348

ON A SECTION IN RAVENSBOURNE VALLEYGRAVELS AT LEWISHAM.

By ....RTHUR L. LE.\CH, F.G.S. , and ALFRED e. YOUNG, F.e.S.

[Read April 411, 1930.J

[Received January 291h, 1930.J

AN excavation in a field beside the River Ravensbourne, andimmediately north-east of Lewisham Junction Station

(Southern Railway) was worked for ballast from 1927 until theautumn of 1929. when the pit was filled up and the site levelled.\Ve conducted an Excursion of the Association to the pit onJune 9th, 1928, but deferred an account of the deposits untilthe development of the section and examination of the shells,plant- and animal-remains had been completed. Ballast wastaken out to a depth of nearly 30 feet below the surface of thefield (15-20 a.D.), the lower limit of excavation being reachedwhen patches of " calcrete" (rubbly chalk and flints) indicatedthe base of the gravel and the nearness of the Chalk. The pitat it s greatest extent showed vertical faces on the north-east,north-west and south-west: the various layers of sand, clayand grave l were cont inuous on all the sides except where thelaminated or striped silt s were interrupted by " wash-outs "(irregularly eroded hollows) into which the overlying ballasthad sunk. Along the north-east face the sect ion showed at it ssouth-eastern end a band of peaty silt which dipped lower inthe gravel towards the north-west and finally split into twolayers separated by ballast. The measurements for thefollowing section were made at the north-western end of the pit ,where the laminated or striped peaty silts appear as twodistinct bands.

Section at the Western End of the Lewisham BallastPit, 1929.

9, ' Soil, lift., the cult ivated top spit of t~ . Heavy muddy clay, l! ft ., passing down H .

into OLOCE t\E

7. Sandy silt, 1 ~-2 ft., with a peaty base I6. Upper ballast, 5ft. '5· Upper striped or laminated peaty silt, 1 ft.} L4. Middle ballast, 0-2 ft. AT E .3. Lower striped peaty silt. 1 ft. PLEISTOCE:-iE

2. Lower ballast, 16 ft.I. Calcrete, seen to 1 ft. (resting probably

on Chalk).The calcrete of chalk rubble with angular and pebbly flints

in a chalky paste, resembled the material commonly found inmany valleys on the Chalk outcrop, where it has formed assub-aerial rubble-drift , a product of frost- shattering comparable

SECTION IN RAVENSBOURNE VALLEY GRAVELS. 349

with the "head" of western Britain. The ballasts, lowerand upper, consisted of sub-angular and pebbly flints with anoccasional piece of sarsen or other sandstone. The lower ballastcontained many large sub-angular flint nodules. No LowerGreensand chert or carstone could be found; one large quartzitepebble (from Blackheath Beds) was picked up. All these con­stituents were such as occur within the present valley of theRiver Ravensbourne. Fragments of wood, rare in the sandylower ballast, were common in the stiffer and more loamy upperballast, which contained large patches of disintegrated plantremains, twigs, branches and parts of large tree-trunks; onepiece, fully 9 feet long, was too tough and fibrous to be cutinto by a hammer. In the upper part of this ballast, partlyembedded in it and partly projecting into the overlying alluviumlay the bole of a large tree, not rooted in the ballast but apparentlydriftwood.

The most interesting beds in the section were the stripedpeaty silts (united into one band at the south-eastern end ofthe pit) which consisted of alternating light and dark layers,t to t inch thick, of calcareous (chalky) and carbonaceous (peaty)material. The colour contrasts made these layers conspicuousby the pronounced horizontal black and white striping.Although the plant-fragments in these silts were almost valuelessfor identification, small freshwater shells, rather crushed, werefairly common. Many delicate rootlets (not from the presentland-surface) ran vertically down through the silt and spreadlaterally along the bedding-planes. The upper band of siltshowed the horizontal striping even more conspicuously thanthe lower layer. These silts were cut off by an irregular planeof erosion upon which lay the upper ballast which again waslimited upwards by another and very hummocky surface oferosion, upon which lay Holocene deposits, sandy and peatybelow, graduating upwards into stiff, dark, alluvial clay ormarsh mud.

Reports on the Fauna and Flora.MOLLUSCS. The freshwater shells were examined by Messrs.

A. S. Kennard, A.L.S., and B. B. Woodward, F.L.S., whostate:

The material on which this report is based was partlycollected during the Excursion (June 9th, 1928) and sampleswere kindly sent to us by A. L. Leach and A. C. Young.Eighteen species were determined, viz. :-Valvata piscinalis (Mull.) Planorbis laevis (Ald.)

" cristata (Mull.) crista (Linn.)Limntea pereger (Mull.) contortus (Linn.)Pupilla muscorum (Linn.) "complanatus (Linn.)Vallonia excentrica (Sterki.) Cochlicopa lubrica (Mull.)

PROC. GEOL. Assoc., VOL. XLI., PART 3, 1930. 23

350 A. L. LEACH AND A. C. YOUNG,

"

Punctum pygmceum (Drap.)Sphcerium corneum (Linn.)Pisidium nitidum (Jenyns)

subtruncatum (MaIm.)

Succinea Pleifferi (Rossm.)Pisidium cinerum (Ald.)

milium (Held.)obtusalastrum

(B. B. Wood.).Nearly all the samples are dwarfed, and this would appear to

indicate uncongenial environment. The facies of this faunaclearly indicates a Late Pleistocene age, certainly later thanCrayford brick earths, and approximating to the age of thewell-known Ponder's End beds. Unfortunately the conditionsof deposition were different, for the Lewisham shells representthe finer rejectamenta of a river, whilst Ponder's End bedswere in the main swamp deposits.

PLANT REMAINs.-Specimens collected from the laminatedsilts and from large peaty and woody patches in the ballast wereexamined by Mr. W. N. Edwards, F.G.S., who states :-

" The following plants have been recognised in the peatymaterial :-

Alnus (Alder)-wood, catkins, pollen, seeds.Corylus (Hazel)-pollen, nuts.Tilia (Lime)-pollen, rare.

Some of the samples consist mainly of fragments ofwood, bark, and twigs, usually much decayed and unidenti­fiable. Pollen is not common, alder being the most frequent;some unidentified fungus spores were also found. The siltthough containing decayed vegetable matter did not yieldanything recognisable."

All these plant-remains" have been recorded from Pleistocenedeposits.

In a specimen of very mossy peat taken from a detached blockin the upper ballast Prof. H. N. Dixon recognised two commonpaludal species :

Calliergon giganteum (Schimp.) Kindle.Drepanocladus fluitans (Linn.) Warnts.

MAMMALIAN BONES.-A few fragmentary mammalian bonestaken from the ballast were submitted to Miss D. M. Bate; theywere too imperfect for exact determination, but may be theastragalus of a large Bos. sp. and a metapodial of ? Cervus sp.

Some remains of horse (skull) and a mandible of a pig werefound in the alluvial clay.

Buried Land Surfaces and "Wash-Outs."

Perhaps the most interesting and geologically valuablefeaturesof this section were the traces of buried land-surfaces,two of which had supported vegetation, each being in turn at alater stage torn up, channelled by "wash-outs" and buried

• C. Reid. .. The Origin of the British Flora."

SECTION IN RAVENSBOURNE VALLEY GRAVELS. 351

beneath still later deposits of river-drift. Including the presentland-surface the sequence may be represented thus :-

LAND SURFACE D = Recent field surface.Clay and loam .~''''''''''''''''''''''''''''-''''''''''''''-''''''''''''''''''''''''''''''''''''''''''--

LAND SURFACE C = H olocene woodland surface.Upper ballast.-~~

LAND SURFACE B = Late Pleistocene marsh surface.Striped shelly and peaty silts and middle ballast.Lower ballast.

-..-...,....,""....................,...........ro.;................."..........,..................,....,....,.......... ro.;.......__,.._

LAND SURFACE A = Subaerial Pleistocene surface.Calcrete and Chalk .

Surface A, the sub-aerial rubble of Chalk and flint s, underliesthe lowest gravel. B, a lat e Pleistocene marshy surface, cameinto existence after the striped silts (with fresh water shells)had been deposited in very quiet water, probably in pools, whichwere slowly silted up. Numerous delicate rootlets ran up throughthe upper striped silt, but all ended abruptly at the plane oferosion marked by the occurrence of the upper " wash-outs."The large blocks of striped silt and masses of twigs with occasionaltree-trunks and patches of mossy peat, dispersed through the

. upper ballast were probably torn up from woodland areas onthis surface higher up the valley of the Ravensbourne, carrieddown by the current and embedded.in the accumulating gravel.

Land surface C developed upon loamy beds on the upperballast. The roots of large trees which had grown on thissurface could be traced through the upper ballast (in someinstances up from the silt bands), but all ended abruptly-atthe plane of erosion which separated the upper ballast from theoverlying Holocene clay and loam. Here and there in the baseof the Holocene deposits were trunks and boles of large trees,but all were drift-wood derived from destroyed parts of land­surface C. Along almost the whole length of the section thi seroded junction was clear, sharp and irregular, and the Holocenedays rested on a very hummocky surface filling and levelling upthe troughs. The only roots which ran through the Holoceneclays were those of plants now or recently growing on the surfaceof the field. "Wash-outs " occurred at two levels: first, duringthe deposition of the middle ballast small channels were cut inthe lower striped silt and filled up with ballast; secondly, duringthe deposition of the upper ballast similar channels were cut inthe upper striped silt. In one instance erosion went deep enoughto cut through the lower striped silt also: here the upperballast came into contact with the lower ballast exposed in thefloor of the" wash-out." These" wash-outs" were probablydue to sudden increases in the volume and velocity of the river;

352 A. L. LEACH AND A. C. YOUNG,

some of its branches changed their courses, broke into the quietmarshy pools and drier marsh land, and deposited large quantitiesof gravel in newly-cut channels.

The deposits show repeated sequences of similar events,viz. (r) erosion of a channel on a land surface; (2) depositionof gravels, sands and silts in the channel; (3) growth ofof vegetation on the deposits in the silted-up channel. Renewederosion manifested by channelling and "wash-outs" theninitiated a new cycle. The present land-surface marks thethird stage in the third cycle of these changes.

Summary and Conclusion.The gravel exposed at Lewisham under the alluvium cannot

be equated with the Taplow (Crayford or Middle) Terrace of theThames valley, but must be assigned to a later date. In the" Geology of South London" Mr. Bromehead suggests a corre­lation " which must not be regarded as more than tentative"of the later Terrace deposits of the Thames valley as follows :-

Alluvium. Neolithic and later.Lower [Younger] Flood Plain. Neolithic (?)Buried Channel. Later Cave Period and

Neolithic (?)Upper [Older] Flood Plain Earlier Cave period

(The terms" Upper" and "Lower," ambiguously used inthe Memoir, are equivalent to "Older" and "Younger" asindicated in the brackets).

No palreolithic or other implements nor any significantmammalian bones were collected from the Lewisham gravel.Only the shells give positive evidence of date, and (accordingto Messrs. Kennard and Woodward) indicate a Late Pleistoceneage. The base of these gravels, 5 to ro feet below OrdnanceDatum, must lie considerably above the base of the excavated" Buried Channel." The deposits may. therefore, be regardedas Ravensbourne valley gravels equivalent to the Older [Upper]Flood Plain of South London and Late Pleistocene in date.The great value of the section consists in the evidence yieldedof repeated periods of erosion alternating with deposition andof the development of vegetation upon temporary land-surfacesat various levels during the building-up of the gravel Terrace.

The alternations of pale chalky mud with carbonaceoussilt in the striped beds suggest comparison with the" varves"or laminse in the post-glacial marine clays of Southern Swedenwhich have been intensively studied by Baron de Geer andshown to be due to changes in the supply of sediment associatedprobably with rapid melting of the glaciers in summer and slowermelting in winter. We do not suggest that the Lewishamlaminated silts are so closely post-glacial as the "varves" ofSweden, but the Pleistocene cold period had not quite ended;

PROC. GEOL. Assoc., VOL. XLI. (1930) . PLATE 25 .

4

6

7

5

2

3

bo-6 4 12

~C=:=--===--==:J_C==-_IE.J. FEET .A.L.L.

0(; 0 oo~:v· o 0 0 0 0 ' 0 0.0o ... 0 ••• 0 0u 0 <> () . 0 0

A .-SECTION ON THE N ORTH-W E STE R N F ACE OF THE L EWISH AM B ALLAST- P IT.J Calcret e . 2 Lower ball ast (full t hick ness not shown). 3 Lower stripe dsilt . 4 Middle ballast. 5 Upper strip ed silt . 6 Upper ballas t . 7 Holoce nea lluvium . Thick lines sh ow p lanes of erosion . A" wash-out " cuts t hroug hthe upper si lt (5) down in t o the lower si lt (3).

7

6

5

3

2

Photo by A . L . Leach.B .-S OUTH-WE STE R N FACE OF TH E PIT.

Showing (7) H olocene a lluviu m ; (6) Upper Ballast; (5) Upper peatysilt; (4) Middle ball ast in a .. wash- out " ; (3) Lower peaty silt (2) Low er

ball ast. The " Calcret e " of F ig . A lies below (2).[To face p. 352.

Paoc. GEOr.. Assoc., VOL. XLI. (1930). PLATE 26.

Photo by A . L . Leach.

PA RT OF TH E U P ? E R BA~D OF S T R IPED S I LT .One fifth of actual size ; a lt er a t ions , probably sea sona l, o f cha lky mud and

peaty silt.

Photo by A . L . L each.

THE UPPER BAND OF S T RI P E D OR LA~IINATED P EATY S ILT .Cut into by a .. wash-out " ; po rtion to t he r ight of the stick en larged above

[T o face p. 352.

SECTION IN RAVENSBOURNE VALLEY GRAVELS. 353

winter frost-action may have been severe, and these chalky mudsmay have been washed into the stream after frost-shattering ofChalk in winter, while the carbonaceous layers represent materialfrom the summer vegetation in the valley. In this sense theIaminre of the striped silts may be expressions of seasonal changes.

We wish to record our thanks to Miss D. N. Bate, Mr. A. S.Kennard, F.G.S., Mr. B. B. Woodward, F.L.S., Mr. W. N.Edwards, F.G.S., and Prof. H. N. Dixon for their identificationsof the fossils, and to Mr. G. Ellson, Engineer's Office, SouthernRailway, for permission to examine the section.

Discussion.MR. A. S. KENNARD said that it was the work of Mr. S. Hazzledinc

Warren at Ponder's End that first drew the attention of geologists to theimportance of these low-lying gravels. The molluscan faunule was avery interesting one, in the main freshwater, with a few drifted landshells. The examples were nearly all dwarfed and the conditions werenot suitable. Probably a climate similar to that of Lapland or Icelandis indicated.