravelaggradations (Gibbard, 1981) and it therefore seemslikely that the Middle and Late Devensian aggrada­tions of the Upper Thames may also have beenseparated by a period of erosion, or at leastnon-deposition. Judging from the grouping of datespresently available, this period may have lasted fromapproximately 29,000-15,000 BP.

It may be tempting, on the basis of the radiocarbondates, to suggest that terrace aggradation is in someway related to interstadial climatic amelioration. It isclear, however, from studies over much of southern

Britain (Castleden, 1977) including the Thames valley(Briggs & Gilbertson, 1980; Gibbard, 1977; Clarke &Dixon, 1981; Bryant, 1982) that terrace gravel andsand sediments are almost entirely laid down under acold climate regime. This is demonstrated by thesedimentology, the association with both periglacialstructures and deposits, and by the fossil assemblagesfound in organic deposits contained in the sediments.Interstadial warming that occurs coincidently withgravel aggradation may result only in a cessation ordiminution of river flow arising from increasedvegetation cover of the land surface. This might beexpected to lead to stabilisation of channels,colonisation of the floodplain by plants and animalsand associated accumulation of organic sedimentswithin floodplain hollows (cf. Kerney, Gibbard, Hall& Robinson, 1982). The return to cold climateconditions in the post-interstadial period appears toresult in rejuvenation of river flow and resultantfloodplain accretion manifested as gravel and sandaggradation. Any organic deposits that accumulatedare eroded and channelled at this time and, if notentirely removed, become buried as isolated lenseswithin the gravel and sand sequence.

8. CONCLUSIONS

The sequence of gravels and sands exposed at ThruppHouse Farm quarry probably accumulated in abed-load dominated braided stream under a predomi­nantly cool or cold climatic regime. Organic clay mudwas deposited in a floodplain channel during the LateDevensian substage, as shown by the radiocarbondates. The preserved fossil remains indicate that thepool and surrounding area were colonised by aquaticand marsh plants and an abundant coleopteran fauna.The latter indicate that deposition occurred underpredominantly cool temperate conditions. On thebasis of the radiocarbon dates and the geologicalsituation the deposits are thought to be the upstreamequivalent of the Late Devensian 'Lower Floodplain'gravel of the Middle Thames Valley.

ACKNOWLEDGEMENTS

We would like to thank Dr. V. R. Switsur (Universityof Cambridge), FL. H. Jugner (University of Helsinki)and Dr. T. Kankainen (Geological Survey of Finland)for the radiocarbon dates; Dr. C. Turner, Dr. R. C.Preece, Dr. A . .T. Stuart and Professor R. G. West,F.R.S. for their help in various ways; and Mr. B.Skellington for bringing the sections to our notice.

The support of research associateships fromN.E.R.C. and the Leverhulme Foundation to P. L.Gibbard are gratefully acknowledged.

78 M. M. AALTO, G. R. COOPE AND P. L. GIBBARD

Appendix

Mollusca from the Thrupp House Farm deposit

D. T. Holyoak

126

4655 shells

After inspection of the sediments three samples were takenand analysed for Mollusca:

69-77 em dry weight 0.26 kg

24-29 em dry weight 0.24 kg

14-19cm dry weight 0.30kg

Mollusca were found only in the sample from 69-77 em,from which the following were extracted using standardprocedures:

Lymnaea truncatula (Muller)Lymnaea peregra (Muller)Anisus leucostoma (Millet)Oxyloma pfeifferi (Rossmassler)

total

L. truncatula, L. peregra and A. leucostoma are aquaticspecies and tolerate shallow bodies of water, as well asoccurring in richer habitats. L. truncatula is oftenamphibious, living on bare mud or mud beneath vegetationin mires, and this species and A. leucostoma commonly occurin pools that dry in summer. O. pfeifferi usually lives ondamp mud, on vegetation in mires or on floating or emergentaquatic vegetation. All four species are common in depositsof Middle and late Devensian age, and all four have modernranges in Europe that extent from the Arctic to theMediterranean.

University of Reading, Reading RC6 2A U, England

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