Correlation of Thames terrace deposits between the Lower Thames,eastern Essex and the submerged olfshore continuation of the

Thames-Medway valley

David R. Bridgland,* Brian D'Olier,t, Philip L. Gibbard:j: and Helen M. Roe:j:

BRIDGLAND, D. R., D'OLIER, B., GIBBARD, P. L. & ROE, H. M. 1993. Correlation ofThames terrace deposits between the Lower Thames, eastern Essex and the submergedoffshore continuation of the Thames-Medway valley. Proceedings of the Geologists'Association, 104,51-57. A modified correlation of terrace deposits between the Lower Thamesand eastern Essex is proposed, based on a reconsideration of the available evidence andcorrecting earlier published versions. It is suggested that the Rochford Gravel and RochfordChannel of the Southend area are not separate features within the fluvial record, but belong,instead, within the Southend Channel-Southchurch Gravel aggradational sequence. A correla­tion between the onshore terrace system and three submerged terraces recognized beneath thesouthern North Sea is proposed here for the first time. An important implication of these newcorrelations is that all terrace deposits lower than the Lynch Hill Gravel are below ordnancedatum east of Canvey Island.

• D. R. Bridgland Earth Science Consultancy, 41 Geneva Road, Darlington DL1 4NE.t D 'Qlier Associates (Consultant Geologists), 64 Hill Rise, Rickmansworth, Hertfordshire WD32NX.:f: Subdepartment of Quaternary Research, Botany School. Cambridge CB23EA.

1. INTRODUCTION

Recent publications have clarified the correlation ofMiddle and Upper Pleistocene Thames terracedeposits between the classic Middle Thames sequence,around Slough and Beaconsfield, and the area east ofLondon, commonly referred to as the Lower Thames(Bridgland, 1988; Gibbard, Whiteman & Bridgland,1988; Bridgland & D'Olier, 1989). As a result of workby Gibbard (1985) in the Middle Thames and London,and Bridgland (1983, 1988) in Essex, it has beenshown that the mapping by the Geological Survey(New Series) of terrace deposits to the east and westof London is not directly comparable. The terracedeposits mapped by the Geological Survey to the eastof London as 'Taplow Gravel' are, in fact, equivalentto the Lynch Hill Gravel of the Middle Thames. Thetrue Taplow Gravel of the Middle Thames 'type area',in its continuation east of London, was mapped by theGeological Survey as 'Floodplain Gravel'. To avoidconfusion, a system of local names has been applied tothe Lower Thames sequence (Table 1), although theMiddle Thames nomenclature has precedence andshould ideally be applied throughout the catchmentonce equivalence has been demonstratedunequivocally.

Correlations have also been suggested between theLower Thames terrace deposits and the sequence ofgravels in eastern Essex (Bridgland, 1988; Bridgland& D'Olier, 1989). Continued work in the areasconcerned has shown that the previously publishedversion of these correlations requires modification.

This modification allows a convincing correlationbetween the onshore and offshore Thames terracesequence to be proposed for the first time.

2. EVIDENCE FROM THE LOWER THAMES

Preliminary studies of the terrace deposits of theLower Thames valley (Bridgland, 1983, 1988;Gibbard, in prep. Gibbard et ai., 1988) haveconfirmed the accuracy of the mapping by theGeological Survey during the early years of thiscentury. Despite the problems in relating the maps ofthis area (New Series Sheet 257, Romford, and 171,Dartford) to those of the districts west of London,only minor modifications are required to the mappingeast of London. Indeed, the problem has arisenlargely because the mapping west of London failed toidentify the important Lynch Hill Gravel as a separateaggradational unit. Data from the existing GeologicalSurvey mapping have recently been supplemented bya reinvestigation of the sedimentary sequences,together with evidence from borehole surveys,allowing an enhanced reconstruction of terracesediment bodies (Gibbard, in prep.).

In this recent work, no evidence has been found forthe decrease in the downstream gradients of theLynch Hill/Corbets Tey Gravel and theTaplow/Mucking Gravel, east of London, that wasimplied by the published correlations between theLower Thames and eastern Essex (see fig. 5 inBridgland (1988». Instead, it appears that these twogravel bodies continue downstream at a gradient

51

52 DAVID R. BRIDGLAND ET AL.

Table 1. Gravel units described in this article, arranged according tothe revised correlation presented here

Middle Thames' Lower Thamesb Southend area" Offshored

Shepperton (Below lowestGravel floodplain) submerged

terraceKempton Park East Tilbury middle

Gravel Marshes Gravel submergedterrace

Taplow Mucking (below coastal highestGravel Gravel alluvium) submerged

terraceLynch Hill Corbets Tey Barling

Gravel Gravel GravelBoyn Hill Orsett Heath Southchurch

Gravel Gravel Gravel

• Gibbard (1985)b Bridgland (1988), Gibbard et al. (1988)"Bridgland (1988)d Bridgland & D'Olier (1989)

similar to the Boyn Hill/Orsett Heath Gravel, thepublished interpretation of which is upheld.

The downstream projections of the Corbets Tey andMucking Gravels, modified in the light of the aboveinformation, suggests correlation of the LynchHill/Corbets Tey Gravel with the Barling Gravel ofthe Southend area. No upstream equivalent of theRochford Gravel of the Southend area can thus beidentified (Fig. 1), nor can a downstream continuationof the Taplow/Mucking Gravel now be recognizedamongst the terrace deposits of eastern Essex,although it may be represented by gravel buriedbeneath coastal alluvium, previously correlated withyounger deposits (Fig. 1). According to this revisedscheme, the post-Boyn Hill terraces have longitudinalprofiles that run broadly parallel to that of the BoynHill/Orsett Heath/Southchurch Gravel (Fig. 1).

The conformity of terrace gradients throughout theThames sequence that this entails contrasts with theprevious scheme, in which the lower terraces hadshallower gradients than the higher ones downstreamfrom London.

The possible reconstruction of a terrace long profiledirectly connecting the Corbets Tey Gravel of thePurfleet-Tilbury area with the Barling Gravel to theeast of Southend has previously been discussed byBridgland (1983). He noted that an important outlierof Thames-Medway gravel at Grain, north Kent,which has a wide, flat upper surface suggestive of aterrace remnant some 12 m above 00, appears to fallexactly on such a profile (Fig. 1). Despite this,Bridgland adopted the correlation scheme publishedin 1988, partly because of the apparent necessity of

identifying an upstream equivalent for the RochfordGravel.

3. EVIDENCE FROM EASTERN ESSEX

The sequence of terrace deposits in eastern Essex iscomplicated by the occurrence of a number of deepburied channels that appear to underlie the fluviatilegravels (Lake, Ellison, Hollyer & Simmons, 1977;Bridgland, 1988). Bridgland showed that the gravelscan be divided into High-level East Essex Graveldeposits, of Medway origin, and Low-level East EssexGravel deposits, laid down by the Thames-Medwaysubsequent to the diversion of the Thames (as a resultof Anglian glacial activity north of London) into theformer valley of the Medway. He also showed that theburied channels occur only in association with thelater Thames-Medway gravels.

In his classification of the Low-level East EssexGravel, Bridgland (1983, 1988) largely followedearlier terrace mapping by Gruhn, Bryan & Moss(1974) and the Geological Survey (New Series, Sheet258/9; Lake, Ellison, Henson & Conway, 1986). Bothof these had recognized three terraces within what wassubsequently defined as Low-level East Essex Gravel,as is shown in Table 2. According to Bridgland (1983,1988), each of these three gravels overlies a differentburied channel (Table 2).

It has been shown earlier in the present paper thatan upstream equivalent of the middle of these threeterrace deposits, the Rochford Gravel of Bridgland(1988), can no longer be recognized. This calls intoquestion the separate existence of the Rochford

CORRELATION OF THAMES TERRACE DEPOSITS 53

'E'E .E '"t: ,.,

8 .E '" ~c: ;::::; Cl 0!l

80 =c:8..

7 j

60

50

-2

-30

-40

-50

'" " " " " "

~ 5 10 'fHorizOOtal s::ale -kIn

Terrace surfaces

ThamesMedwayThames-Medway

'l 'l 'l 'l Base of channels

'" '" '"

High - levelEast Essex Gravels ­of Medway origin(Bridgland 1988)

Fig. 1. Longitudinal profiles of terrace deposits to the east of London, showing the revised interpretation proposed in thisarticle. Only the reconstructed upper surfaces of the gravel bodies are shown, with the exception of theSouthend-Rochford/Clacton Channel and the Buried Channel, the bases of which are indicated. Note that the terms 'highestsubmerged terrace', 'middle submerged terrace' and 'lowest submerged terrace' are intended to be purely descriptive. Thelithostratigraphic terms Mucking Gravel, East Tilbury Marshes Gravel and Shepperton Gravel (respectively) should beapplied to the deposits forming these offshore features.

Gravel and the associated Rochford Channel. It isnotable that Gruhn et al. recognized their RochfordTerrace only in the Roach valley, a view reinforced bythe Geological Survey mapping, although Bridgland(1983) identified a possible upstream continuation tothe northeast of Southend. None of the variousremnants of Rochford Gravel gives rise to anyconvincing evidence of terrace morphology. Anotherproblem with this gravel is that it is difficult to confirm

that it overlies the Rochford Channel; instead, thetype outcrop at Rochford (TQ 877910) is only 4-5.5 mthick and has a situation suggestive of a meander-corewithin the arcuate course of the Rochford Channel(see fig. 8 in Bridgland (1988». Thus it is possiblethat the gravel at Rochford pre-dates the cutting ofthe channel and is part of the thick Southend ChannelGravel-Southchurch Gravel aggradation (Table 2).Without unequivocal evidence that the Rochford

54 DAVID R. BRIDGLAND ET AL.

Table 2. The Pleistocene fluviatile sequence in eastern Essex; a summary ofrecent research development

Correlation withMiddle Thames,

according to:

Gruhn et al. Geologicala Bridgland Bridgland Present(1974) Survey (1983, 1988) (1988) article

Shoeburyness Terrace 1 Barling Taplow LynchTerrace Gravel Gravel Hill Gravel

Shoeburyness ShoeburynessBuried ChannelChannel Deposits

Rochford Terrace 2 Rochford Lynch Boyn Hill/Terrace Gravel Hill Black

Gravel Park GravelRochford Rochford

Buried ChannelChannel Deposits

Asheldhamb Terrace 3 Southchurch Boyn Hill/ Boyn Hill/Terrace Gravel Black Black

Park Park GravelGravel

Southend SouthendChannel Channel

Deposits

a New Series Sheet 258/9; Lake et al. (1986).b The Asheldham type locality is situated on the Dengie Peninsula, north of theRiver Crouch.

Gravel is a separate aggradational terrace, there is noreason to conclude that the Rochford Channel 'loop'was only occupied by the river after downcutting fromthe Southchurch Gravel floodplain level; the pos­sibility thus exists that the Rochford Channel ismerely an overdeepened section of the Southend­Asheldham Channel and that it too belongs to theSouthend Channel Gravel-Southchurch Gravel ag­gradation. The Rochford Channel deposits would thusbe correlatives of those filling the Swanscombe LowerGravel-Lower Loam channel and the ClactonChannel (see Bridgland, 1988). It is hoped that aninvestigation of the fossiliferous sediments containedwithin the Rochford Channel and other buriedchannels in eastern Essex (Roe, in prep.) will shedmore light on this subject.

4. EVIDENCE FROM OFFSHORE

D'Olier (1975) used reflection seismic profilingtechniques to trace the drowned extension of theThames-Medway river system into the southernNorth Sea (Figs 2 & 3). On the sides of this valley

system a number of aggradational terraces were alsorecognized, their constituent sand and gravels beingeasily distinguished from the underlying Palaeogeneclays in the seismic profiling records (Fig. 3).

The altitudinal relations of three submergedoffshore terrace gravels to the onshore terraces of theThames-Medway system were illustrated by Bridg­land & D'Olier (1989). At that time it was notpossible to suggest direct correlations between theonshore and offshore sequences. One problem wasthat the offshore terraces appeared to have a steeperdownstream gradient than the lower terraces in theonshore sequence, comparable to the gradient of thehigher onshore terraces. A ready explanation forsteeper gradients in the offshore area was at hand: thepossibility that subsidence of the Noth Sea Basin hasbeen highly active during the latter part of thePleistocene. Bridgland (1988), however, haddismissed subsidence as a major influence on theterrace record in eastern Essex. Balson & Cameron(1985), furthennore, had established that the south­ern North Sea Basin has been tectonically stable sincethe Middle Pleistocene. The modified scheme for

CORRELAnON OF THAMES TERRACE DEPOSITS 55

No data

.of>

No-data

,-------------I

II

II

II

I

II

II

I

tN

Fig. 2. Map showing the offshore extension of the Thames-Medway valley (see text). Bedrock surface contours are shown.Remnants of submerged terrace deposits are indicated.

correlating the Lower Thames Terraces with those inthe Southend area presented here now invites areconsideration of the offshore evidence, without theneed to invoke differential subsidence.

Offshore from the modern estuary, the buried andsubmerged valley of the Thames-Medway, with itsassociated terraces, extends eastwards to approxim­ately 51°30'N/l°6'E, where it turns to the north (Fig.2). It then turns eastwards at approximately51°36'N/1"7'E and runs into the Southern Bight of theNorth Sea, where it joins the larger Rhine/Lobourgvalley (Fig. 2). At the present day the floor of thislatter valley slopes gently northwards, but thedistribution of sediments in the Netherlands sector ofthe North Sea suggests that the Rhine drainagesystem flowed southwards through the Straits of

Dover during the last glacial (Gibbard, 1988).Fragments of three submerged aggradational terraceshave been recognized along the flanks of the drownedThames-Medway valley, their component sedimentsbeing recognized in seismic profiling records (Fig. 3).Occurring predominantly on the northern side of thevalley, these have suffered considerable dissection,particularly the highest (oldest) terrace.

As Fig. 1 shows, the revised projections of thevarious gravel body long profiles into the southernNorth Sea have a downstream gradient comparablewith those of the terrace deposits recognized offshoreby D'Olier. The three offshore terraces illustrated byBridgland & D'Olier (1989) appear, from theirelevations and locations, to be continuations of theTaplow, Kempton Park and Shepperton gravels, which

.,

.,

, ..,....... ,)......••,f·

'.:

...... /., ..

"':.. ,.r

.... '

.... ,.. ~.~ .,,:...,' , ~ .

Fig. 3. An example of the type of information from which Fig. 2 has been compiled. The figure shows a seismic profile of thenorthern side of the Thames-Medway valley in an area where it is buried by F1andrian marine sediments. All threesubmerged terraces and the buried channel are represented in this traverse, but only the highest and middle terraces retainftuviatile sediments, the former in a very degraded state. The lowest terrace and the base of the buried valley are representedhere only by erosional features in the bedrock surface. The information is shown in (a) raw and (b) annotated forms.

CORRELATION OF THAMES TERRACE DEPOSITS 57

form the lowest three terraces in the onshore valley(Fig. 1).

5. CONCLUSIONS

The correlations, proposed here, between the Thamesterrace deposits in the London area and theirdownstream equivalents in Essex and beneath thesouthern North Sea represent a revision of thescheme published previously by Bridgland (1988) andBridgland & D'Olier (1989). The previous schemeoffered correlations only for the deposits recognizedonshore. Correlations with the submerged terracedeposits that lie off the Essex coast are offered herefor the first time. One of the merits of the revisedscheme is that the suggested terrace long profiles are

now broadly parallel throughout the sequence. Thisseems inherently more likely than the earlier scheme,in which the lowest terraces had very differentlong profiles from the higher and earlier aggradationalunits. An important implication of the revised schemeis that Thames deposits younger than the LynchHill/Corbets Tey Gravel, or at least those youngerdeposits laid down during periods of cold climate andlow sea-level, would not be expected to be preservedabove present ordnance datum in the areadownstream from Canvey Island (Fig. 1).

ACKNOWLEDGEMENTS

The illustrations were prepared at the CityCartographic Unit, City of London Polytechnic.

REFERENCES

BALSON, P. S. & CAMERON, T. D. J. 1985. Quaternarymapping offsho~e East Anglia. Modern Geology 9,221-239.

BRIDGLAND, D. R. 1983. 'The Quaternary fluvial depositsof north Kent and eastern Essex.' PhD thesis, City ofLondon Polytechnic.

-- 1988. The Pleistocene fluvial stratigraphy andpalaeogeography of Essex. Proceedings of the Geologists'Association, 99,291-314.

--, & D'OLIER, B. 1989. A preliminary correlation ofonshore and offshore courses of the Rivers Thames andMedway during the Middle and Upper Pleistocene. In(Henriet, J. P. & De Moor, G., eds) Quaternary andTertiary geology of the Southern Bight, North Sea. BelgianMinistry of Economic Affairs Geological Survey.

D'OLIER, B. 1975. Some aspects of late Pleistocene­Holocene drainage of the River Thames in the eastern partof the London Basin. Philosophical Transactions of theRoyal Society, A279,269-277.

GIBBARD, P. L. 1985. The Pleistocene History of the

Middle Thames Valley. Cambridge University Press,Cambridge.

-- 19118. The history of the great northwest Europeanrivers during the past three million years. PhilosophicalTransactions of the Royal Society, 8318, 559-602.

-, WHITEMAN, C. A. & BRIDGLAND, D. R. 1988. Apreliminary report on the stratigraphy of the LowerThames. Quaternary Newsleller, 56, 1-8.

GRUHN, R., BRYAN, A. L. & MOSS, A. J. 1974. Acontribution to Pleistocene stratigraphy in southeast Essex,England. Quaternary Research, 4, 53-71.

LAKE, R. D., ELLISON, R. A., HENSON, M. R. &CONWAY, B. W. 1986. Geology of the Country AroundSouthend and Foulness. Memoir of the British GeologicalSurvey.

-, -, HOLLYER, S. E. & SIMMONS, M. 1977.Buried channel deposits in the south-east Essex area; theirbearing on Pleistocene palaeogeography. Report of theInstitute of Geological Sciences, 77/21.