Temporary exposures of the Eocene London ClayFormation at Highgate, north London: rediscovery of

a fossiliferous horizon 'lost' since thenineteenth century

Steve Tracey', Stephen K. Donovan", Diana Clements!", Paul Jeffery3,

John Cooper', Phil Rye4 & Caroline Hensley'

TRAC EY, S., DONOVAN, S. K., C LEMENTS, D., JEFFER Y, P., COOPER, J., RYE, P. &HENSLEY, C. 2002. Temporary expos ures of the Eocene Lond on Clay Fo rmation atHighgate , north London : rediscovery of a fossiliferous horizon 'lost' since the nineteenthcentury. Proceedings of the Geologists ' Association, 113, 319-33 1. Highgate, north London, washistorically an impor tan t area for collectors of well-preserved macrofossils, particularly benthicmolluscs, of the London Clay Formation. Howeve r, exposures have not been generall yavailable since the nineteenth century. Auger holes and excavations made recently in the baseof the cutting on the for mer branch railway line from Finsbury Park to Highgate have shownrelatively unweathered , fossiliferous London Clay Fo rmat ion to occur within 1 m of thesurface. Thi s is the lowest unit of division E. A comprehensive faunal list is presented for thebasal clays, which are considered to represent a muddy marine shelf pa laeoe nviro nment duringthe deepest phase of the Londo n Clay sea.

I S choo l of Earth S cien ces. University of Greenwich, Chath am Maritime, Kent. M E4 4TB, UK2Departll1ent ofPalaeontology , National Natuurhistorisch Mu seum. Postbus 9517. N L-2300 RA.Leiden, The NetherlandsJDepartment of Palaeontology , The Natural History Mu seum. Cromwell R oad, LondonSW75 BD, UK"Park Crescent. Rounday, Leeds LS8 I DH, UK"Corresp onding author

1. INTRODUCTION

The Highgate area of north Lond on was a well-knownsite in the nineteenth century for fossils of the LondonClay Fo rmation (Arkell et al., 1954, p. 69). It formerlyyielded an abundant and diverse warm water faunawith an unusually robust pre servat ion for this for­mation listed in Wetherell (1842) and Newton (1891).Despite this, the strata containing this 'Highgatefauna' are unknown to most modern collectors, as theor iginal exposures are no longer ava ilable for studyand Highgate localities have never feat ured in relevanttwentieth century field guides (such as Blezard et al.,1967). The earliest nineteenth century collections fromHighgate were made during construction of theArchway Road [TQ 292 874], but exposures here arerarely seen today. In the following years of thenineteenth century, new railway cuttings and tunnel sprovided geologists with more of this fossiliferou ssandy clay. In particular, the richness of the fossilfaun a of the railway tunnel at the edge of HighgateWood [TQ 284 883- TQ 287 880] was remarked on inglowing terms by several author s. One such rich sourceof fossils was said to be in the cutti ng near the outermou th of the tunnel southeast of Highgate Stat ion

Proceedings of the Geologis ts ' Association, 113, 319-33 1

[TQ 2875 8800]. This line is no longer operat ing, but itsforme r course can still be followed in the long cuttingwhich now forms the part of Parkl and Walk betweenHighgate and Finsbury Park. The area around thetunnel mouth was considered by the authors to bea worthwhile prospect for re-excavation if the clayshere were still fossiliferous (Fig. I) . This would afforda rare opportunity to assess the stratigr aphy andfauna of this classic site in detail using modernobjectives for the first time; in particular the smallerfauna often missed by early workers. Con sequently ,several explorato ry holes were augered and, in thosenear the floor of the cutt ing, unweath ered LondonClay with scatt ered fossils was found less than I mbelow ground. In due course a trench was excava tedto 2 m at the same site. Our prelimina ry notes on theobserved biostratigraphy, together with notes on thehistory and biostratigraphy of the Highgate area,form the subject of the present paper. The firstpubli cat ion resultin g from this study was a shortaccount of the geology and fossils of the ParklandWalk (Clements, 2001), issued as a public infor­mation leaflet by Haringey Parks Service and obtai n­able at their Railway Fields Nature Park Exhib ition ,Green Lanes, Lond on N4.

0016-7878/02 S15.00 © 2002 Geologists' Association

320 s . T RACEY E T AL.

I ~ ~ ~ IBagshot Sands (Virginia Water Formation)

D Claygate Beds (London Clay Formation)

D London Clay (London Clay Formation)

0.5 km

Fig. I. Map of Highgate area , nort h London , redr awn and sim plified aft er British Geological Survey (1994). Asterisk mar ks theSt Aloysius' borehole. Inset shows deta ils of par t of the Parkland Wal k with locations of tr ial holes (A- D) and excava tion (X) .© Crown Co pyr ight NC/A 7.

EXPOSUR E S OF T HE EOCEN E LONDO N C L A Y A T HIGHGAT E 321

2. PREVIOUS WORK ON LONDON CLAYEXPOSURES AROUND HIGHGATE

Archway Road

Owing to the pressures of urb anizat ion and the rapidweathering of these soft sedimenta ry rocks, the onlyfresh exposures of London Clay in this area tendto have appeared in the course of deep temporaryexcava tions, most notably for roads and railways. Theearliest of these to att ract the attention of palaeontol­ogists was the construction of Archway Road inHighgat e (1810-1 813 [TQ 292 874]). Fossils collectedfrom the construction site were figured by JamesSowerby (1812-1822) and James de Carle Sowerby(1823-1 846) in The Mineral Conchology of GreatBritain, and were claimed to be one of the maininspirati ons for writing this important work . NathanielWetherell was a local resident at this time and collectedmany of these fossils himself, but he also stimulated'...the friendly aid of the workmen, who were delightedto keep for "the doctor" every shell and every fragmentof orga nism that pickaxe and spade revealed ' (Lobley,1889). Wetherell gave a talk to the Geo logical Societyon the Archway section on 13 June 1832, an abstractbeing publ ished in that year (Wetherell, 1832). Thi swas subsequently followed by a fuller account of theexposures and correlative sites in the Londo n Basin(Wetherell. 1836). The library of Th e Natura l HistoryMu seum , Lond on (BMNH), has an additional unpub­lished plate (drawn by J. de C. Sowerby) of some of themore unu sual Highgate shells in Wetherell's collection,all now housed in the museum. The Archway sectionwas exposed again between 1889 and 1890 in trenchesdug for the found ations of buildings. Abbott (1893)gave an account of the section, although the somewhatunfam iliar succession described suggests that it mayinadv er tently have included some of the clay backfillfrom the original Arch way construction .

More recently (1969), a 26 m borehole was sunk atSt Aloysius' College [TQ 291 873], a short distancewest of the original Archway Road cutting (Fig . I).Starting in overlying sands and gravels of theBagshot Formation, and bottoming in stiff siltyLond on Clay (division D) , the borehole passedth rough various units of division E, including 3 m ofsandy silty clay containing cha racteristic species ofthe 'Highgate fauna ' (see Lloyd's, 1880, appendix).Fossiliferous spoil and cores were investigated on siteby members of the Tertiary Research Group andtheir collected findings were summa rized by Coo per(1970). Material taken away for processing enabledsome rath er more precise stra tigra phic record ing ofthe limited meio- and microfau nas recovered (King,1981, p. 40).

Hampstead and Finchley area

Wethe rell also figured some Lond on Clay molluscsand microfossils from a well excava tion on nearby

Hampstead Heath , which were compared to theHighgate fauna (Wetherell, 1837).

Evans (1873) noted that the fossiliferous sandy clayoccurred not only in the High gate Wood and Arch wayRoad sections, but also at a definite level on the south,the west and the northwest sides of Hamp stead Heathin a continuous band underlying the Bagshot Sand. Acompiled list of the shell fauna of this bed, said to bemost abunda nt at one Finchley Road site ncar Child'sHill [TQ 252 865], was presented in the same work(Evans, 1873).

Underground Railway extension - Highgate toFinchley

The Lond on Clay spoil obtained by tunnelling for theUnderground Northern Line Extension from Highgateto Finchley [TQ 286 882-TQ 254 907] was thesource of a number of molluscs described and listedby Wrigley (I940a). These were also compared by himto some earl ier Highgate collections (see 'Discussion ­stratigraphical position ' section below), althoughmuch of Wrigley's collection was from deeperhorizons.

Mainline tunnels - Highgate Wood area

Th e fossil collectors ' bon anza most relevant to thepresent project had been pro vided much earlier duringthe main period of local railway construction in the1830-1 8605. At this time clay spoil from the tunnelsand cuttings out of King's Cro ss and St Pancras hadbeen laid out on nearb y fields to weathe r before beingmade into bricks. According to Arkell et al. (1954,p. 69), such temporary sites, includin g Highgate Wood,were the source of man y of the shells described andfigured by Edwards (1855-1 861).

Construction of the Edgware, Highgate and LondonRailway (par t of the Great Northern Railway North­ern Heights branch) was begun in 1864, opened in 1867(not 1830s as sta ted by Arkell et al., 1954), closed topassenge rs in 1954 and finally lifted in 1972 (Davies,1980; Con nor, 1997; Smith, 2002). The originalHighgate high-level station was built in the shelter ofa small, steep-sided valley below Shepherds Hill[TQ 286 882, Fig. 2]. The platforms are still visibletoda y above the entrance to the present HighgateUnderground Station , whose tube lines run at a deeperlevel. Trains on the former main (overground) linewould approach and leave Highgate high-level stat ionthrou gh two short pairs of single bore tunn els, onerunning northwest th rou gh High gate Wood, the otherleadin g out into a cutt ing on the southeast. The tunnelconstru ction sites and their spoil heaps were inevitabl yvisited by local geologists. Whitaker (1872, p. 303)reported that much clayey greensand had been foundwithin the southeastern tunnel and many beaut ifulfossils obtained from it. More of this sand was'scattered in the sandy clay near the bott om' next to

322 S. TRACEY ET AL.

Fig. 2. Highgate Station c.1868. The station opened in 1867 on the new Edgware, Highgate and London Railway. Passengerservices ran on the 1873 branch line to Alexandra Palace until 1954. The newer Highgate underground station is situateddirectly below this. View shows the northwestern tunnels to Highgate Wood, once a rich source of fossils, seen from a pointnear the southeastern tunnels, about 150 m from the present excavation. Postcard no. 9 in the 'Old Highgate Series'; reproducedby kind permission of the publishers: Homsey Historical Society, The Old School House, 136 Tottenham Lane, London N8.

the southeastern mouth. The location was later la­belled 'fossiliferous sandy clay' on the old 6-inchgeological map (Cameron, 1934). This segment of thecutting is now a relict and openly wooded cul-de-sac atthe western extremity of this section of the ParklandWalk. Being close to the tunnel and at the sameelevation, it was selected as the most promising site tolook for the classic fossil horizon (Fig. 1).

The locality name 'Highgate', as used for materialfrom the earliest collectors such as Sowerby andWetherell, usually referred to the original ArchwayRoad exposures (see above). However, among the localvariations used on old collection labels and by earlyauthors (e.g. Newton, 1891) were 'Highgate WoodTunnel' and 'Highgate Wood Tunnel, Finchley'. Sinceno part of Highgate Wood was ever located inFinchley, both these names presumably refer to therailway tunnel leading northwest from Highgatestation, which is the only tunnel within HighgateWood. Whitaker also used the 'Finchley' version forsome of his own fossils, whose precise sources werepublished in detail (Whitaker, 1872, p. 303; 1889,p. 257). Although the corresponding southeasterntunnel is outside Highgate Wood, the same names mayalso have been used for material from this site. Inaddition to all of the above, Newton (1891) also usedthe variations 'Highgate Wood, Finchley', 'HighgateWood near Finchley', and simply 'HighgateWood'. These would have been taken from labels in

the F. E. Edwards collection and were perhaps the onlylocality data supplied to Edwards by those who hadcollected for him. In the absence of any other evidencewe can assume that they may all refer to materialexcavated from the same pair of railway tunnelsand perhaps temporarily heaped in Highgate Wood.However, maps show another railway cutting on thenorthwest side of the wood towards Finchley thatcould feasibly have been an additional source of someof Edwards' material (British Geological Survey, 1994[grid reference TQ 281 888]). This area is still in use asa head shunt for the Northern Line depot betweenHighgate and East Finchley, and is not accessible forgeological investigation.

3. EXCAVATION PROJECT AT HIGHGATE

Auger survey

A preliminary survey of this area, now part of theParkland Walk, was made by five of the authors(DC, rc, SKD, Pl, ST) together with David Bevan(Haringey Conservation Officer) and lain Fletcher(Surrey RIGS - Regionally Important GeologicalSites) on 27 April, 1999. To assess the present-daygeological potential of the site, several trial holes wereaugered by hand in the sides and floor of the cutting.Despite a build-up of slumped loams and topsoil onthe sides of the cutting, it was possible to assess the

EXPOSURES OF THE EOCENE LONDON CLAY AT HIGHGATE 323

overall succession; the following lithologies were notedin auger holes A-D (Fig. 1):

A. At the top of the eastern slope of the cutting, wellbelow the road level of Shepherds Hill: brown inter­laminated sands and clays corresponding to the localequivalent of the Claygate Member were augered to1.8 m below 0.4 m of rubbly topsoil, i.e. to a totaldepth of 2.2 m.

B. Halfway down the overgrown eastern slope of thecutting, directly below hole A, about 4.5 m above thetrack base. Oxidized brown sandy clay, seeminglyunfossiliferous, relatively undisturbed in the lowerI m+ but its upper limit obscured by a capping ofslumped sandy material and topsoil. At 1.7 m a sharpcontact with fresh grey clay containing indeterminateshell fragments below the top 0.3 m.

C. In the floor of the cutting, in mid-track base,2.6 m southeast of the tunnel mouth: fresh grey claywith scattered shells augered to 1.6 m.

D. In the floor of the cutting, again in mid-trackbase, 27 m from the tunnel mouth: bluish-grey, slightlysandy clay augered to a depth of 2.5 m; sparselyfossiliferous in the upper part, noticeably greater pyritecontent below 2.4 m. Fossils found in the auger coreincluded indeterminate plant debris in nodular pyrite,and both aragonitic and calcitic shells which weremostly crushed and fragmentary. Internally pyritizedVaricorbula pairs were found uncommonly from 0.2 mdownwards; a fish vertebra and the small gastropodGemmula below 1 m; associated fragments of the thick­shelled bivalve Pitar and one tube of the polychaeteworm Ditrupa below 2.25 m. This spot was earmarkedfor further excavation (see below) in view of theeasy access and the promise of a relatively diversefossil fauna. The lack of fossils in the more intenselyweathered clay and the steeply wooded sides of thecutting ruled out excavating auger sites A and B.

Main excavation

With the support of Haringey Council, a mechanicalmini-digger was hired to make an excavation in thetrack base in the general area of trial hole D (Fig. I[TQ 288880]). On 16 September, 1999, the mini-diggeropened up a trench 2 m long and initially 1.5 m deep(Fig. 3a), eventually widened and taken down to 1.8 mwhich was the maximum operational depth of thedigger (Fig. 3b). Below 0.3 m of made ground (com­pressed brick rubble comprising the track base) was theunit noted earlier, of fresh medium blue-grey silty clay,slightly sandy in places, with small « 1 em) pyritenodules and scattered shells; more or less oxidized inthe top 0.4 m. Samples of this were bagged fromapproximately 1 m downwards (HGB). At 1.5 m deptha septarian nodule 15 em in diameter was seen in situ,and further similar mudstone fragments collected fromthe spoil. From 1.5-1.8 m shell material was moreapparent than above. Most of the bagged clay, selectedfrom the excavated pile (HGA), came from this

interval. One of the team (PJ) increased the depth toapproximately 2 m with a spade. This was consideredby the director of the dig (SKD) to be about themaximum depth to which a temporary excavationcould safely be dug without shuttering. Several kilo­grams of moderately shelly clay, were hand-collectedhere at the base of the section (1.9 m ± 0.1 m; sampleHGA). A composite section constructed from all theabove measurements is given in Figure 4.

Larger macrofossils were most obvious and easiestto collect in the material dug from below 1.5 m.Lithological samples of oxidized and unoxidized clay,and concretions, were also taken. Altogether 50 bags,each of approximately 10 kg of clay, were removed tobe processed for fossil content in the laboratory. Whenthe section had been recorded and samples loaded up,the trench was backfilled and the original turf replaced.

4. RESULTS

Sampling and processing

The finer pyritic silty clays from our excavation belowtrack level unexpectedly revealed a significant andlargely unrecorded biota, overlooked by early workersbecause of its small size. Clay spoil from the upperc. 1.5 m of this excavation at first appeared unfossil­iferous except for widely scattered small, fragile, andmostly crushed molluscs, and pyrite debris. Thespoil from 1.5-1.8 m, associated with the horizon ofseptarian nodules, was more fossiliferous. However,the clay dug by hand, at a depth of 1.9 m, was seen tocontain even more fragmentary shell material than the1.5-1.8m interval. It also contained occasionaladult examples of the gastropods Volutospina nodosa(1. de C. Sowerby, 1823) and Wrigleya complanata(1. de C. Sowerby, 1823), up to 45 mm long. Samplesof clay from this c. 20 em interval were bagged andtreated separately. Owing to difficulties imposed by theweather, by the mechanized method employed and bythe scattered distribution of fossils, most of the largevolume of clay needed to produce an adequatequantity of residue for a meaningful faunal analysiswas gathered ex situ from the spoil heap. Although thedepth at which the clay was currently being dug couldbe estimated at any given moment, it was often moreconvenient to recognize the sources of excavatedblocks by their appearance (i.e. shelly clay on the topof the heap=lower unit; more-or-Iess unfossiliferousclay=probably upper unit). Consequently, there is astrong possibility that some contamination of samplescould have occurred and that the exact figuresobtained from any analysis based on such sampleswould be compromised. However, regarding thegenerality of trends inferred from one such analysis(species frequency chart, Table 1), the effect of asmall degree of sample impurity is considered to beinsignificant.

A preliminary examination by one of us (ST) ofsome of the material from each of these two horizons

324 S. TRACEY ET AL.

Fig. 3. Work in progress 16 September 1999. (a) One of the authors (Pl) examining the eastern face of the trench excavatedinitially; this shallow excavation was not considered to require shuttering for the brief period «2 hours) that it was exposed.(b) Mini-digger widening the trench viewed from the western end, with spoil heap to the left.

involved washing samples of the dried clay throughgraded sieves with a mesh size of I mm and 0.25 mm.The residues recovered by this process were examinedunder a low-power binocular microscope, and alldeterminable fossils were picked out, identified andcounted. Additional smaller residue samples downto 75/lm grain size were retained and searched formicrofossils.

Most of the bulk samples were reduced to >0.5 mmgrain size in a clay-washing machine (Ward, 1981)donated to the BMNH by its designer, David Ward,who also kindly processed some of our initial samplesin his own updated model.

Statistics of faunal distribution

Excluding indeterminate pyrite debris, some of whichseemed to represent burrows and/or plant fragments,molluscs clearly dominated the macrofauna (>0.5 mm)

in both diversity and number of individuals (Table I).Other fauna represented included coelenterates, anne­lids, decapod crustaceans, brachiopods, echinoderms,and fish teeth and otoliths (Table 2), but no species ofany of these groups exceeded 0.5%, of the total macro­fauna in either sample.

The statistical calculation (Table I) shows the rela­tive frequencies of mollusc species found in eachsample, and compares the two intervals of deposition.Frequencies of the commonest species are given aspercentages of the total number of individuals in eachsample. The less common species, marked r (rare, 2--4examples foundlless than 0.4% of total fauna) or u(uncommon, 5-10 examples found) and included in thecalculation, together comprised only 3% of the molluscfauna in each sample. The same lettering system is alsoused to indicate the overall rarity in the samples ofspecies of other phyla (Table 2), although these wereexcluded from the statistical count in view of theirscarcity.

EX POS URES OF T H E EOCENE LON D O N C L A Y AT HIGH G A T E 325

Borehole atSt Aloysius' Collegenr.Hlghgate Arch way,

after Cooper (19701

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(describe d herein)

Parkland Walk .cutting SE of

Highgate Stat ion

Silty sandy clay (weathered)

Laminated sands & clays

Blue-grey silty clay with scatteredsmall shells and pyritized plant debris

Irregularly spaced septaria at top of siltyshelly clay with larger benthonic gastropods

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Fig. 4. Composite section (right) based on trial holes and excavation in the Parkland Walk southeast of Highgate stat ion ,corr elated (left) with part of the more complete log of St Aloysiu s' College borehole, I km to the south (after Cooper, 1970).Stra tigraphic al units are based on King (1981, p. 35, text-fig. 8).

The respective clay samples provided the following:

HGA: Lower unit - shelly clay betwee n and belowscattered septaria 1.5-2.0 m below gro und : 50 kg clay(dry weight) - to ta l 873 mollusc specimens represent ­ing 44 species. The 23 most freq uent of these com­pri sed 97'% of the total populat ion [average c. 17.5individua ls per I kg clay].

HGB : Upper unit - stiff silty clay 0-1.5 m belowgro und : 20 kg clay - tot al 440 mo llusc specimensrep resenting 39 species [average c. 22 individ uals perI kg clay) . The 26 most frequent of these comprised97% of the total population.

A furt her c. 400 kg of undifferentiated spoi l(sample HGU) was subsequently sieved to 0.5 mm andexamined, although this increased the moll usc list byonly 6 addi tio na l species.

M ollusca: changes in frequenc y and faunal notes

As noted above, on ly mollu scs provided sufficientnumbers of indiv iduals to highlight the differencesbetween the two main intervals sampled. T he percent­age compositions of the respective mollusc faunas inthe two samples were close ly similar, consisting of

326 S. TRACEY ET AL.

Table 1. Mollusca from the Parkland Walk excavation, Highgate: species extracted from samples of lower shelly clay (HGA)and upper clay (HGB) units.

trend HGA HGB (HGU)

Mollnsca - GastropodaHaustator sp. indet.Alvania sp.*Adeorbis aff. lucidus Cossmann, 1881*Solariorbis sp* [Teinostoma priscum Newton non Desh.]Chevallieria cf. cylindroides Cossmann, 1907Entomope semipunctata Jeffery & Tracey, 1997Eotibia lucida (1. Sowerby, 1815)xenophorid? sp. [protoconch only]Euspira glaucinoides (1. Sowerby, 1812)Eocypraea? sp. [protoconch only]Ficopsis multiformis (Wrigley, 1929)Crassiscala subterranea (Wrigley, 1940b)Wrigleya complanata (J. de C. Sowerby, 1823)Wrigleya conifera (J. Sowerby, 1818)Daphnobela juncea (Selander, 1766)Volutospina nodosa (J. de C. Sowerby, 1823)Amalda cf. arenaria (Cossrnann, 1889) [protoconch only]Bonellitia laeviuscula (J. Sowerby, 1822)Conilithes concinnus (1. Sowerby, 1821) [iUY.]Cochlespira pulcherrima (Edwards, 1857)Turricula teretrium (Edwards, 1857)Eopleurotoma simillima crassilinea (Edwards, 1861)Eopleurotoma abnormis (Edwards, 1861)Eopleurotoma sp. *Amblyacruml sp. [protoconch only]Gemmula longaeva (Edwards, 1861)Gemmula (s.l.) [asciolata (Edwards, 1861)Mathilda sororcula Wrigley, 1940bOdostomia sp. *Crenilabium elongatum (1. de C. Sowerby, 1824)Ringicula turgida (J. Sowerby, 1817)Roxania hiumbilicata (Deshayes, 1863)Cylichna aff. censors (Deshayes, 1863)*Cylichna aff. uniplicata (1. de C. Sowerby, 1850)*Volvulella oxyacrum (Cossmann, 1889)Limacina mercinensis (Watelet & Lefevre, 1880)Limacina taylori (Curry, 1965)Limacina aff. taylori (Curry, 1965)'Altaspiratella bearnensis (Curry, 1981)Mollusca - BivalviaNucula consors Wood, 1864Leionucula sp. [iUY.]Yoldiella prisca (Deshayes. 1860)?Ledina amygdaloides (1. de C. Sowerby, 1827) [mould]Striarca wrigley! (Curry, 1958) [Glycymeris]Trigonodesma lissa (Bayan, 1873)Cucullaria impolita (J. de C. Sowerby, 1837)Semimodiola elegans (1. Sowerby, 1812)Amygdalum sp. [iUY.]Lentipecten corneus (J. Sowerby, 1818)?Atrina affinis (1. Sowerby, 1821) [fragments]Pteria media (J. Sowerby, 1812) [iUY.]Anomia anomialis (Lamarck, 1819)Heteranomia scabrosa (Wood, 1861)Nemocardium nitidulum (Tremlert, 1950)Abra splendens (1. de C. Sowerby, 1837)Pitar sulcatarius (Deshayes, 1825)Varicorbula globosa (1. Sowerby, 1818)Mollusca - ScaphopodaLaevidentalium nitens (1. Sowerby, 1814)

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The number of individuals of each species is expressed as a percentage of the total number of individual molluscs per sample, rounded to the nearest integer. Species whoseindividuals make up less than 0.51% of the total in either sample are indicated only by their overall rarity: r, rare (2--4examples/less than 0.4(/"0 of total fauna); u, uncommon(5-10 examples); c, common; [rl, species identified from minor fragments only; *, species probably undescribed; -, bivalves with articulated paired valves present.Additional taxa that occurred in the remaining unlocalized samples (HGU), but not in the counted samples, are recorded in the right-hand column. Where the relativefrequency of a species differed significantly between the two units, the increase or decrease trend (with time) is indicated by symbols in the left-hand column.

EXPOSUR E S OF TH E E OCE N E LONDON C L A Y AT HIGHGATE

Table 2. Non-molluscan phyla from the Parkland Walk excava tion, Highgate: faun a extractedfrom all samples (HGA, HGB , HG U).

327

Protozoa - ForaminiferidaSp irople ctinella gr. carinata (d ' Orbigny. 1846)miliolid sp.Brizal ina angl ica (Cushma n, 1936)dentalinid sp. indet .Pulsisiph onina prima (Plummer, 1926)Euuvigerina hatjesi (Kaasschieter, 1961)Cihi cides tallahatt ensis Band y, [949Cihicidoides aff. alieni (Plummer, 1926)'Lenticulina spp.Anom alinoides aff. nobilis Brotzen, 1948'Anomalinoides acutus (Plummer, 1926)Porifera?scleractinian [spicule]Coelenterata - PennatulaceaGraphularia wetherelli Milne-Edwards & Haime, 1850Annelida - PolychaetaDitrupa plana (1. Sowerb y, 1815)Crustacea - Decapoda?pagurid sp. indet. [limb fragment]'!calianassid sp. indet. [burrowed intern al cast of left chela]Glyp hi thy reus wetherel li (Bell 1858) [carapace & chelae]Z anthopsis leachii (Desmarc st, 1822) [right dact ylus]?Xanthilites sp. [xanthid chela]Portun ites sp. [chela]Crustacea - OstracodaCytheridea newburyensis Gokcen, 1971Eucytherura sp."Lox oconcha aff. nyst iana (Bosquet, 1852)'Eopaijenhorchella cf. loma taTri cbcl, 1949Echinocy thereis reticula tissim aEager, 1965Brachiopoda - LingulidaLingula tenuis (J . Sowerby, 18 13)Echinodermata - Echinoideaspatangoid - spines and test platesEchinodermata - Asteroideaastero id [indet .]Echinodermata - OphiuroideaOph iura weth erelli Forbes, 1852 [lateral arm plate s]Coulonia colei (Forbes, 1852)Ichnotaxacf. Glyphichnus sp. [pyritized burrow infills 1-3 mm diam.]Vertebrata - ChondrichthyesPhysogaleus secundus (Winkler, 1874) [shark - teeth]Rhin obatos bruxelliensis (Jaekel, 1894) [guitar ray - teeth]Vertebrata - OsteichthyesPycnodus sp. [teeth]Pterothrissus angulatus Stinton, 1966 [bonefish - otoliths]Mu raenesox cymbium (Stinton, 1966) [pike-conger - oto liths]Hildebrandia eircularis (Stinton, 1966) [conger eel - otoliths]Synodus davisi (Frost, 1925) [lizard fish - otolith s]Ophidypterus obtusus (Frost, 1925) [cusk eel - oto liths]Glyptophidium po lli (Ca sier, 1946) [cusk eel - oto liths]serranid sp. [indet.] [sea bass - otoliths]pomada syid sp . [indet .] [grunt - otoliths]Cepo la densa (Frost , 1934) [ribbon-fish - otoliths]A rdiodus marriotti White, 1931 [mackerel - teeth ]Cybium proost i Sto rms, 1897 [mackerel - teeth]PlantaeWetherellia variabilis Bowerban k em. Reid & Chandler, 1933

General indication of frequency as in Table I

u

u

u

r

u

U

I'

I'

U

U

u

rur

ruurrrrr

328 s . TRACEY ET AL.

sma ll to medium-sized benthic gastro pods andinfau nal bi~alves , of genera cha rac teris tic of Earl yEoc~ne man ne clays, together with a significant pro­portion . of holoplanktonic gastro pods and plank­tot rop hic lar val shells showing little or no subsequentadult gro wth. The main differen ces observed betweenthe two samples were the relati ve frequencies of a fewgastropod species (Table I, ' trend' column), of whichthe two most significant communities were as follows.

(a) Several carnivores, Volutospina , Bonellitia andEuspira, and a detritus feeder Eotibia, some ofwhich had survived long eno ugh to grow to adult­hood. Thes e were more commo n in the lower shellbed with septa ria, their rela tive frequencies beingreduced by up to 50°;() in the overlying clays .

(b) Two of the four species of holoplanktonic opistho­bran chs (pteropods) present: one an undescribedLim.acina, similar to L. taylori (Curry, 1965), buthaving a flattened spire ; the other Altasp iratellab~arnell:~is \Cu~ry, 1981), rarely record ed despite itsWIde distribution. Unlike Limacina mercinensis(Wate let & Lefevre, 1880) and L. tavlori, the othertwo pterop ods in the sample, whose frequenciesremained constant, these two species increased by upto 50% in the upper homogeneous clays. Pterop odshave been used with some success for cor relation ofC.en~zoi~ strata, in view of their geographically widedistribution, albeit restri cted in time, which impliesthat they could show a significant rate of evolution­ary change. The fact that two par ticular species inour samples appear to be thri ving while two otherclosely re!ated species are declining, though all livingtogether 111 the plankton , may therefore indicate anevolutionary event. Altaspiratella bearnensis, whilevery common in our sample , was not recorded fromEngland at . all in Curry's (1965) monograph ,although, as It happened, it was he himself who firstdescribed the species some years later from beds ofslightly younger age at Gan (NPI3), southernFrance (Curry, 1981). In England it is now seen tobe locally abundant in the lower part of division E of~he London Clay Formation (NPI2). There areIsolated records of the species (ex situ - perhapsdivision C or D) from Sheppey , Kent (Janssen, 1990;Hodgkinson et al., 1992) and (ex sit ll - but of similarage or older) from Burnham-on-Crouch, Essex (A.Janssen, pers. comm.). It is also recorded fromsomewhat younger stra ta (basa l Lutetian , NPI 5) inthe Weches Form ation of Texas, USA). Janssen(1990) also confirmed its presence in division Ematerial from St Aloysius' College, Highgate (seeabove), and its brief appearance in the No rth SeaBasin. Its apparent overall scarc ity may have beendue partly to its being mistaken for L. tutelina(Curry, 1965), which characteristica lly occurs at alower horizon in the Lond on Clay.

Bivalves were not so well represented as gastro podsaJ~d genera lly belong to lon g-lived species of generaWIdely distribut ed in this facies of the Londo n Clay .

Th e ubiquit ous deep burrowers Nunda and Yoldiellaare the most commo n. Scaphopods were barelyrepresented a t a ll, only a solitary example being found .

5. DISCUSSION

Stratigraphical position

Perh aps the combination of rich fossil beds and rela­tively homogeneou s lithology was responsi ble for thelack of detailed stra tigra phic records of previou sexposures around Highgat e. Assessments of the bio­stra t ig~aphy of the area and its corre la tion, byPrestwich (1854) and Wrigle y (1940a), have beenreviewed by King (1981). Whereas all agreed that thesebeds 1c~y near the top of the London Clay Form at ion ,they dl!fere~ !n. regard ~o which boundarie s separatedt~e maJo.r d.lvIS!On~. Wngley (1940a) not ed con trastingdlag~n.esl~ 111 fossil material from the old Highgatelocaliti es 111 museum collections. The se were consideredto represent an upper horizon con tai ning well­preserved uncru shed shells with a sand y matrix, and alower !evel with shells mostly interna lly pyri tized,ot h~rwlse crushed or damaged, in a clay matrix(Wng ley, 1940a , p. 235). Th ese two lithofacies alsoappeared to suppo rt somewhat dissimilar faunas. Inthe a bsence of any adequa te stra tigra phical da ta forthe area, the clay unit was mistakenly correlated withra ther older sites to the south of Lond on and the baseof his upp erm ost division (5) was drawn between thetW? Highgate beds. In a subsequent assessment,Bns~~w et al. (1980, p. 265) pre ferred to gro up thefossiliferous sandy clay with the overlying Clayga teBeds,. on the basis of its coarser grained lithology andal~o Its suppose dly typical Claygate fauna . However,Km g (1981, p. 39), found the faun al con tinuitybetween .the two fossil beds more convincing, anddefined hIS own upp erm ost division (E) of the Londo nClay as extending from the base of the pyritic clay (unitI) to ~he top of the fossil iferous sandy clay (unit 2),overlain by the Claygate Member (unit 3). Also units 1and 2 of division E were considered (Cooper , 1970)to be the source of the 'Highgate faun a ' and thisconclusion is supported by the result s of our ownstudy.

The present section can be correlated with Cooper'sO?70) log of the St Aloysius' Co llege borehole a tHIghgate Archway, I km to the south, and also withKing's (1981) generalized Eocene stra tigra phy of thearea (see Fig. 4).

The bio- and lithost rati graph y of the fossilifero usclay from our main excava tion equates it to unit I ofdivision E, probabl y within 3 m of the base, which hasbe~~ dated. as within NP I2 (Ali et al., 1993). Th eong mal ? bJect of ~~is excavation had been to samp lethe claSSICglauco nit ic sandy clay hor izon with the rich'Highga te fauna' of macrofossils. Alth ough our tri alholes were augered in the sides of the cutti ng close tothe tunn el and on the same horizon tal level, theyencountered only weathered sandy clay appa rently

EXPOSURES OF THE EOCENE LONDON CLAY AT HIGHGATE 329

without fossils. This suggested that the shelly horizonhere might now be decalcified, at least superficially,although local dip and lenticular development of theshell bed(s) were alternative explanations to consider.

Palaeoenvironment

The homogeneous silty clays seen in the upper shell bedlacked any sedimentary structures, presumably owing tototal bioturbation. The large proportion of planktonicmaterial and the low survival rate of many juvenilegastropods in the muddy substrate is characteristic ofsedimentation in a relatively deep-water shelf environ­ment. As at Sheppey the common plant debris compareswell with that from modern mangrove environments. Itspresence in a muddy shelf facies suggests flotsam carriedout to sea through a major estuary.

The higher proportion of more adult benthic gastro­pods in the lower shell bed with septaria can best beinterpreted as an interval of reduced sedimentationallowing development of intermittent hardgroundshospitable to benthic gastropods. Their relative fre­quencies were reduced by up to 50'%, however, with theonset of steady sedimentation.

6. CONCLUSION

The sandy bed exposed in the nineteenth centurycontaining the 'Highgate Fauna' had been notable forits uncrushed fossils and lack of pyrite. Larger fossilsin the clays we examined were mostly pyritized, fragileand somewhat crushed. This supports Wrigley'sobservation that two different faunas in separate hor­izons were represented in the old Highgate collections,

ours being the lower. It is hoped that we will get theopportunity to locate a suitable site within the uppersandy level in this area in order to compare the fossilsfrom the two horizons. A fuller account of the fauna,including descriptions of the eight or so new gastro­pods, will be presented when the remaining sampleshave been processed.

ACKNOWLEDGEMENTS

For taxonomic determinations of certain groups withinthe fauna we are grateful to several experts in theirrespective fields: Allan Lawson (Vertebrata), ProfessorAlan Lord (Crustacea - Ostracoda), Joe Collins(Crustacea - Decapoda), Dr Adrian Rundle (Echino­dermata; miscellaneous invertebrates), and Dr ChrisKing (Foraminiferida and specialist advice on ptero­pod distribution). Thanks also to Dr Arie Janssen foradditional advice on the pteropods and Dr JohnWhittaker for help with the microfossil references. Wethank David Ward for processing some of our initialclay samples, and David Bevan, Conservation Officerfor Haringey, who arranged payment of the WayleavePermission to Haringey Council and gave this projecthis enthusiastic support throughout. Our thanks alsoto Tommy Smyth of Orbital Equipment, WalthamAbbey, for his efficient excavation and to lain Fletcherfor providing the augers. This research was madepossible by a grant from the BMNH's PalaeontologicalResearch Fund, which we gratefully acknowledge. Thispaper was improved in the light of insightful reviewcomments by Dr Eric Robinson (University CollegeLondon) and David Bone.

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Manuscript received 3 May 2002; revised typescript accepted 8 July 2002