434

EXCURSION TO WOODSIDE (LONDON CLAY).

J UNE 6TH, 1925.

REPORT BY A . G. D AVIS AND ARTH UR WRIGLEY.

Directors of the E xcursion.

A SMALL group of members met at Norwood Junction sta tionand walked via Birchanger Road and Woodside Green to Mr.Handley's Brickworks. Usually, the extensive section of LondonClay here is obscured by the clawing and smearing of an excava­ting machine, but, fortunately, we found that a good length ofthe working face had been exposed for some time to weathering,which enabled us clearly to see the nature of the sedimentsexposed.

The section is 24 ft . deep: the lower 8 ft. ha s the familiardark grey colour of unaltered London Clay, while the upper16 ft. is weathered brown; these two divisions being separatedby a distinct horizontal line which has no relation to the con­stit ution of the sediments, but merely marks the limit of atmos­pheric influences.

At 2 ft. above the base of the section there is a line ofspheroidal septaria ; at 8 ft ., 13 ft. , and 18 ft. from the base arethree continuous layers of tabular concretionary stone about3 in. thick, which break up into discoidal pieces, happily describedas " cheeses " by our lat e friend, Mr. Whitaker. At 2 2 ft. fromthe base is a fourth layer of tabular claystone, which, however,is discontinuous.

Throughout the whole of the section, both in the unweatheredand in the weathered parts, there is a regular alternation ofrz-inch beds of clay and r-inch sandy seams, with crusts of finelydivided selenite on the upper surfaces of the claybands. Towardthe base of the pit the sandy seams contain lumps of pyritesand of pyritised sand which, in the weathered part ab ove, areseen to be changed into hollow ferruginous nodules or boxstones,accompanied by nest s of selenite crystals. All the boxstoneswe have seen in various parts of the London Clay and of thepassage-beds above it appear to have originated by th e decay ofpyritic nodules.

Fossils are scarce and mostly poorly preserved; the majorityof those listed below were ext racted from the tabular septaria .The Microzoa were decidely rare in proportion to th e largeamount of unweathered clay which was washed to obtain th em.Leda partimstriata is fairly common ; many of the " cheeses "are covered with flat spiral worm tubes or casts . Th e assemblageof fossils resembles that found about 1 0 0 ft. ab ove the BasementBed at Beddington Lane and Hendon.

435 A. G. DAVIS AND ARTHUR WRIGLEY,

LIST OF FOSSILS.R = rare. C = relatively common ..

MOLLUSCA.Area (Fo ssul arca) ? impolita Wood .-RNucula botoerban ki J. de C. Sow.Leda oblata Wood .L . partimstriata W ood.-C.P inna aflin is J . Sow.A xinus angulatus J. Sow .Cryptodon goodhalli (J . de C. Sow .)-CProtocardia sp .- RSurc ula z spp. (Imper fect ly preserved)-RS urcula aff . terebrali s (Lam k.)- RHemipleurotoma sp .- RStreptolathyrus cymatodis (Edw.)-?Ptychatractus a ff. interru ptus (Pi lk.) - RNatica sp . (casts only)Sc aphander sp .- RDentalium anceps J. de C. Sow.

VERMES .Ditrupa sp.Granularia sp .Sp iral and othe r worm tra cks or cas ts -s-C,

O STRACOD A.Cy therella compressa Muns t-e-RCytheridea sp. - RKrithe londiensis J o . & Sher b.-R

FORAMINIFE RA .Cristellaria inornata d 'Orb .- R

C. rotu lata (Lam k.)-CDentalin a comm unis d 'Orb .---C

D. pauperata d'Orb.-CDis corbina rugosa d'Orb.-RMiliola trigonula (Lamk.)- CNodosaria humilis (Roem.)---CPlanorbulina ammonoides Reuss-e-RPullenia quinqueloba (Rellss)-RTe xtu laria agglu tinans d 'Orb .- RVerneu lina tr icarina ta d 'Or b.- R

VARIOUS .E chinoid radioles-RCrino id ossicles (? RhizOCTinus)-RCoral (indet .)S calpellum quadratum Darwin*

To determine th e horizon of London Clay seen in this pit wehave plotted all the available da ta from local wells, etc, on twovertical section s passing through the bri ckyard :-One fromThornton Heath to Addington ; the other from Beddington Laneto Woodside . From these it is plain that Handley's brickyardis in the lower part of the London Clay, being probably from 25 ft.to 50 ft. ab ove the base. In constructing th ese sections wenoticed some anomalies in the Geological Survey Map (sheet 270 ,S. London, 1921). Some of the .. tongues " of BlackheathBeds mapped in the neighbourh ood of Stroud Green must reallybe .. dr ift " washes, for they are underlain by London Clay,

• Kindly determined by Mr. H. With ers.

EXCURSION TO WOODSIDE (LONDON CLAY). 436

unless we assume wholly improbable structures. The sectionacross the Tertiary escarpment at Addington obtained by usingthe outcrops marked on the geological map, gives to the Black­heath Beds an impossible thickness of about 100 ft. By makinga liberal allowance for downhill washes of these pebble bedswe contract their real outcrop and raise their base, but we arestill confronted with an improbable thickness which we canreduce only by assuming an anticline along the line of theAddington hills.

By the kindness of the Council of the Croydon Nat. Hist.and Sc. Soc. we are able to quote from MS. notes in their posses­sion a notice of an exposure of Woolwich Beds, yielding manyoysters and other mollusca, seen in 1882 in excavations at the­south end of Holmesdale Road at the corner of Newhaven Road.At another spot (now built upon) t mile N.W. of Selhurst station"fossiliferous Woolwich Beds were seen 7 ft. below the surface.Mr. E. A. Martin* also has recorded the presence of WoolwichBeds and their usual fossils seen, in 1878, in Holinesdale Road(at 8ft. below the surface) and Boswell Road, i- mile S. of Thornton,Heath railway station. These data confirm the sections by whichwe have attempted to estimate the horizon of London Clayseen in Handley's brickyard. Tabular concretions much like'those described above have been noticed in temporary sections,in Ritchie Road, Woodside, and in Wharfedale Gardens, Norbury..By projecting these exposures on to our sections it appears thatthe concretions belong to one or other of the four strata seenin Handley's brickyard, which thus are found to be continuous.for three miles.

The regular alternation of clay bands and seams of coarsersediment is a remarkable feature of this section; when weatheredit 'recalls the Claygate Beds, excepting that, at Woodside, theclay bands are thicker and predominant.

We have submitted to Mr. Berdinner four samples of thesediments found in this pit; the results of his examination ofthem, together with a series of useful comparisons are given inan appendix (p. 437.)

After tea at Croydon we continued the excursion to theA.P.C.M.'s pit (formerly Hall's) at Beddington Lane, where we'found in the London Clay most of the common fossils listed invol. xxxv., pp. 254-6 of these PROCEEDINGS.

• National History and Antiquities of Croydon, 1925, P. 48.

437

MECHANICAL CONSTITUTION OF LONDONCLAY,

FROM HANDLEY'S BRI,CKWORKS, WOODSIDE, WITHANALYSES OF COMPARABLE HORIZONS AT

OTHER LOCALITIES.BY HERBERT C. BERDIKNER, B.Sc.

1. LONDOK CLAY, WOODSIDE.REF.No. LOCALITY. M.S. E.S.

1 Woodside

34

,Cs. Es. c. E.G. G.E. HORIZON.11.24 32.12 56.64 .02 .107 Stiff blue-grey clay

3 ft. above base section8.0 37.7 54.3 .017 .15 Stiff blue-grey clay

10 ft. above base section.8..8 35.3 55.9 .018 .11 Loamy top horizon

36 A 18 ..95 44 .65 .035 .H " Sand " band near topof section. Associatedwith selenite

.5 Newington,Kent

6 Upnor

'; Shome

8 Swanscombe

JI. L01\DOK CL<\.Y, BASEMEKT BED.5.5 10 28 24.7 31.8 .067 .09 Basement Bed

5 ft. above base2.9 9.6 35.7 22.2 29.6 .055 .251 Basement Bed

9 ft above base27.9 49 12.8 10.3 .093 .54 Basement Bed

4 ft. above base7.6 61.5 6.9 24 .058 .46 Basement Bed

2 ft. above base

e. =C1ay less than .01 mm.E.G. = Equivalent Grade.G.P. = Grading Factor.

JI1. LONDO" CLAY, MAIN MASS.2.4 14.5 83.1 .012 .606 100 ft. above base

(eire.)1.0 24.1 74.9 .0117 .6 100 ft. above base

(eire.)

9 Pinner,Middlesex

10 Regent St.(well)

Waterloo Place

In No. 4 the material treated was free from selenite, and so represents the mechanical compo-sition of the sediment as it was originally laid down.

Limits of grades :-M.S.=Medium Sand 0.5 - O.:!5 m.m.F.S.=Eine Sand 0.25 - 0.1 rnm.Cs =Coarse Silt 0.1 - 0.05 rnrn.Fs =Fine Silt 0.05 - 0.01 mrn.

The figures in the first table (I.) express the mechanicalcomposition of horizons in the London Clay not far above thebase at Woodside.

Analyses I, 2 and J represent the normal lithology of thispit in London Clay, and it will be seen that rather more thanhalf of the sediment is composed of true clay and that there isalso present a high percentage of the fine silt grade. The vir­tually constant figures for each grade are significant and theconsistently low grading factors express even percentage distri­bution through the grades.

Similar figures have been obtained by the writer for ClaygateBeds at Rayleigh, Chingford and Brentwood in Essex* and atTolworth in Surrey.] though a greater range of composition ismet with in the case of these beds.

* H. C. Berdinner, Geol. of Brentwood and Shenfield Sections, Proc. Geol. Assoc., xxxvi.,1925, pt. 2, p. 180.

t A. Wrigley, Excursionto Tolwor.th, Peoc. Geol, Assoc., XXXV,) 1924, pt. 2) p. 153.

MECHANICAL CONSTITUTION OF LONDON CLAY. 438

Boswell,* in one of the few attempts to investigate this­subject, has given figures for low horizons in the London Clayof East Anglia. These results (particularly those for Felixstowe;Cornard, near Sudbury, and Bramford) show fairly close agree­ment with those here presented.

Tables II. and III. show comparative results for the London'Clay basement bed and the London Clay main mass.

A careful examination of the sediments comprising thebasement bed at many localities in the London Basin, shows aclose conformity of lithological change to conditions of deepeningwater. Of these results four representative types (5, 6,7 and 8»)from the southern outcrop have been selected. In these thecoarser grades become important, though the combined per­centage of the two silt grades always constitutes betweenone half and two-thirds of the sediment.

Finally, elutriations (9 and TO) of samples of the main mass­of the London Clay are added. These show a bulk percentage ofclay, while the remainder, (approximately one-sixth ofthe total),is silt-chiefly of the fine grade. The average or equivalentgrades are .012 and .or rz, and a relatively high tendency to­grade perfectly is shown by the figures .606 and .60.

To sum up, the Lon don C lay lit h 0 log y atWoo d sid e con sis t s 0 f fi n e s i 1t Y c lay S,.

intermediate in character between thec 0 a r s e bas erne n t bed f a c i e sand the fi n estwell - g r a d e d Lon don C lay t y pes. In general.there is a likeness between the beds at Woodside and the Clay­gate Beds of Brentwood type (i.e., those in a massive condition).The likeness here demonstrated is one of mechanical compositionand this in turn suggests similarity of marine conditions-indepth of water for example-tending to produce such types oflithology.

In some places in the London Basin it is evident that thestiff clay lithology of the main mass occurs at the lowest horizonsof the London Clay.

It seems probable that the foregoing conclusions could be­extended in application to other localities. But in view of theevidence given it would not be justifiable to make any general.proposition to this effect.

As regards the" sand bands" (No.4) in the London Clay atWoodside, they are found in association with finely dividedselenite, which imparts a spurious coarse appearance to the­weathered surface of the lenticles.

In the cliffs at Minster, Sheppey, occur distinct and sometimesrelatively thick " sand bands" in a very finely laminated con­dition. Ninety-five per cent. of a sample of this bed are divided

* P. G. H. Boswell, Lower Eocene Deposits of London Basin, Quad. [ourn, Geol. Soc., lxxL,.1915. p, 569·

439 MECHANICAL CONSTITUTION OF LONDO!' CLAY.

between the silt and the clay fractions and the whole band hasan equivalent grade as low as .03 IDID. The" sand" at Wood­side yielded an analysis so similar to that just summarised thatthe two may be regarded as identical in lithological character.

The " sand bands" of the London Clay are in reality thinspreads of silt and clay, exhibiting low grading and they are metwith in several parts of the London Basin, both near the baseand near the top of the London Clay sequence.

The following table expresses in a condensed form the mamconclusions arising out of this investigation ;-

HEIGHT ABOVEBASE. HORIZON. DESCRIPTION.

'"ft . LONDON CLAY A fine deposit as a whole, but coarser

.~ (}-10 Basement Bed constituents (sand and silt) always presen t.

"'-'" 50 LONDON C LAY Fine clayey silts ill-graded and with" sand -!l ~" "- [circ.} Woodside bands I t associated.~O -\jI

LONDON CLAY Very fine, well-graded sediment, ehieHy100 + Main Mass consisting of true clay .