The stratigraphical and geomorphological significance of the Red Crag fossils at Netley Heath, Surrey: a review and re-appraisal

The stratigraphical and geomorphological significance of the Red Crag fossils at Netley Heath, Surrey: a review and re-appraisal D. T. John and P. F. Fisher JOHN, D. T. & P. F. FISHER, 1984. The stratigraphical and geomorphological significance of the Red Crag fossils at Nelley Heath, Surrey: a review and re-appraisal. Proc. Geol. Ass., 95(3),235-247. The presence of Red Crag-type fossils at Nelley Heath, on the North Downs of Surrey, is of major significance for the geomorphological history of southeast England. The fossils occur in association with an outlier of sand and gravel, which until recently was incorporated into the 'Netley Heath Deposits', Their stratigraphic relationship to the associated sand and gravel is far from certain. New excavations provide more reliable information on their mode of occurrence. Fossiliferous ironstone was retrieved from a periglacial structure surficial to the main mass of the sand. Petrographic analysis reveals that whilst the sand belongs to the remnant Headley Formation, the fossiliferous ironstone is mineralogically distinct. The fossils cannot be used to provide an age for the various patches of the Headley Formation preserved on the western part of the North Downs. The accepted views on Wealden denudation, in so far as they relate to this formation, may be in need of major revision. The School of Geography, Kingston Polytechnic, Penrhyn Road, Kingston-upon- Thames KTl 2EE.

1. INTRODUCTION Patches of supposed Pliocene and early Pleistocene sand and gravel are scattered over the Chilterns and North Downs, but are known on the South Downs only from Beachy Head. These outliers generally occur at altitudes between 120 and 200 m O.D. The deposits at Netley Heath, Surrey are one such patch on the North Downs. Red Crag-type fossils have been recovered from this outcrop, but there is doubt as to the mode of their occurrence within the deposits. The site has therefore been re-investigated to establish the precise relationship between the fossils and the deposits. This paper first reviews the history of previous investigations, and then presents an account of the recent excavations. The discussion and conclusion focus on the possible significance of the new information for Wealden denudation history. 2. A REVIEW OF EARLIER WORK (a) The supposed Plio-Pleistocene marine sands and gravels on the Chalk.

With the exception of Monkton (1892) and Salter (1897-8, 1905-6), who interpreted certain of the Surrey outliers as products of a westward-flowing 'Bagshot stream', and of Sherlock (1924, 1929) who reasoned that the deposits might constitute a continental facies of the Reading Beds, all other investigators have agreed on the marine character of the sands and gravels, and of the fossils the former sometimes contain. In about 1854 (Reid, 1890) the sands were discovered in pipes in the Chalk at Lenharn, Kent, and have since become known as the 'Lenharn Beds'. They were initially regarded as basement beds of the

London Clay, as the Oldhaven and Blackheath Beds were then classified. However, ill-preserved fossil casts found in the sands led Prestwich (1858) to assign them to the Coralline Crag. As corroboration he cited the lack of corresponding Eocene Beds; the fact that Tertiary outliers on the crest of the Downs are lithologically identical to the main mass (i.e. it cannot be argued that the sands are coastal variants of Eocene formations); and the existence of matching Diestian deposits at similar elevations on the Chalk of northern France. Whitaker (1862) initially accepted a Pliocene age for the fossils, but later (1872) rejected their stratigraphic value because of the poor state of preservation, and referred the deposits instead to the Lower Tertiary-the fine sand to the Oldhaven and Blackheath Beds, the coarse sand to the Woolwich Beds. Further scrutiny of the fossils by Reid (1890) vindicated Prestwich's dating, and suggested that they were organisms which lived in a sea approximately 70 m deep. In 1900, Stebbing obtained very imperfect fossils from the sands on Netley Heath, Surrey, that were held to confirm the link with the Lenham Beds suggested by both Prestwich and Reid. Additional finds by Wright (quoted in Davies, 1917), and by officers of the Geological Survey (Fig. 1) were examined by Chatwin (1927). He conduded that the assemblage was distinct from that at Lenham, and most probably of Red Crag age. This discovery, together with one of a similar kind in the Chilterns, at Rothamsted, a few years later (Dines, 1930), was taken by Wooldridge and Linton (1955) to indicate a transgressive Pliocene sea, younger in the west of the London Basin than in the east. It has until recently been widely referred to as the 'Calabrian Transgression', because the Red Crag was tentatively correlated

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RED CRAG FOSSILS AT NETLEY HEATH : REVIEW & RE· APPRAI SAL

with the Earl y Pleistocene Calabri an Stage of the Italian marine succession (Baden-P owell , 1955) . Since both the correlation and the dating of the Red Crag as early Pleistocene are now in doubt (R . W. Hey, pers. comm ., 1983) , it seems advisable to discard the term Calabri an . The name ' Red Crag Tr ansgression' is therefore used in this paper without prejudice as to whether this was a Pliocene or Pleistocene event. Fossils have not yet been found at any of the other outliers , but a possible alterna tive means of correlating them was furnished by Davies (1915- 16). He showed that in Surrey the sands possess a diagnostic suite of hea vy minerals, distinguished from Tertiary counterparts, particul arly by an abundance of coarse andalusite and chiastolite, and the constant though minor presence of monazite and a near absence of garnet. Later, a more comprehensive survey by Wooldridge (1927) confirmed that virtually all the occurrences of these sands in the London Basin have a distinctive mineralogy. Even so , in the absence of fossils, Dines and Edmunds (1933) did not accept as conclusive either the dating or the correlat ion of these deposits in Surre y. That the y were wise not to do so can be demonstrated for two oth er reasons. Firstly, as Sherlock (1929) observed , the miner al evidence is not entirel y convincing, as the same mineral species are found at Lenham and Netley Heath in beds which are now thou ght to be of different ages . In any case , he noted , ' the constituents of a sand depend on the source of supply and the same source of supply may be tapped at different peri ods, while on the other hand different sources of supply may be tapp ed at one and the same period' (Sherlock, 1929, p. 359) . In addition, adequ ate petrographical descriptions of the sands sampled , especially of their grain-size characteristics, were not given , nor is it clear whether all the varieties of sand pre sent at a site were examined. In the latter context it is pertinent to recall that Groves (1931) encountered a 'crop' of heavy minerals from the sands on Netley Heath that differed in many important respects from the typical 'Lenharn ' suite. Secondly, the fossil evidence from Netley Heath is equivocal. Chatwin believed that the comminuted state of the shell casts and the isolated conglomeratic nature of the boulders in which they occur , were not inconsistent with their having been acquired from a source other than that from which the associated deposit originated. Indeed , Sherlock (1929) viewed the shelly conglomerates as glacial erratics. Significantly, Boswell (reported in Din es and Edmunds, 1929) found the heavy miner als of the 'boulders' to be strikingly dissimilar to those of the enclosing sands . On the other hand , Green (in discussion of Sherlock's 1929 paper) described the fossiliferou s conglomerates as being ferruginous horizons within the outlier of sand, and Wooldridge (1927) also referred to them as slabby ironstones formed within the sand. Thus , on

237

the basis of the evidence present ed , the case for the distinctive high-level assemblage of sands and gravels on the Chalk of southeast En gland having been deri ved from a Pliocene and/or Pleistocene marine incursion is considera ble , but by no means incontro vertibl e. (b) The nature of the 'Netley Heath Deposits' Th e term 'Netley He ath Dep osits' , coined by Din es and Edmunds (1929) 'is one implying location only' (p. 110). It embr aces a group of deposits, namely presumed Red Crag sand and grave l, the 'clay-withflints' of the Geological Survey , and two quite separate suites of gravels. According to Dines and Edmunds the se various deposits are so intermi xed, both stratigraphically and spatially, as to defy separate mapping. Unlike the Red Crag(?) materi al and the clay-with-flints, the remaining gravel deposits are unique to the area. The more exten sive of the two is best developed at Newlands Corner, wher e a 7 m thick mass of dark , well-rounded flint pebbles , containing lenses of sand and mottled clay, is pr esent (Jo hn , 1974) . It contrasts markedly with the smaller gravel-spread in Mountain Wood , which consists almost entirel y of Lower Gr eensand detritus and trenches deeply into undisturbed Red Crag(?) sand. Th e age and formation of the presumed Red Crag sediments have been conside red above , whilst the comp osition and der ivation of the so-called 'clay-withflints' has been extensively discussed elsewhere (e.g . Loveday, 1962; Hodgson et al. , 1974; John , 1980). Before de aling with the origin of the Netley He ath Deposits as a whole , therefore , it is first necessary to review the literature on the two more restricted constituent facies, namel y the two grave ls. Both Prestwich (1890) and Monk ton (1904, 1923) belie ved that the gravel at Newlands Corner testifies to earl y glacial conditions: not to glaciation proper, but to rivers and floods rising in the Wealden area. Bury (1922) correlated this gravel with a similar deposit capping the Hale Plateau, near Aldershot , as did Monkton (1904), but the form er exte nded the correl ation to include the supposed Pliocene sands and gravels, because of their bro ad lithological resembl ance and comparable physiograph ic location . This was suppo rted by Wooldridge (1927) who not iced that despite its confused state the Newland s Corne r depos it cont ains 'unmistakable Diestian sand' (p. 69), although Da vies (1917) had previously recorded the 'Lenham' suite of hea vy miner als as being absent. Din es and Edmunds (1929) subsequently re-classified the Hale Plateau gravels as the 'Caesar's Camp Gravels' , having failed to find in them any convincing evidence of a marine origin . In their opinion, the pre sent arrangement of the Caesar 's Camp Gravels accord s with glacial disturbance , and it is interes ting to note that Montford (1966) has suggested that they are

238

D. T. JOHN AND P. F. FISHER

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true glacial gravels. Most recently, however, Clarke and Fisher (1983) have ascribed the Caesar's Camp Gravel to fluvial activity in a periglacial environment. Following its inclusion with the Netley Heath Deposits, the gravel at Newlands Corner was not examined again for over 50 years (John, 1974). The Lower Greensand detritus overlying presumed Red Crag sand in Mountain Wood was commented upon specifically only by Bury (1910) and Wooldridge (1927). A drawing of a recent section revealing the junction between the two deposits is shown in Fig. 2. The gravel comprised several layers, each consisting of variable mixes of tabular chert and pitted sandstone, with presumed Red Crag sand, but containing also a small number of flints and pieces of ironstone. Regardless of lithology the gravel was overwhelmingly subangular to rounded. The upper part of the gravel was horizontally stratified, whereas below 1.3 metres the stratification was inclined at about 30 degrees towards the north-northeast, i.e. the same orientation as the interfluve on the spur of which the gravel is situated. The lowermost part of the gravel plunged first through weathered, presumably Red Crag, sand and then into its unaltered, apparently in situ, counterpart. Both Bury and Wooldridge reasoned that the Lower Greensand had been exploited by a consequent river that flowed across the site of the present Gault strike vale immediately to the south of the Chalk escarpment (the Vale of Holmsdale), and left the gravel in its existing position. Bury believed that the gravel was discharged onto a sea-shore as this provides an

explanation of the latitudinal continuity of chertbearing formations on the North Downs as a whole, especially between the major consequent valleys. He dated the shoreline to the Pliocene because the Netley Heath fossils were then assigned to that period. From its stratigraphic position and apparent imbricate structure Wooldridge inferred instead that this particular chert gravel is wholly fluviatile, having accumulated after the retreat of the sea which was thought to have deposited the underlying sand. He assumed that this happened immediately prior to glacial times, since the excavation of the Vale of Holmsdale could not have been accomplished over too short a time-span. It is difficult to equate the last interpretation with that of the origin of the Netley Heath Deposits given by Dines and Edmunds (1929). They invoked a small local glacier, dying in place, to make sense of the present arrangement of the constituents. As noted above, Sherlock went further and stated that the deposit contains true glacial erratics. Similarly, Shepard-Thorn (1975) has suggested the possibility that the marine Coralline and Red Crag fossils on the North Downs were emplaced by ice from the North Sea. (c) Deletion of the name 'Netley Heath Deposits' A recent, detailed survey of the soils and superficial deposits on the North Downs of Surrey (John, 1974, 1980) revealed little point in retaining the name 'N etley Heath Deposits'. Apart from the restricted gravel mass in Mountain Wood, the other constituent deposits are wholly typical of the formations that mantle much of the Chalk to the east. In particular, the so-called Red Crag sediments were shown to be the in situ remnants of a much dissected solid formation which was designated the 'Headley Formation'. This comprises a basal gravel-the Ranmore Member-and an overlying sand-the Headley Member, both named after the type localities, which are immediately west of the Mole Gap and three kilometres to the east of it, respectively. In the same way, the 'clay-with-flints' incorporated into the 'Netley Heath Deposits' could be subdivided into characteristic Plateau Drift with a margin of clay-with-flints sensu stricto, as defined by Loveday (1962). The Newlands Corner gravel was found to be merely a pebbly variant of the Plateau drift. Not only could these various deposits be petrographically differentiated, it was possible to map them separately as well (Fig. 1). Thus no special explanations are needed for the 'Netley Heath Deposits'. Moreover, with the exception of the gravel in Mountain Wood, the deposits which occur in this locality are also widely developed elsewhere on the Chalk, and there is now broad agreement on their nature and origin. In Surrey, John

RED CRAG FOSSILS AT NETLEY HEATH: REVIEW & RE·APPRAISAL

(1974, 1980) showed that the Plateau Drift is largely derived from Woolwich and Reading Beds residues more or less in place, and that the associated perimeter of clay-with-f1ints is similarly associated with Eocene material that has been more radically re-arranged. He also accepted a relatively shallowwater marine origin for the sediments comprising the Headley Formation, on various petrographic grounds. Because the Red Crag fossils on Netley Heath had been described by earlier workers as occurring within the sands on Netley Heath, and since the latter deposits were shown to be in situ, it was likewise accepted that the sands were Red Crag or younger. In the same survey it was demonstrated that the seemingly unique gravel of Lower Greensand debris in Mountain Wood is in fact a typical periglacial slope deposit. This was evident not only from the sedimentological character of the deposit, but also from its confused yet distinct layering on what is the lower part of a steeply-inclined spur. The deposit was probably derived from earlier gravel spreads at higher elevations, as evidenced by the frequent presence of Lower Greensand material in the uppermost soil horizons on the higher parts of Netley Heath. The presence of this Lower Greensand detritus on the presumed Red Crag sands at Netley Heath has long been taken to mean that the Gault Vale of Holmsdale, which separates the Chalk escarpment from the Lower Greensand cuesta immediately to the south, could not have existed in early Pleistocene times, at least not in its present pronounced form. Similarly from the paucity of flints it was claimed that the Chalk must have been extensively obscured by overlying, presumed Red Crag, deposits at the time the Greensand-derived gravel was laid down. From the foregoing review, it is clear that the mode of occurrence of the Red Crag-type fossils at Netley Heath is crucial to not only the dating of the associated shallow-water sands and gravels of the Headley Formation, but also the excavation of the Vale of Holmsdale and the low ground of the western Weald in general. The Headley Formation is at present widely regarded as Red Crag in age, and as it occurs in situ up to elevations of about 185 metres a.D., it is assumed that the post-Red Crag uplift of the North Downs has been roughly of this order. If, on the other hand, it can be shown that the fossils are actually erratics, no such inference can properly be made. In that case the fossils could conceivably have been derived from a former projection of the Chalk, well to the south of, and much higher than, the contemporary North Downs escarpment. Wooldridge and Linton's view that the present-day Low Weald was sculpted during the Pleistocene, from an uplifted 'surface' with a base level approximating to the summit of the Downs, rests heavily on the belief that the Red Crag fossils are in their original position. Should the reality be otherwise, then the possibility of

239

uplift in excess of 185 metres cannot be discounted in the absence of reliable stratigraphic evidence to the contrary. Obviously this implies that the erosion to be attributed to the Pleistocene is correspondingly greater than that envisaged by Wooldridge and Linton. In fact this applies even more strongly to Jones' (1980) recent reconstruction of Wealden denudation. He accepts Small and Fisher's (1970) contention that by the end of the Tertiary the rivers around the Weald may already have been incised down to 150-200 metres O.D. According to his interpretation, the Pleistocene merely accentuated earlier landform patterns. In summary, there is still considerable uncertainty surrounding the depositional status of the Netley Heath fossils. This makes it prudent to leave open the possibility of much greater Wealden uplift since Red Crag times than is allowed for in the above models of relief development, together with the concomitant greatly enhanced role for Pleistocene erosion. In support of this position it is pertinent to record Worssam's (1973) observation that despite intense late Pleistocene erosion in the Weald there is still appreciable relief. This led him to reject the notion that parts of the Weald had already been reduced to 200 metres O.D. by the start of the Pleistocene. Similarly, Fisher (1982) has argued that when the oldest of the Blackwater terraces, the Caesar's Camp Gravels near Farnham, was deposited, the Lower Greensand of the western Weald was still largely sealed by the Chalk, because flints account for about 99 per cent of this early Pleistocene deposit. Evidently, this is difficult to reconcile with both of the denudational models referred to above. The remainder of this paper, therefore, is concerned with the description and interpretation of a number of sections made with a mechanical excavator on Netley Heath, in December, 1982, to establish reliably the stratigraphic position of the presumed Red Crag fossils. The findings are considered in relation to the ideas that have been considered here on the post-Red Crag development of the Wealden landscape.

3. THE 1982 EXCAVATIONS AT NETLEY HEATH (a) Field Description Three pits were opened in an attempt to locate the enigmatic fossiliferous ironstone (Fig. 1). They were sited a short distance from each other and revealed that the contact between the Chalk and the overlying sediments is highly irregular. In Pit 1, chalk had not been encountered when digging stopped at a depth of just over 6 metres, whereas in Pit 3, only 20 metres to the southeast, disturbed, rubbly chalk was proved at depths varying between 1 and 2.5 metres (Fig. 3). In the absence of borehole data, the precise character of the Chalk surface could not be determined, and the

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irregularities noted here may be due to solution of the Chalk, cryoturbation or differential erosion . Pit 1 was for the most part excavated in a medium to fine, almost stoneless, reddish yellow (7.5YR7/6-6/6) sand , with frequent inclined, planar layers of brown (7.5YR5/6), clayey sand (Fig. 3). The continuity of the latter was sharply disrupted at irregular inter vals by near-vertical faults which may be cryogenic phenomen a. The sand was clearly cross-bedded in places , and the bedding planes were roughly parallel to the clayey horizons , and were affected by the same faults . Wheth er the clay segregation is an original depositional feature , or due to subsequent differentiation , is unclear. Very occasional fine-gravel size pebbles of chalk were retrieved from the sand , but there were no facies variations apparent from the base of the pit to the base of the shallow surface drift of flinty sand, in which podzolic soils are developed .

However , in the north face of the pit occurred a roughly W-shaped emplacemen t of highly heterogeneous material, encased by the sand and buried by the thin surface drift (Fig. 3). The constituent materials included: angular and nodular flints ranging from fine gravel to cobble-size as well as Eocenederived flint pebbles ; gravel- to cobble-size plat y fragment s of at least two varieties of ironstone, with long axes near to the vertical; some gravel-size, sub-rounded Lower Gre ensand clasts; and dark yellowish brown (lOYR3/6) sand with variable admixtures of clay. The form and poor sorting of this body of heterogeneous sediment are evidently due to geliturbation. This is especially significant, since specimens of one of the ironst one types retrieved from a depth of roughly 3.5 metres were found to contain numer ous fossil casts of bivalves and gastropods. Unfortunately the casts were too fragmentary for more specific identification (R . Markham , pers. comm ., 1983). Even so, their comminuted state , the nature of the ironstone matrix, and the heavy minerals from the latter (see below) , are all consistent with descripti ons of the Red Crag materi al found at the same site earlier this century, and now stored with the Institute of Geological Sciences. There can be no doubt that the fossils are from the assemblage described by Chatwin (1927) and other workers . The new specimens are now in the Ipswich Museum collection (Number R .1983-86). Pit 2 was dug about 10 metres to the south of Pit 1, and again approximately 6 metres of undisturbed stoneless sands were encountered with identical clayey sand horizons, and almost ver tical faults, to those described above. Conversely, there were no drift depo sits let into the sand , althou gh once more a thin surface drift of podzolised flinty sand was present. Apart from exposures of much-broken chalk at depths of between 1 and 2.5 metres, Pit 3 also revealed a more complex set of lithological relationships than was found at the other two sites (Fig. 3) . Immediately above the surface of the Chalk came a layer of very dark grey clay with numerous dark-stained nodular flints. This in turn gave way to a variably flinty strong brown (7.5YR5/6) clay, which in one part of the pit was overla in by the flinty, sandy drift noted earlier, but which elsewhere was succeeded by a stoneless band of strong brown (7.5YR5/6) clay which defined a U-shaped structure. This contained sand with clayier zones; both textural types were grey to white in colour, presumably as a result of gleying. Except for colour , the sand with its clayier facies was like the sand and associated clay-rich horizons that lay immediately next to the clay in the western part of the pit. This in turn was indistinguishable from the main body of sand at Pits 1 and 2. In summary , the deposits at Netley Heath resting irregularly on the Chalk display a variety of depositional and probable cryogenic featur es. The

241

RED CRAG FOSSILS AT NETLEY HEATH: REVIEW & RE·APPRAISAL

principal observation which emerges, however , is that the casts of the Red Crag-type fossils on Netley Heath clearly occur in periglacial structures wedged into the outlier of sand preserved on the Hea th, and not in the sand itself .

(b) Laboratory Analyses A total of 14 samples were collected from the freshly excavated sections. T he relation of the samples to the sections is shown in Fig. 3. Seven of the samp les (1 to 6, 14) were from facies of the Headley Sand, five from periglacially disturbed deposi ts (7, 8, 11 to 13), and two were irons ton es (9 and 10). These samples were air dried upon ret urn to the laboratory, and submitte d to a variety of petrographic tests, including: particle-size analysis, by dry sieving at half-phi intervals th rou ghout the gravel and sand fractio ns, and then by the pipet te method for the silt and clay fractions (British Standards, 1967); heavy mineral analysis after separation in bromoform; viewing by scanning electron microscope of the 0.18 to 0.125 mm size fraction of some samples; and assessment of the lithological content of the one gravel-rich sample (7). Ironstone samples were prepared for heavy mineral and particle-size analysis by gentle grinding and subsequent cleaning of iron coats by warming with 2NHCl. The particle -size distribution curves are shown in Fig. 4. It is immediately apparent that the Headley Sand samples are a group of moderately well sorted sands more akin to each other than to the periglacial materials. Of the latter, only one sample (11) has any textural similarity with the sands . Folk and Ward (1957) statistics were calculated for the total particle-size distribution (Table 1). The clay content of the ironsto ne samples is very difficult to measure, being grossly affected by diagenetic iron and pretreatment in hydrochloric acid. Therefore, so that all deposits can be compared , Folk and Ward statistics were also pre pared for only the sand-sized fraction of all samples (Table 2). From these (Tab les 1 and 2) it is apparent that the qualitative observations on the distribution curves (Fig. 4) are confirmed. For the Headley Sand, the mean and median sizes are generally from 1 to 2 phi, only one sample being finer. In contrast, the size parameters of the periglacial material are more variable: only one sample lies in the range of the Headley Sand samples when calculated for the total distribution, and only two when sand alone is employed in the calculations. In both tables, the sorting is invariably better (numerically lower) in the Headley Sand, whereas skewness and kurtosis are variable for both sets of calculations. Furthermore, the Head ley Sand curves are very similar to the curves given by John (1980, fig. 6) for that formation, although the mean size is slightly coarser here.

TABLE 1. Folk and Ward summary statis tics of particle-size analysis (calculated for the whole range of sizes) Sample 1 2 3 4 5 6 7 8 9 10 11

12 13 14

Mean Sorting Skewness Kurtosis 0.692 0.445 1.451 1.66 0.754 0.321 1.75 1.368 0.814 -0.106 1.242 2.49 0.798 0.323 1.189 1.35 0.327 0.821 0.998 1.96 1.383 2.932 1.62 0.450 -2.78 3.709 0.792 0.883 1.46 1.767 -0.267 4.168 n.d .-ironstone n.d .-ironstone 1.384 0.275 0.851 2.98 1.876 2.94 0.386 1.187 n. d .-clay 5.59 0.932 0.224 1.852 1.28

Median 1.51 1.61 2.61 1.24 1.83 1.50 -4.55 1.79

2.74 2.39 1.28

The results of heavy mineral analysis are presented in Table 3 for the total sand fraction, and Table 4 for the 0.09 to 0.075 mm size fraction. In traversing a slide all non-opaque grains were identified , while opaques were counted as a single group. About 2000 grains were counted in all, but where the number of non-opaques was small (under 200) , the count of non-opaques only was increased to between 400 and 500. In all samples the suite is dominated by zircon, as is typical of many Pleistocene deposits of southeast England (Chartres, 1981; Davies, 1915-16; Fisher, 1982; John , 1980). Muscovite is usually highly variable, but in the ironstone samples (9 and 10) it is the most important single mineral. Minerals with a consistently high proportion include rutile , tourmaline , staurolite and kyanite, generally in that order of importance, although in the ironstone samples tourmaline is most plentiful. Monazite is a significant component of the fine fraction, but less important in TABLE 2. Folk and Ward summary statistics of particle size analysis (calculate d for the sand-sized fraction) Samp le 1 2 3 4 5 6 7 8 9

10 11

12 13 14

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Sorting Skewness Kurtosis 0.547 0.325 1.201 0.244 1.229 0.653 0.970 -0.306 0.675 1.022 0.666 0.266 0.835 0.695 0.237 1.194 0.643 0.053 -0.086 1.155 1.259 -0.168 2.110 0.736 -0.308 1.752 0.825 1.445 -0.240 0.883 - 0.048 0.875 0.736 0.929 0.696 0.227

Median 1.48 1.59 2.57 1.21 1.79 1.42 1.48 1.82 2.77 2.22 2.35 1.99

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242

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B.

100...----=----r----..------,,-------,----r-----r-----,.----..----..------, 90

12

IE:

~

80 70

7

60 50 40 30 20

10

Fig. 4. Particle-size distribution curves for (A) samples of Headley Sand, and (B) samples of periglacial deposits .

the total sand fraction. Sillimanite is the only other mineral always present. Remaining minerals are variably present, but always at concentrations of less than 1 per cent. The single exception to this generalisation is the garnet content of the ironstone samples (9 and 10), which is from 1 to 4 per cent in the fine sand, although it is never more than 0.5 per cent in any other sample. All minerals are available in local deposits , particularly those of the Weald to the south (Davies , 1915-16; Wood, 1956). Garnet, however, is best represented in the Thanet Sand of the Palaeocene Beds and some Plateau Drifts of the North Downs (John, 1980, table I; Morton, 1982). Taken as a whole, there are three suites of heavy

minerals that can be distinguished in this study, the characteristics of which follow. In samples 1,3, 11 and 14 much zircon is accompanied by a lower amount of rutile (zircon to rutile ratio is always greater than 2). This group is indistinguishable from the Headley Sand reported by John (1980, table III) . In contrast , samples 7 and 8 have equal concentrations of zircon and rutile . Ancillary minerals in both groups are similar, and no doubt imply a broadly similar provenance. The third group relates to samples 9 and 1Q-the ironstones. It is characterised by anomalously high garnet , muscovite and tourmaline contents. The first had previously been noted as distinctive of the Netley Heath fossiliferous ironstone by Boswell (in

243

RED CRAG FOSSILS AT NETLEY HEATH: REVIEW & RE-APPRAISAL

TABLE 3. Heavy minerals in the total sand fraction Samples Minerals zircon rutile tourmaline staurolite kyanite monazite andalusite sillimanite brookite spinel garnet corundum muscovite titanite topaz epidote opaques number counted

44.48 15.58 13.96 1.94 4.54 2.27 0.65 0.97

3

7

8

9

10

11

14

43.36 9.39 5.64 3.35 2.55

36.18 24.12 8.04

21.45 18.Q1 9.19 6.51 5.36 1.15

18.41 6.65 9.26 2.85 1.66 1.07

43.77 14.89 14.22 4.82 6.00 1.77 0.67

51.69 16.20 3.70 2.93

0.47

9.38 5.48 9.70 3.13 1.88 0.62 0.46 1.09

1.34

4.52 4.02

0.54 0.40 0.27

0.50 1.00

1.53

1.00

0.38 0.38 36.03

0.32 0.32 14.96

7.03

33.15

13.58

1.33

0.92

0.92 0.45 0.15

0.44 0.30

1.66 0.23 56.17 0.23 0.23 1.07

0.46 63.22 0.46 0.62 1.25

0.32 11.79

2.03

22.38 0.15 0.15

53.19

42.69

89.86

95.27

90.77

98.34

55.22

70.33

673

1300

1929

924

2049

2349

1005

2184

Dines and Edmunds, 1929, pp. 114-116). Of the local deposits, the mineralogy of the fossiliferous ironstone is most similar to the Thanet Sand (John 1980; Morton, 1982), although the other petrographic attributes of that formation are very different (John, 1974).

Scanning electron microscope examination of sand grains from samples 1, 3, 7, 8, and 11 show grains to be characterised by v-notches with recrystallisation and conchoidal fractures (Fig. SA, B). These properties are so intermixed that they can only be taken as indicative of deposition in sub-aqueous environments

TABLE 4. Heavy minerals in the sand fraction from 0.075 to O.09mm Samples Minerals zircon rutile tourmaline staurolite kyanite monazite sillimanite brookite corundum garnet titanite hypersthene epidote dumortierite muscovite biotite topaz enstatine andalusite opaques number counted

52.04 17.17 10.97 4.95 2.30 1.59 0.71 0.18 0.18 0.35

3

7

8

9

10

11

14

46.5 20.99 7.12 5.1 2.25 1.42 0.59

34.62 32.27 10.53 5.01 2.51 1.00 0.84 0.16

33.54 29.65 3.89 4.76 1.51 1.29 0.43

28 14.90 11.64

4.95 3.96 6.43 1.48 0.74 0.50 0.50

50.47 24.16 7.48 3.2 3.73 1.07 1.47

57.08 14.59 7.3 1.72 3.00 1.29

0.12 0.12

0.16 0.22 0.16 0.22

15.78

12.70

24.48

10

5.27 1.09 1.64 0.36 3.81 0.18 0.18 1.45 0.18 20.36 0.36 0.26 0.18

0.50

0.43

0.99

0.25 0.25 0.74 77.97

0.13 8.28

13.73 0.86

0.25 0.5

65.65

64.05

90.07

89.70

92.50

91.55

64.32

73.00

1645

2345

2001

1962

1881

2272

2099

863

244

D. T. JOHN AND P. F. FISHER

.'

B

Fig. 5. (a) V-notches and associated recrystallisation on a rounded quartz grain from sample 7 (framelength 20 microns); (b) conchoidal fractures on quartz grains from sample 7 (framelength 550 microns); (c) worn sponge spicule from sample 7 (framelength 550 microns); (d) abraded foraminifera probably of the species Heterohelix cf. Histriata, which is probably no older than Turonian, and suggests a source in the Chalk, together with (i) coccoliths, Prediscosphaera cretacea, which range throughout the Chalk, and (ii) possible calcispheres (R. Stokes, pers. comm., 1983; framelength 220 microns).

(Krinsley & Doornkamp, 1973). The most striking finding is the presence of microfossils in one of the samples from the periglacial deposit (sample 7), in contrast with the complete absence of any such remains in the other samples, either from the periglacial deposits (8 and 11) or from the Headley Sand (1 and 3). This fossil material is all worn and generally broken, and therefore problematic to identify at the species level. Sponge spicules are present probably derived from the Lower Greensand (Fig. 5C). The fossils identified include the foraminifera Heterohelix d. Histriata, probably no older in age than Turonian, and suggesting a source in the Chalk, and the coccolith Prediscosphaera cretacea which ranges throughout the Chalk (R. Stokes, pers. comm., 1983; Fig. 5D). All appear to be derived from the Cretaceous strata to the south. The lithology of stones greater than 8 mm in sample

7 was also examined. From a total of 237 stones, two were silica-cemented sandstones with and without sponge spicules, probably derived from the Lower Greensand to the south. One vein quartz pebble was present, and of the remainder 79 per cent were flint (31% rounded, and 48% sub-angular), and 20 per cent ironstone. The stone lithologies again suggest a southern provenance for the periglacial deposit.

4. DISCUSSION AND CONCLUSIONS

In the new sections at Netley Heath it was possible to identify three deposits: a shallow surface drift with a podzolic soil profile; the Headley Sand; and in places a separate intervening periglacial deposit. Heavy mineral and partizle-size analyses of the last two sediments have shown their essential dissimilarity, although the mineral suite of one sample of the periglacial material

RED CRAG FOSSILS AT NETLEY HEATH : REVIEW & RE-APP RA ISAL

is akin to the Headley Sand , and so may be derived directly from it. Furthermore , the iron stone s, which contain the fossils, are incorporated within this periglacial deposit, but the samples anal ysed show the ironstones to be distinct from any of the associated formations-both the containing periglaci al debris and the Headley Sand. The fossiliferous ironstones at Netley Heath are surficial to the Headley Formation. Therefore , the Red Crag-like faun al assemblage preserved in one of the ironstone types found on Netley Heath , cannot be used to furnish an absolute age for the underlying Headley Sand . The latter could be either younger or older. This fact has important implications for the denudational histor y of the Weald , especially the westernmost parts . As noted in the review of earlier work, Wooldridge believed that the Lower Greensand detritus resting on the Headley Sand in Mountain Wood is fluviatile in origin , and must have been derived from the source area to the south, at a time when the intervening Gault Vale of Holmsdale did not exist. Since the sand bene ath the gravel s had already been att ributed on the basis of the Netley fossils to the Red Crag transgression , the initial excavation of the present strike vale in the Gault , and indeed of the Weald sensu stricto, was related to post-Red Crag uplift. Such a reconstruction is now seen to be insecure . The fossiliferous ironstone is neither in situ nor was it incorporated into the Headley Sand at the time of its deposition . Similarly, the supposed fluviatile debris of Lower Greensand origin in Mountain Wood has been reworked from an older accumulation and now has the form of a typical slope deposit (John, 1974, 1980). All that can definitely be said about the He adley Sand is that it has many of the sedimentary attri butes of a marine deposit , it is derived princip ally from the Lower Greensand , and it has obvious mineralogical affinities with the Lenh am Beds. Howev er , it will be recalled that analogous heavy mineral suites occur in a range of Pleistocene dep osits-for example in the Plateau Gravels of Surrey (Fisher, 1982)-and that Sherlock (1929) stressed that the parent strata of younger sediments can be exploited on several different occasions . Whether the present association of the apparentl y marine Headley Sand with the Red Crag fossils is merely fortuitous is hard to say, but it is equally difficult to reject the view tha t the fossils derive from an unrelated deposit of which only the ironstone remains. The original geogr aphical and altitudin al location of this former deposit in relation to the existing North Downs cuesta is impossible to determ ine, alth ough ther e is no obvious reason why it could not have been much farther to the south and at a greater elevation. The same can be said of the gravel deposits from which the Lower Greensand debris in Mountain Wood was ultim ately derived. The widely-accepted belief that the sculpt ing of the

245

Weald below the level of the surrounding Chalk rims comm enced at or befor e post-R ed Crag uplift, may well be in need of significant revision. Wooldridge and Linton's interpretation in particular relies on two assumptions: (i) the Red Crag fossils, which are shallow-water marine organism s, are in situ ; and (ii) the Lower Greensand material on Netley Heath was laid down directl y by rivers tr aversing an emergent Red Crag shore . In view of the new evidence, there is a possibility of the fossiliferou s ironstone having been der ived from greater elevations to the south , on a form er projec tion of the Chalk dip-slope. If this is so, it is also possible that the erosion of the Weald could be a much more recent event than is generally supposed. With this in mind, it is worth emphasising that chronologies of Wealden ero sion have been based in no small part on extrapolations from the encircling Chalklands. Less weight has been given to the superficial deposits of the Weald itself, or to those in nearb y area s which are derived wholly or in part from its denudation . In this cont ext , Worssam 's (1973) study of cert ain late Pleistocene Wealden drifts is pertinent. They are the legacy of intensive periglacial mass wasting , and are of appreciable extent and thickness. Accordingl y he queried why so much relief remains if the same kind of degradation had operated throughout earl ier parts of the Pleisto cene, on a land scape which had already been reduced to a level broadly coincident with the summits of the adjacent Chalk cuestas? Stated otherwise , it would seem difficult to accommodate the full potential of the Pleistocene in the models of landform development advan ced to date. Similar questions arise from Kern ey, Brown and Chandlers' (1964) dem onstration that immen se de struction of part s of the Nor th Downs escarpment was achieved solely in Zone III of the Late De vensian. As for drift s der ived from the Weald , refe rence has alre ady been made to the recent work of Fishe r (1982). His study of the widespread Plateau Gravels on the northwest flank of the Weald reve aled two major points of relevance. Firstly, the proportion of Lower Greensand detritus in the gravel ofte n decreases with altitude of the deposit , so that the highest remnants are virtually all flint, much of it still part ially ret aining a nodular form. Secondly, even the lowest Plateau Gra vels are preponderantly of fiint, and although it has been derived in part from the destruction of Palaeogene formations, the majorit y agains seems to have come first-hand from the Chalk . Respectively, these observations imply that the final unroofing of the western Weald was dela yed until the Pleistocene, and that the Chalk outcrop was thereafter progressively reduced to wher e we find it tod ay. Finally, it is stressed that it is not the purpose of this paper to suggest an alterna tive model of landform evolution for the western Weald . Rather it is to show

246

D. T . JOHN AND P. F. FISH ER

that one of the key assumptions from which current reconstructions proceed (i.e. the amount of post-Red Crag erosion and uplift) is invalid, or at least unproven . Moreover, the assumption is inconsistent with newer lines of evidence obtained from studies of drift deposits within and around the Weald . Further detailed examination of such deposits is essential before the problems raised in this paper can be resolved .

ACKNOWLEDGEMENTS We wish to acknowledge the cooperation of the Nature Conservancy Council in funding the excavations, and The School of Geo graphy, Kingston Polytechnic for providing facilities. Bob Stoke s, E . M. Finch and Bob Markham gave paleontological advice, David Bridgelands assisted with logistic support, Bill Edwards assisted with S.E .M. work , and Tim Aspden prepared the drawings.

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of Southern England. Inst. Br . Geo gr. Spec . Pub . No . 11, Academic Press, London, 101-30. JONES , D. K. C. 1980. The Tertiary evolution of south-east England with particular reference to the Weald . In (Jones, D. K. C.; Ed .) . The Shaping of Southern England .Inst. Br. Geogr . Spec. Pub. No. 11, Academic Pre ss, London, 13-47. KERNEY, M. P., E . G . BROWN & T. J. CHANDLER. 1964. The Late-glacial and Post-glacial history of the Chalk escarpment near Brook , Kent. Phil. Trans. Roy . Soc. 8248, 135-204. KRINSLEY, D. H . and J . DOORNKAMP . 1973. A tlas of Quartz Sand Surfa ce Textures. Cambridge Univer sity Press , Cambridge. LOVEDAY , J. A. 1962. Plateau depo sits of the southern Chiltern Hills. Proc. Geol . A ss. , 73, 83- 102. MONKTON, H. W. 1892. On the gravels south of the Thames from Guildford to Newbury. Q. Jl. Geol. Soc. London . 48, 29-47. - -. 1904. Excursion to Farnham Gr avel Pits, etc . Proc. Geol. Ass. , 18, 409-14. - -. 1923. Excursion to Clandon and Chilworth , Surre y, Proc. Geol. A ss.. 34, 67-9 . MONTFORD , H . M. 1966. Field Meeting to Farnham, Surre y. Proc. Geol. A ss., 77,381-3 . MORTON , A. C. 1982. The provenance and diagenesis of Palaeogene sandstones of southeast England as indicated by heavy mineral analysis . Proc. Geol. Ass. , 93, 263-74. PRESTWICH, J. 1858. On the age of some sands and iron-sandstones on the North Downs. Q. JI. Geol. Soc. London, 14,322-35. - -. 1890. On the relation of the Westlcton Shingle to other pre-glacial drifts in the Thames Basin, and on a Southern Drift, etc. Q. Jl. Geol. Soc. London, 46, 151-81. REID , C. 1890. The Pliocene deposits of Britain . Mem . Geol. Surv. G.B. SALTER, A . E . 1897-8. Pebbly and other gravels in southern England . Proc. Geol. Ass., 15, 264-76. - - 1905-6. On the superficial depo sits of central and parts of southern England. Proc. Geol. Ass., 19, 1-56. SHEPHARD-THORN, E . G . 1975. The Quaternary of the Weald-a review . Proc. Geol. Ass ., 86, 537-47. SHERLOCK. R. L. 1924. The superficial depos its of s. Bucks. and s. Herts . and the old course of the Thame s. Proc. Geol. A ss. , 35, 19-28. - -. 1929. Discussion on the alleged Pliocene of Bucks. and Hert s. Proc. Geol. Ass ., 40, 357- 72. SMALL, R. J. & G . C. FISHER. 1970. Th e o rigin of the secondary escarpment of the South Downs. Trans. lnst.

RED CRAG FOSSILS AT NETLEY HEATH: REVIEW & RE·APPRAISAL

Br. Geogr., 49, 97-107. STEBBING, W. P. D. 1900. Excursion to Netley Heath and Newlands Corner. Proc. Geol. Ass., 16, 524-6. WHITAKER, W. 1862. On the western end of the London Basin; on the westerly thinning of the Lower Eocene Beds in that Basin; and on the greywethers of Wiltshire. Q. Jl. Geol. Soc. London, 18, 258-74. - - . 1872. The geology of the London Basin, Part I. Mem. Geol. Surv. G.B. WOOD, G. V. 1956. The heavy mineral suites of the Lower

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