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

a fossiliferous horizon ‘lost’ since thenineteenth century

Steve Tracey1, Stephen K. Donovan2, Diana Clements3*, Paul Jeffery3,John Cooper3, Phil Rye4 & Caroline Hensley3

TRACEY, S., DONOVAN, S. K., CLEMENTS, D., JEFFERY, P., COOPER, J., RYE, P. &HENSLEY, C. 2002. Temporary exposures of the Eocene London Clay Formation atHighgate, north London: rediscovery of a fossiliferous horizon ‘lost’ since the nineteenthcentury. Proceedings of the Geologists’ Association, 113, 319–331. Highgate, north London, washistorically an important area for collectors of well-preserved macrofossils, particularly benthicmolluscs, of the London Clay Formation. However, exposures have not been generallyavailable since the nineteenth century. Auger holes and excavations made recently in the baseof the cutting on the former branch railway line from Finsbury Park to Highgate have shownrelatively unweathered, fossiliferous London Clay Formation to occur within 1 m of thesurface. This 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 palaeoenvironment duringthe deepest phase of the London Clay sea.

1School of Earth Sciences, University of Greenwich, Chatham Maritime, Kent, ME4 4TB, UK2Department of Palaeontology, National Natuurhistorisch Museum, Postbus 9517, NL-2300 RA,Leiden, The Netherlands3Department of Palaeontology, The Natural History Museum, Cromwell Road, LondonSW7 5BD, UK4Park Crescent, Rounday, Leeds LS8 1DH, UK*Corresponding author

1. INTRODUCTION

The Highgate area of north London was a well-knownsite in the nineteenth century for fossils of the LondonClay Formation (Arkell et al., 1954, p. 69). It formerlyyielded an abundant and diverse warm water faunawith an unusually robust preservation for this for-mation listed in Wetherell (1842) and Newton (1891).Despite this, the strata containing this ‘Highgatefauna’ are unknown to most modern collectors, as theoriginal exposures are no longer available for studyand Highgate localities have never featured 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 tunnelsprovided geologists with more of this fossiliferoussandy clay. In particular, the richness of the fossilfauna of the railway tunnel at the edge of HighgateWood [TQ 284 883–TQ 287 880] was remarked on inglowing terms by several authors. One such rich sourceof fossils was said to be in the cutting near the outermouth of the tunnel southeast of Highgate Station

[TQ 2875 8800]. This line is no longer operating, but itsformer course can still be followed in the long cuttingwhich now forms the part of Parkland 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. 1). This would afforda rare opportunity to assess the stratigraphy andfauna of this classic site in detail using modernobjectives for the first time; in particular the smallerfauna often missed by early workers. Consequently,several exploratory holes were augered and, in thosenear the floor of the cutting, unweathered LondonClay with scattered fossils was found less than 1 mbelow ground. In due course a trench was excavatedto 2 m at the same site. Our preliminary notes on theobserved biostratigraphy, together with notes on thehistory and biostratigraphy of the Highgate area,form the subject of the present paper. The firstpublication resulting from this study was a shortaccount of the geology and fossils of the ParklandWalk (Clements, 2001), issued as a public infor-mation leaflet by Haringey Parks Service and obtain-able at their Railway Fields Nature Park Exhibition,Green Lanes, London N4.

Proceedings of the Geologists’ Association, 113, 319–331 0016-7878/02 $15.00 � 2002 Geologists’ Association

Fig. 1. Map of Highgate area, north London, redrawn and simplified after British Geological Survey (1994). Asterisk marks theSt Aloysius’ borehole. Inset shows details of part of the Parkland Walk with locations of trial holes (A–D) and excavation (X).� Crown Copyright NC/A7.

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2. PREVIOUS WORK ON LONDON CLAYEXPOSURES AROUND HIGHGATE

Archway Road

Owing to the pressures of urbanization and the rapidweathering of these soft sedimentary rocks, the onlyfresh exposures of London Clay in this area tendto have appeared in the course of deep temporaryexcavations, most notably for roads and railways. Theearliest of these to attract the attention of palaeontol-ogists was the construction of Archway Road inHighgate (1810–1813 [TQ 292 874]). Fossils collectedfrom the construction site were figured by JamesSowerby (1812–1822) and James de Carle Sowerby(1823–1846) in The Mineral Conchology of GreatBritain, and were claimed to be one of the maininspirations 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 organism that pickaxe and spade revealed’ (Lobley,1889). Wetherell gave a talk to the Geological Societyon the Archway section on 13 June 1832, an abstractbeing published in that year (Wetherell, 1832). Thiswas subsequently followed by a fuller account of theexposures and correlative sites in the London Basin(Wetherell, 1836). The library of The Natural HistoryMuseum, London (BMNH), has an additional unpub-lished plate (drawn by J. de C. Sowerby) of some of themore unusual 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 foundations of buildings. Abbott (1893)gave an account of the section, although the somewhatunfamiliar succession described suggests that it mayinadvertently have included some of the clay backfillfrom the original Archway 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. 1).Starting in overlying sands and gravels of theBagshot Formation, and bottoming in stiff siltyLondon Clay (division D), the borehole passedthrough various units of division E, including 3 m ofsandy silty clay containing characteristic 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 summarized by Cooper(1970). Material taken away for processing enabledsome rather more precise stratigraphic recording ofthe limited meio- and microfaunas recovered (King,1981, p. 40).

Hampstead and Finchley area

Wetherell also figured some London Clay molluscsand microfossils from a well excavation on nearby

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

Evans (1873) noted that the fossiliferous sandy clayoccurred not only in the Highgate Wood and ArchwayRoad sections, but also at a definite level on the south,the west and the northwest sides of Hampstead Heathin a continuous band underlying the Bagshot Sand. Acompiled list of the shell fauna of this bed, said to bemost abundant at one Finchley Road site near Child’sHill [TQ 252 865], was presented in the same work(Evans, 1873).

Underground Railway extension – Highgate toFinchley

The London 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 (1940a). These were also compared by himto some earlier Highgate collections (see ‘Discussion –stratigraphical position’ section below), althoughmuch of Wrigley’s collection was from deeperhorizons.

Mainline tunnels – Highgate Wood area

The fossil collectors’ bonanza most relevant to thepresent project had been provided much earlier duringthe main period of local railway construction in the1830–1860s. At this time clay spoil from the tunnelsand cuttings out of King’s Cross and St Pancras hadbeen laid out on nearby fields to weather before beingmade into bricks. According to Arkell et al. (1954,p. 69), such temporary sites, including Highgate Wood,were the source of many of the shells described andfigured by Edwards (1855–1861).

Construction of the Edgware, Highgate and LondonRailway (part of the Great Northern Railway North-ern Heights branch) was begun in 1864, opened in 1867(not 1830s as stated by Arkell et al., 1954), closed topassengers in 1954 and finally lifted in 1972 (Davies,1980; Connor, 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 visibletoday 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 stationthrough two short pairs of single bore tunnels, onerunning northwest through Highgate Wood, the otherleading out into a cutting on the southeast. The tunnelconstruction sites and their spoil heaps were inevitablyvisited by local geologists. Whitaker (1872, p. 303)reported that much clayey greensand had been foundwithin the southeastern tunnel and many beautifulfossils obtained from it. More of this sand was‘scattered in the sandy clay near the bottom’ next to

EXPOSURES OF THE EOCENE LONDON CLAY AT HIGHGATE 321

the southeastern mouth. The location was later la-belled ‘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, JC, SKD, PJ, ST) together with David Bevan(Haringey Conservation Officer) and Iain 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

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: Hornsey Historical Society, The Old School House, 136 Tottenham Lane, London N8.

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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 inter-laminated 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 lower1 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 thick-shelled 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. 1[TQ 288 880]). 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 (com-pressed brick rubble comprising the track base) was theunit noted earlier, of fresh medium blue-grey silty clay,slightly sandy in places, with small (<1 cm) 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 cm 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 kilo-grams 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 unfossil-iferous 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(J. de C. Sowerby, 1823) and Wrigleya complanata(J. de C. Sowerby, 1823), up to 45 mm long. Samplesof clay from this c. 20 cm 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-less 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

EXPOSURES OF THE EOCENE LONDON CLAY AT HIGHGATE 323

involved washing samples of the dried clay throughgraded sieves with a mesh size of 1 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 µm 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 1).Other fauna represented included coelenterates, anne-lids, decapod crustaceans, brachiopods, echinoderms,and fish teeth and otoliths (Table 2), but no species ofany of these groups exceeded 0.5% of the total macro-fauna in either sample.

The statistical calculation (Table 1) shows the rela-tive 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 found/less 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.

Fig. 3. Work in progress 16 September 1999. (a) One of the authors (PJ) 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.

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The respective clay samples provided the following:

HGA: Lower unit – shelly clay between and belowscattered septaria 1.5–2.0 m below ground: 50 kg clay(dry weight) – total 873 mollusc specimens represent-ing 44 species. The 23 most frequent of these com-prised 97% of the total population [average c. 17.5individuals per 1 kg clay].

HGB: Upper unit – stiff silty clay 0–1.5 m belowground: 20 kg clay – total 440 mollusc specimensrepresenting 39 species [average c. 22 individuals per1 kg clay]. The 26 most frequent of these comprised97% of the total population.

A further c. 400 kg of undifferentiated spoil(sample HGU) was subsequently sieved to 0.5 mm andexamined, although this increased the mollusc list byonly 6 additional species.

Mollusca: changes in frequency and faunal notes

As noted above, only molluscs provided sufficientnumbers of individuals to highlight the differencesbetween the two main intervals sampled. The percent-age compositions of the respective mollusc faunas inthe two samples were closely similar, consisting of

Fig. 4. Composite section (right) based on trial holes and excavation in the Parkland Walk southeast of Highgate station,correlated (left) with part of the more complete log of St Aloysius’ College borehole, 1 km to the south (after Cooper, 1970).Stratigraphical units are based on King (1981, p. 35, text-fig. 8).

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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)

Mollusca – GastropodaHaustator sp. indet. rAlvania sp.* 1%Adeorbis aff. lucidus Cossmann, 1881* 1% 1%Solariorbis sp.* [Teinostoma priscum Newton non Desh.] 28% 28%Chevallieria cf. cylindroides Cossmann, 1907 2% 2%Entomope semipunctata Jeffery & Tracey, 1997 rEotibia lucida (J. Sowerby, 1815) : 6% 3%xenophorid? sp. [protoconch only] rEuspira glaucinoides (J. Sowerby, 1812) : 15% 11%Eocypraea? sp. [protoconch only] rFicopsis multiformis (Wrigley, 1929) r rCrassiscala subterranea (Wrigley, 1940b) 1% 1%Wrigleya complanata (J. de C. Sowerby, 1823) 1% 1%Wrigleya conifera (J. Sowerby, 1818) rDaphnobela juncea (Solander, 1766) r 1%Volutospina nodosa (J. de C. Sowerby, 1823) : 1% rAmalda cf. arenaria (Cossmann, 1889) [protoconch only] r rBonellitia laeviuscula (J. Sowerby, 1822) : 2% 1%Conilithes concinnus (J. Sowerby, 1821) [juv.] rCochlespira pulcherrima (Edwards, 1857) rTurricula teretrium (Edwards, 1857) rEopleurotoma simillima crassilinea (Edwards, 1861) 1% 1%Eopleurotoma abnormis (Edwards, 1861) rEopleurotoma sp.* rAmblyacrum? sp. [protoconch only] rGemmula longaeva (Edwards, 1861) r 1%Gemmula (s.l.) fasciolata (Edwards, 1861) uMathilda sororcula Wrigley, 1940b r rOdostomia sp.* 5% 4%Crenilabium elongatum (J. de C. Sowerby, 1824) rRingicula turgida (J. Sowerby, 1817) : 2% 1%Roxania biumbilicata (Deshayes, 1863) ; r 1%Cylichna aff. consors (Deshayes, 1863)* ; 2% 4%Cylichna aff. uniplicata (J. de C. Sowerby, 1850)* : 4% 2%Volvulella oxyacrum (Cossmann, 1889) rLimacina mercinensis (Watelet & Lefèvre, 1880) 3% 4%Limacina taylori (Curry, 1965) 3% 4%Limacina aff. taylori (Curry, 1965)* ; r 1%Altaspiratella bearnensis (Curry, 1981) ; 8% 16%Mollusca – BivalviaNucula consors Wood, 1864 4% ~ 3% ~Leionucula sp. [juv.] rYoldiella prisca (Deshayes, 1860) : 3% 1%?Ledina amygdaloides (J. de C. Sowerby, 1827) [mould] r ~Striarca wrigleyi (Curry, 1958) [Glycymeris] r ~Trigonodesma lissa (Bayan, 1873) [r] rCucullaria impolita (J. de C. Sowerby, 1837) rSemimodiola elegans (J. Sowerby, 1812) [r] r ~Amygdalum sp. [juv.] r ~Lentipecten corneus (J. Sowerby, 1818) r ~?Atrina affinis (J. Sowerby, 1821) [fragments] [r]Pteria media (J. Sowerby, 1812) [juv.] rAnomia anomialis (Lamarck, 1819) r ~Heteranomia scabrosa (Wood, 1861) [r]Nemocardium nitidulum (Tremlett, 1950) r ~ rAbra splendens (J. de C. Sowerby, 1837) 1% 1%Pitar sulcatarius (Deshayes, 1825) 1% 2%Varicorbula globosa (J. Sowerby, 1818) 2% 2%Mollusca – ScaphopodaLaevidentalium nitens (J. Sowerby, 1814) r

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 0.5% of the total in either sample are indicated only by their overall rarity: r, rare (2–4 examples/less than 0.4% of total fauna); u, uncommon(5–10 examples); c, common; [r], 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.

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Table 2. Non-molluscan phyla from the Parkland Walk excavation, Highgate: fauna extractedfrom all samples (HGA, HGB, HGU).

HGA HGB HGU

Protozoa – ForaminiferidaSpiroplectinella gr. carinata (d’ Orbigny, 1846) umiliolid sp. rBrizalina anglica (Cushman, 1936) rdentalinid sp. indet. rPulsisiphonina prima (Plummer, 1926) rEuuvigerina batjesi (Kaasschieter, 1961) uCibicides tallahattensis Bandy, 1949 cCibicidoides aff. alleni (Plummer, 1926)* rLenticulina spp. cAnomalinoides aff. nobilis Brotzen, 1948* cAnomalinoides acutus (Plummer, 1926) rPorifera?scleractinian [spicule] [r]Coelenterata – PennatulaceaGraphularia wetherelli Milne-Edwards & Haime, 1850 r uAnnelida – PolychaetaDitrupa plana (J. Sowerby, 1815) u rCrustacea – Decapoda?pagurid sp. indet. [limb fragment] r?calianassid sp. indet. [burrowed internal cast of left chela] rGlyphithyreus wetherelli (Bell 1858) [carapace & chelae] r rZanthopsis leachii (Desmarest, 1822) [right dactylus] r?Xanthilites sp. [xanthid chela] rPortunites sp. [chela] rCrustacea – OstracodaCytheridea newburyensis Gokçen, 1971 uEucytherura sp.* rLoxoconcha aff. nystiana (Bosquet, 1852)* rEopaijenborchella cf. lomataTriebel, 1949 uEchinocythereis reticulatissimaEager, 1965 uBrachiopoda – LingulidaLingula tenuis (J. Sowerby, 1813) r rEchinodermata – Echinoideaspatangoid – spines and test plates uEchinodermata – Asteroideaasteroid [indet.] uEchinodermata – OphiuroideaOphiura wetherelli Forbes, 1852 [lateral arm plates] u uCoulonia colei (Forbes, 1852) rIchnotaxacf. Glyphichnus sp. [pyritized burrow infills 1–3 mm diam.] uVertebrata – ChondrichthyesPhysogaleus secundus (Winkler, 1874) [shark – teeth] rRhinobatos bruxelliensis (Jaekel, 1894) [guitar ray – teeth] rVertebrata – OsteichthyesPycnodus sp. [teeth] rPterothrissus angulatus Stinton, 1966 [bonefish – otoliths] uMuraenesox cymbium (Stinton, 1966) [pike–conger – otoliths] rHildebrandia circularis (Stinton, 1966) [conger eel – otoliths] rSynodus davisi (Frost, 1925) [lizard fish – otoliths] rOphidypterus obtusus (Frost, 1925) [cusk eel – otoliths] uGlyptophidium polli (Casier, 1946) [cusk eel – otoliths] userranid sp. [indet.] [sea bass – otoliths] rpomadasyid sp. [indet.] [grunt – otoliths] rCepola densa (Frost, 1934) [ribbon-fish – otoliths] rArdiodus marriotti White, 1931 [mackerel – teeth] rCybium proosti Storms, 1897 [mackerel – teeth] rPlantaeWetherellia variabilis Bowerbank em. Reid & Chandler, 1933 r

General indication of frequency as in Table 1

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small to medium-sized benthic gastropods andinfaunal bivalves, of genera characteristic of EarlyEocene marine clays, together with a significant pro-portion of holoplanktonic gastropods and plank-totrophic larval shells showing little or no subsequentadult growth. The main differences observed betweenthe two samples were the relative frequencies of a fewgastropod species (Table 1, ‘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 enough to grow to adult-hood. These were more common in the lower shellbed with septaria, their relative frequencies beingreduced by up to 50% in the overlying clays.

(b) Two of the four species of holoplanktonic opistho-branchs (pteropods) present: one an undescribedLimacina, similar to L. taylori (Curry, 1965), buthaving a flattened spire; the other Altaspiratellabearnensis (Curry, 1981), rarely recorded despite itswide distribution. Unlike Limacina mercinensis(Watelet & Lefèvre, 1880) and L. taylori, the othertwo pteropods in the sample, whose frequenciesremained constant, these two species increased by upto 50% in the upper homogeneous clays. Pteropodshave been used with some success for correlation ofCenozoic strata, in view of their geographically widedistribution, albeit restricted in time, which impliesthat they could show a significant rate of evolution-ary change. The fact that two particular species inour samples appear to be thriving while two otherclosely related species are declining, though all livingtogether in 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 (NP13), southernFrance (Curry, 1981). In England it is now seen tobe locally abundant in the lower part of division E ofthe London Clay Formation (NP12). There areisolated records of the species (ex situ – perhapsdivision C or D) from Sheppey, Kent (Janssen, 1990;Hodgkinson et al., 1992) and (ex situ – but of similarage or older) from Burnham-on-Crouch, Essex (A.Janssen, pers. comm.). It is also recorded fromsomewhat younger strata (basal Lutetian, NP15) inthe Weches Formation of Texas, USA). Janssen(1990) also confirmed its presence in division Ematerial from St Aloysius’ College, Highgate (seeabove), and its brief appearance in the North SeaBasin. Its apparent overall scarcity may have beendue partly to its being mistaken for L. tutelina(Curry, 1965), which characteristically occurs at alower horizon in the London Clay.

Bivalves were not so well represented as gastropodsand generally belong to long-lived species of generawidely distributed in this facies of the London Clay.

The ubiquitous deep burrowers Nucula and Yoldiellaare the most common. Scaphopods were barelyrepresented at all, only a solitary example being found.

5. DISCUSSION

Stratigraphical position

Perhaps the combination of rich fossil beds and rela-tively homogeneous lithology was responsible for thelack of detailed stratigraphic records of previousexposures around Highgate. Assessments of the bio-stratigraphy of the area and its correlation, byPrestwich (1854) and Wrigley (1940a), have beenreviewed by King (1981). Whereas all agreed that thesebeds lay near the top of the London Clay Formation,they differed in regard to which boundaries separatedthe major divisions. Wrigley (1940a) noted contrastingdiagenesis in fossil material from the old Highgatelocalities in museum collections. These were consideredto represent an upper horizon containing well-preserved uncrushed shells with a sandy matrix, and alower level with shells mostly internally pyritized,otherwise crushed or damaged, in a clay matrix(Wrigley, 1940a, p. 235). These two lithofacies alsoappeared to support somewhat dissimilar faunas. Inthe absence of any adequate stratigraphical data forthe area, the clay unit was mistakenly correlated withrather older sites to the south of London and the baseof his uppermost division (5) was drawn between thetwo Highgate beds. In a subsequent assessment,Bristow et al. (1980, p. 265) preferred to group thefossiliferous sandy clay with the overlying ClaygateBeds, on the basis of its coarser grained lithology andalso its supposedly typical Claygate fauna. However,King (1981, p. 39), found the faunal continuitybetween the two fossil beds more convincing, anddefined his own uppermost division (E) of the LondonClay as extending from the base of the pyritic clay (unit1) to the top of the fossiliferous 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 fauna’ and thisconclusion is supported by the results of our ownstudy.

The present section can be correlated with Cooper’s(1970) log of the St Aloysius’ College borehole atHighgate Archway, 1 km to the south, and also withKing’s (1981) generalized Eocene stratigraphy of thearea (see Fig. 4).

The bio- and lithostratigraphy of the fossiliferousclay from our main excavation equates it to unit 1 ofdivision E, probably within 3 m of the base, which hasbeen dated as within NP12 (Ali et al., 1993). Theoriginal object of this excavation had been to samplethe classic glauconitic sandy clay horizon with the rich‘Highgate fauna’ of macrofossils. Although our trialholes were augered in the sides of the cutting close tothe tunnel and on the same horizontal level, theyencountered only weathered sandy clay apparently

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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 environ-ment. 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 gastro-pods 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 fre-quencies 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 hor-izons 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 gastro-pods, 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 (Echino-dermata; miscellaneous invertebrates), and Dr ChrisKing (Foraminiferida and specialist advice on ptero-pod 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 Iain 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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