Temporary exposures of the Eocene London Clay Formation at Highgate, north London: rediscovery of a fossiliferous horizon ‘lost’ since the nineteenth century

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<ul><li><p>Temporary exposures of the Eocene London ClayFormation at Highgate, north London: rediscovery of</p><p>a fossiliferous horizon 'lost' since thenineteenth century</p><p>Steve Tracey', Stephen K. Donovan", Diana Clements!", Paul Jeffery3,John Cooper', Phil Rye4 &amp; Caroline Hensley'</p><p>TRAC EY, S., DONOVAN, S. K., C LEMENTS, D., JEFFER Y, P., COOPER, J., RYE, P. &amp;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.</p><p>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</p><p>1. INTRODUCTION</p><p>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 for-mation 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</p><p>Proceedings of the Geologis ts ' Association, 113, 319-33 1</p><p>[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 infor-mation leaflet by Haringey Parks Service and obtai n-able at their Railway Fields Nature Park Exhib ition ,Green Lanes, Lond on N4.</p><p>0016-7878/02 S15.00 2002 Geologists' Association</p></li><li><p>320 s . T RACEY E T AL.</p><p>I ~ ~ ~ IBagshot Sands (Virginia Water Formation)D Claygate Beds (London Clay Formation)D London Clay (London Clay Formation)</p><p>0.5 km</p><p>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.</p></li><li><p>EXPOSUR E S OF T HE EOCEN E LONDO N C L A Y A T HIGHGAT E 321</p><p>2. PREVIOUS WORK ON LONDON CLAYEXPOSURES AROUND HIGHGATE</p><p>Archway Road</p><p>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 palaeontol-ogists 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 unpub-lished 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 .</p><p>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).</p><p>Hampstead and Finchley area</p><p>Wethe rell also figured some Lond on Clay molluscsand microfossils from a well excava tion on nearby</p><p>Hampstead Heath , which were compared to theHighgate fauna (Wetherell, 1837).</p><p>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).</p><p>Underground Railway extension - Highgate toFinchley</p><p>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.</p><p>Mainline tunnels - Highgate Wood area</p><p>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).</p><p>Construction of the Edgware, Highgate and LondonRailway (par t of the Great Northern Railway North-ern 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</p></li><li><p>322 S. TRACEY ET AL.</p><p>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.</p><p>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).</p><p>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</p><p>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.</p><p>3. EXCAVATION PROJECT AT HIGHGATEAuger survey</p><p>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</p></li><li><p>EXPOSURES OF THE EOCENE LONDON CLAY AT HIGHGATE 323</p><p>overall succession; the following lithologies were notedin auger holes A-D (Fig. 1):</p><p>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.</p><p>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.</p><p>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.</p><p>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...</p></li></ul>