On Rotten Flint Pebbles in thePalaeogene of Southern England

by DENNIS CURRY

Received 1 February 1963. Taken as read 3 March 1964

CONTENTS1. INTRODUCTION page 4572. ROTTEN FLINT PEBBLES: GENERAL DESCRIPTION 4583. SOME FAUNAS FROM ROTTEN FLINT PEBBLES AND STRATIGRAPHICAL

CONCLUSIONS 4584. CAUSES OF THE DECAY OF FLINT PEBBLES 459

REFERENCES 459

ABSTRACT: This note discusses the occurrence at certain horizons in the Palaeogenebeds of the south of England of flint pebbles which are porous and friable. Such pebblesmay contain microfaunas which can be extracted and identified. From these identifica­tions the pebbles may be dated and new stratigraphical information may be forth­coming. A case is recorded of a pebble which has yielded a microfauna of UpperMaestrichtian age, indicating derivation from Cretaceous beds which are considerablyyounger than any now known from southern England. The author speculates onpossible causes of the alteration of such pebbles.

1. INTRODUCTION

IT HAS WNG been realised that the pebbles which are found from time totime in sedimentary sequences can provide valuable geological information.This information clearly includes, for example, evidence of the erosion ofthe beds from which a particular pebble was derived; and may includeevidence of the method of derivation and direction from which thematerial came. In addition, as in the well-known case of the boxstonepebbles from the base of the East Anglian crags, it may provide the onlyevidence for the former existence of beds which have otherwise beencompletely destroyed.

Pebbles of Chalk flint are extremely widespread and often abundant inpost-Cretaceous strata, and the macrofossils found in them have sometimesbeen used to provide information of the kinds mentioned (e.g. Reid (inUssher), 1913, 103).However, the proportion of flint pebbles which containsrecognisable macrofossils is small, and even when these fossils occur theymay not be specifically identifiable or may be of species which are strati­graphically unimportant. Flint, like the Chalk with which it is associated,commonly contains abundant microfossils which could be used for strati­graphical interpretation. The technical difficulties of examining andidentifying these in unaltered flint are, however, so great that work of thiskind has rarely been carried out.

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458 DENNIS CURRY

2. ROTTEN FLINT PEBBLES: GENERAL DESCRIPTION

There are relatively thick beds of flint pebbles at several horizons in thePalaeogene succession of southern England and, in some of these beds,pebbles may be found in which part or the whole of the flint is bleached andporous and much softer than usual. These pebbles preserve their originalform and, commonly, external markings such as those due to beach­battering, but may be so soft that they can be crushed between the fingersand so incoherent that they disintegrate when placed in water. The propor­tion of such rotten pebbles varies greatly. In the Boscombe Pebble Beds(Eocene) they comprise perhaps 2 per cent of the whole; of the rather rarepebbles in the Brockenhurst Bed (lowest Oligocene) at Brockenhurst andWhitecliff Bay, most are rotten (cf. Davis, 1952, 216).

In addition to the localities and horizons already quoted, rotten flintsare known in England from the (?) Blackheath Beds of Knockmill, Kent(Chandler & Leach, 1936); the London Clay Basement Bed of Pebble Hill,near Newbury (Prestwich, 1850, 258); London Clay of Highgate,Hampstead and Stanmore (Reid, 1904, 119), of Sheppey and Bracknell(A. G. Wrigley in Chandler & Leach, 1936, 247); and the BrackleshamBeds of Bracklesham, Sussex (Fisher, Bed 11).

3. SOME FAUNAS FROM ROTTEN FLINT PEBBLESAND STRATIGRAPHICAL CONCLUSIONS

Davis (1952, 217) reported that after extremely rotten flints had beendried and placed in water they collapsed into a mud, from which micro­fossils could be recovered by sifting. Even when decay is only slight,however, it is often possible to extract microfossils by crushing the flint andkneading the resultant powder in a fine-meshed nylon cloth under runningwater. The larger microfossils will be retained in the cloth and, afterdrying, can be picked under a low-power microscope. The fossils soobtained are silicified and their state of preservation varies considerably.In some pebbles they are very well preserved and readily identifiable; inothers the specimens are more or less overgrown with secondary silica andcan only be determined generically, if at all.

About twenty rotten pebbles were collected recently from the BoscombePebble Beds of the sea-cliff at Southbourne, Hampshire (40j(SZ)146912),and five of these yielded recognisable microfaunas. All but one of thefaunas were consistent with derivation from the mucronata Zone (UpperCampanian-Lower Maestrichtian) of the Upper Chalk. This was of courseto be expected as Chalk from the lower part of this zone underlies theLower Tertiary beds almost throughout the southern part of the Hampshiresyncline. There was, however, one pebble which yielded a different fauna.This proved to be rich in Foraminifera, including Racemiguembelina

ROTTEN FLINT PEBBLES IN SOUTHERN ENGLAND 459

fructicosa (Egger), Gavelinella incerta Hofker and Globotruncana area(Cushman). This pebble appears to be of Upper Maestrichtian age. Chalksof this age are not known on the mainland of Britain, but have recentlybeen reported on the floor of the English Channel in two areas. The first(Curry, 1962, 184) is south of the Isle of Wight, in the neighbourhood of50°08' N., 1° 35' W. The second (Curry, 1963, 5, 6,) is south of Plymouthnear 49°35'N., 4°20'W. The discovery of an Upper Maestrichtian micro­fauna in the pebble from the Boscombe Pebble Beds provides additionalevidence of the deposition of Upper Maestrichtian beds in the neighbour­hood, and evidence of their removal by denudation.

Derived microfossils of Danian age have been found in Palaeogene bedsfrom the Channel. The Danian of Denmark and its presumed equivalent,the Tuffeau de Ciply of Belgium, both contain beds of flint similar to thosewhich occur in the Chalk of England. It seems probable that Danian bedswere deposited in parts of what is now the central English Channel and itis at least possible that these contained beds of flint. If so, flint pebblesyielding a Danian microfauna may well await discovery in the LowerTertiary beds of the Hampshire area.

4. CAUSES OF THE DECAY OF FLINT PEBBLES

Possible causes were discussed by Chandler & Leach in a series ofreports on field meetings at Knockmill (for references, see Chandler &Leach, 1936). Pertinent facts seem to be:

(i) the alteration is often only partial and affects only some pebbles;(ii) it has occurred subsequently to the burial of the pebbles, as shown by

the presence of batter marks on the surface of the latter;(iii) rotten flint pebbles commonly occur in association with well­

preserved molluscan shells of calcite or aragonite. Action by acids,therefore, seems ruled out.

Flint is known to be composed of a mixture of amorphous and crypto­crystalline silica. It appears that the most probable explanation of the decayis that the flint has been attacked by an alkaline solution which hasdissolved the colloidal content without affecting the cryptocrystallineportions.

REFERENCESCHANDLER, R. H. & A. L. LEACH. 1936. The Structure of the Eocene Outlier near

Knockmill, Kent. Proc. Geol. Ass., Lond., 47, 239-48.CURRY, D. 1962. A Lower Tertiary Outlier in the Central English Channel, with

Notes on the Beds Surrounding it. Quart. J. geol. Soc. Lond., 108, 177-205.---. 1963. Age Determinations of Some Rocks from the Floor of the English

Channel. Proceedings of the Ussher Society, 1, 5-6.DAVIS, A. G. 1952. The Brockenhurst Beds at Victoria Tilery, Brockenhurst, Hamp­

shire. Proc. Geol. Ass., Lond., 63, 215-19.

460 DENNIS CURRY

PRESTWICH, J. 1850. On the Structure of the Strata Between the London Clay and theChalk, etc. Quart. J. geol. Soc. Lond., 6, 252-81.

REID, C. 1904. On the probable Occurrence of an Eocene Outlier off the Cornish Coast.Quart. J. geol. Soc. Lond., 60,113-19.

---. 1913. in W. A. E. USSHER. The Geology of the Country around NewtonAbbot. Mem. geol. Surv. U.K.

D.CurryEastbury GrangeWatford RoadNorthwood, Middlesex