Late Pleistocene interglacial deposits at Pennington Marshes, Lymington, Hampshire, southern England

Late Pleistocene interglacial deposits at Pennington Marshes, Lymington, Hampshire, southern England Lorraine G. AlIen*, Philip L. Gibbard*, Mary E. Pettit*, Richard C. Preece t & J. Eric Robinson* ALLEN, L. G., GIBBARD, P. L., PETTIT, , M. E., PREECE, R. C. & ROBINSON, J. E. 1996. Late Pleistocene interglacial deposits at Pennington Marshes, Lymington, Hampshire, southern England. Proceedings oj the Geologists' Association, 107, 39-50. Fossiliferous organic sediments interstratified within fluvial gravels at Pennington Marshes, Lymington, have been recovered in boreholes and investigated. The organic deposit, here defined as the Pennington Organic Bed, occurs between -3.9 to -5.3 m 00 and has been traced 200 m across the immediate area. Pollen analyses indicate a temperate flora of interglacial character. Molluscan and ostracod assemblages contain no brackish elements and are typical of a shallow, freshwater stream or abandoned channel. A change from an aquatic to a terrestrial molluscan fauna indicates progressive drying out of the water body. The Pennington Organic Bed cannot be confidently attributed to any particular stage, but since it occurs within a lower terrace than that at Stone Point, 15 km to the NE, it is probably younger and an early Ipswichian age (Ip IIa?) is suggested. The Pennington Lower Gravel, below the organic deposit, is therefore probably Wolstonian and the Pennington Upper Gravel, above them, Devensian in age. The estuarine interglacial deposits at Stone Point, previously believed to have been Ipswichian, are likely to belong to an earlier stage. It is possible, although less likely, that they accumulated during a later part of the Ipswichian as the transgression aggraded to the level of the higher terrace surface. Similarly, if the gravels at Stone Point resulted from a tributary river, rather than the Solent River itself, this could also explain the altimetric differences and allow the organic deposits to be attributed to different parts of the same stage. However, there is no evidence to support either of these alternative possibilities.

* Subdepartment oj Quaternary Research. Botany School. University oj Cambridge, Downing Street. Cambridge CB2 3£A. (Current address: Godwin Institute oj Quaternary Research, Department oj Geography, University oJ Cambridge. Downing Place. Cambridge CB2 3£N.) t University Museum oJZoology. University oJ Cambridge, Downing Street. Cambridge CB2 3£1. t Department oj Geology. University College. University oj London, Gower Street. London WC1£ 6BT.

1. INTRODUCTION As part of the investigations for the construction of a water outfall in 1977, several boreholes were put down by the Southern Water Authority at Pennington Marshes, south of Lymington, between Lymington River and Avon Water on the Hampshire coast of southern England (Fig. I). These boreholes encountered a bed of fossiliferous organic sediments within gravel deposits (Fig. 2) that was originally reported by Nicholls (1985, 1987). The present paper reports the results of detailed biostratigraphical investigations of the same bed that was recovered from a new borehole put down in 1988. The organic deposits occur within a wide spread of gravels and sands that range in height from 125 m OD to below sea-level and underlie the coastal region of south Hampshire and south-east Dorset. The gravels and sands have been investigated for over 100 years and various explanations of their origin proposed. Since they are disposed in a series of terrace-like aggradations and show typical fluvial sedimentary structures, they have generally been interpreted as river sediment. The most recent evaluation of the sequence (Allen, 1991; Allen & Gibbard. 1993; Gibbard & Allen, 1994) confirms that the deposits Proceedings oJ the Geologists' Association, 107,39-50.

represent the remains of a large eastward-flowing Solent River system. 2, GEOLOGICAL SETTING AND SITE STRATIGRAPHY The organic sediments lie between -3.9 and -5.3 m OD within a sequence of sands and gravels. The organic sediments are underlain by 1.8 m of gravel and sand, which in tum rest on Headon Beds (Eocene), and are overlain by a further 3-5 m of gravel and sand. Together these gravels are assigned to the Pennington Gravel Member of the Solent River New Forest Formation and termed the Pennington Upper Gravel and Pennington Lower Gravel respectively, depending on whether they lie above or below the organic sediments, here defined as the Pennington Organic Bed. The gravels are lithologically indistinguishable and can only be divided into Lower and Upper Gravel subunits (Fig. 2) where the Pennington Organic Bed occurs (Allen, 1991; Allen & Gibbard, 1993; Gibbard & Allen, 1994). The organic deposits consist predominantly of silt and clay with some plant remains, and were recorded in three boreholes (A, B and C) over a distance of 200 m (Figs 1 & 2). They rest on an irregular gravel surface. The upper surface of the 0016--7878/96 $07·00 © 1996 Geologists' Association

40

L. G. ALLEN ET AL.

Lower Pennington

N

t 500 m

- o- 8-

E Keyhaven Marshes

C

X·.

1

A'

Solent

-

6

•

8

92

• Bembridge

Isle of Wight Fig. 1. Location map, showing the alignment of the borehole transect at Pennington Marshes, together with other sites mentioned in the text.

Pennington Organic Bed is also irregular and appears to have been truncated by the overlying Pennington Upper Gravel, causing it to be variable in thickness. Its maximum thickness occurs in borehole A (Fig. 2), where it reaches 1.3 m. It thins to the northwest, where it is only 40 cm thick (borehole C), and also towards the southeast, and is absent from boreholes I or G. Landward of the sea-wall the Pennington Gravel is discontinuously overlain by up to 1.2 m of inorganic silt and clay with occasional pebbles. The sequence is commonly capped by modern fill (made

ground). Beyond the sea-wall the gravel is overlain by modern intertidal deposits. The 1988 borehole (Borehole X [SZ 3240 9235]; ground surface -0.4 mOD), was located approximately 15 m northwest of Southern Water Authority borehole A (Figs I & 2), where the Pennington Organic Bed is at its thickest. This new borehole confirmed the sequence recorded in earlier ones. The detailed stratigraphy of the organic sediments is described below (Munsell colours for moist sediment are given in parentheses):

41

INTERGLACIAL DEPOSITS IN HAMPSHIRE, UK

Borehole X

F E o mOD ,---'-_ _--.JL--_ _-'-_ _.....L_ _..J-._.ll...-'-"'----_ _-'-....ll

8

7 L-

....L_ _---'

....L_--,

mOD 4

4

3

3

2

2 Post-glacIal intertidal sediment

o

0

-1

-1

-2

-2

-3

-3

-4

-4

-5

-5

-6

-6

-7

-8

-7

100 '--------'

[8] .'.

.-.

~ [ZJ

[;J

-8

Headon Beds

m

Sand and gravel

[1I] .- -

Clay with Slones

Clayey gravel

iii

Silty sediment/clay and silt with occasIonal stones and sand Silty deposIt wIth organic content

Pebbly sarod Clayey sand

Q]

Fill

Fig. 2. Geological cross-section across Pennington Marshes, based on borehole data supplied by Southern Water Authority. The location of boreholes is given in Fig. 1.

Gravel and sand [Pennington Upper Gravel; 3.7 m] Silty clay, not recovered. Olive grey (5Y 5/2) silty clay with some sand, containing occasional plant fragments and Mollusca. 71-78.5 cm Very dark grey (5Y 3/ I) sand with some silt and clay, with divided plant and wood fragments. 78.5-90cm Very dark grey (5Y 3/1) clay and silt with some sand, interbedded with thin black (5Y 2.5/1) bands of plant remains. Contains fragments of Salix up to 4 cm in diameter. A band of olive grey silty clay occurs at 87.5-88 cm. Gravel and sand [Pennington Lower Gravel; 1.8 m]. +90cm 0-13 cm 13-71 cm

Both the form and sedimentary sequence suggest that the organic deposits represent either a depression or channel-fill on the floodplain developed on the underlying gravels and sands. The abundance of sand and plant fragments towards the base indicate flowing water. The abrupt transition from an essentially sand-dominated deposit to a silty clay at 71 cm, and the accompanying decrease in plant fragments, suggests quieter conditions with substantially reduced flow.

3. PALAEONTOLOGY The organic sequence recovered by the 1988 borehole was subdivided for palaeontological analyses. Pollen, plant macrofossils, shells and ostracods were all present and are discussed below.

(a) Pollen

Five samples, taken at depths of 16, 26, 41, 69 and 84 cm through the Pennington Organic Bed, were analysed for pollen and spores. Samples were prepared using standard chemical methods (West, 1977; Bates, Coxon & Gibbard, 1978); pollen type conventions follow Andrew (1970). In all the samples, the palynomorphs were poorly preserved and there was a high proportion of degraded, broken, corroded and unidentifiable grains. Consequently, the pollen analyses were very time consuming and only a relatively small number of grains were counted from each

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Fig. 3. Pollen diagram from the organic sediments in borehole X at Pennington Marshes.

Clav ond ,ilt wlch plan, 'ragrMntl

130

Trees

Shrubs

I Herbs

50

~o

10

40

Percent total pollen .. spores-aQuatics

176

+ Sino'e grain

Percent total pollen spores - aquatics

+

I

0

10

20

I

Percent totlll poUen ... spores + pre-Pleistocene types

Percent totel pollen + spores· equalics

INTERGLACIAL DEPOSITS IN HAMPSHIRE, UK

sample. In addition, there was a significant number of prePleistocene grains, probably reworked from nearby Tertiary outcrops. The lowest part of the profile is dominated by high frequencies of Betula, Salix and Grarnineae, together with significant amounts of Pinus and Cyperaceae pollen (fig. 3). Betula shows an upward decrease and Pinus appears to replace it as the dominant tree pollen in higher spectra. Alnus is consistently present in small quantities throughout most of the profile. Quercus first appears at 69 em but is more common in the upper levels where !lex and Corylus first appear. Salix persists but has declined markedly from its high frequency towards the base. Herb taxa are well represented throughout but are most abundant in the middle part of the profile. They are dominated, especially towards the base, by Gramineae and Cyperaceae, but a variety of other types occur. Aquatic plants are represented by Sparganium type and Potamogeton pollen. Filicales spores also occur abundantly throughout and show a marked increase between 84 and 69 em, coinciding with the sedimentological transition to silty clay. Both Filicales spores and Pinus pollen are resistant to decay and their high frequencies, particularly in the upper levels, are likely to result from preferential preservation. The overall pollen assemblage suggests a mosaic of wet, damp and drier habitats such as might be expected on a floodplain of a river. Herb taxa of both open and marshy conditions are present, while the aquatic pollen demonstrates a local freshwater environment. The occurrence of Alnus also suggests wetland, typical of a valley bottom. The apparent transition to pine dominance and corresponding decrease of both Salix and Gramineae most probably reflect differential destruction of less resistant palynomorphs resulting from drying out of the floodplain surface (see below). The presence of small amounts of pollen of the maritime indicators Plantago maritima and Glaux hints at the proximity to the coast. The presence of pollen of frost-susceptible taxa, such as !lex, indicates a temperate climate with mild winters (cf. Iversen, 1944). Other thermophilous taxa at the top of the sequence include Quercus, Corylus and Alnus. Their occurrence, albeit at low frequency, with Betula and Pinus, which dominate the boreal forest of the pre-temperate substage of interglacial periods (Turner & West, 1968), suggest that the deposits represent the early part of an interglacial rather than the post-temperate substage. The pollen spectra provide inconclusive evidence of the age of the organic bed, but the relative abundance of Pinus and Quercus pollen suggests that it might represent the beginning of the Ipswichian Stage. Indeed, the pollen assemblages resemble Ip IIa, as defined at the stratotype of this stage at Bobbitshole, Ipswich (West, 1957, 1980). However, this correlation must be tentative because of the brevity of the record represented and the poor pollen preservation. (b) Plant macrofossils The core was divided into 5 samples, the boundaries between each corresponding to the lithological changes.

43

Plant macrofossil remains were extracted after disaggregation of the sediments in water (Table I). The nomenclature used follows Flora Europaea (Tutin et al., 1964-80). Only two of the samples contained significant numbers of well preserved plant fossils. In the remaining samples, preservation was poor, and only a few unidentifiable plant fragments were recovered from the uppermost sample (13-30 em: weight 200 g). Samples 30-47.5 em and 47.5-69 em contained only the occasional oospore of Characeae and a possibly reworked megaspore. The two lowest samples yielded a range of plant remains. Both assemblages were dominated by aquatic plants, particularly Scirpus lacustris and/or Scirpus sp. and aquatic algae (Characeae). Mentha aquatica, a plant of fen and reedswamp communities, is also abundant. The remaining taxa are also typical of those surrounding a channel or pool on a river floodplain. Of the trees, only Betula is represented, supporting the pollen evidence (section a) for its local presence. The absence of plant macrofossils from the overlying sediments reflects a marked change in the depositional environment towards drier conditions. The Characeae oospores might be reworked, as they are particularly common in the Headon Beds, or alternatively they might indicate the continued presence of pools on the floodplain.

(c) Mollusca Non-marine Mollusca were extracted and identified from 11 samples (Table 2; Fig. 4). The gap between 55 and 50 em corresponds to the level where the sediment was retained in the borehole shoe, and was not analysed because of the risk of contamination. Juvenile shells of Vallonia pulchella are impossible to distinguish from those of V. excentrica and therefore are listed in Table 2 as Vallonia spp. However, as all of the adult specimens were V. pulchella, the juveniles have been grouped with this species on the mollusc diagram (Fig. 4). The mollusc assemblage in the lowest part of the profile, between 76 and 68 em, is almost exclusively composed of Bithynia tentaculata opercula, which suggests accumulation in a stream. Belgrandia marginata occurs throughout the sequence but peaks between 61.5 and 43.5 em. As the frequency of Bithynia tentaculata falls, Lymnaea truncatula and Carychium minimum increase in importance and together dominate the fauna between 43.5 and 28.5 em. In the upper part of the core (28.5-13.5 em), Vallonia pulchella becomes the most common species, although L. truncatula and C. minimum remain in significant but declining numbers. The overall molluscan assemblage suggests deposition on the floodplain of a river. There is a gradual transition from a freshwater-dominated fauna at the base to a terrestrialdominated fauna at the top (Fig. 4). This is reflected most clearly in the ratio of freshwater to land shells. The aquatic species indicate slow-flowing or still water but not stagnant conditions. The terrestrial fauna is ecologically restricted and is composed entirely of species indicative of a marsh habitat. At no point in the sequence are any species

44

L. G. ALLEN ET AL.

Table 1. Plant macrofossils from Pennington Marshes Sample depth (em) 30-47.5

47.5--69

69-78

78-84

Trees and shrubs Betula sp. Salix sp. cf. Salix sp.

fr

5

c

4.5

b

7

Herbs (open ground) fr

Agrimonia eupatoria

Herbs (damp ground) Eriophorum sp.

n

Muddy substrates and shallow water Alisma sp. Alisma sp.

Alismataceae Sparganium sp.

a

fr e frst

Fen and reedswamp helophytes n a n a n

(d) Ostracoda

Lycopuseuropeaus Mentha cf. aquatica Mentha cf. aquatica Urtica dioica Urtica dioica

Aquatics Characeae Hippuris vulgaris Potamogeton sp. Scirpus lacustris Scirpus cf. lacustris Scirpus sp. Menyanthes trifoliata Nuphar lutea

0

+

I 2

2

59 2 2

+

fr fr n n n s

++ I

++

2 61

30

1 2 4

4

10

1 I

Unclassified Gramineae cp Cyperaceae n Carex sp. n Potentilla sp. a Ranunculus sp. a Subgenus Ranunculus Viola sp. s Dryopteris type sp ]uncus sp. s

I 2

10

I I 2

3

13 2

Miscellaneous megaspore (?reworked) wood inidentified b sample weight

+

+ 3

200

suggestive of brackish conditions. This demonstrates that the site was located upstream of any tidal influence. The overall succession therefore appears to reflect changing local conditions and the gradual drying out of a shallow pool or abandoned channel. The majority of the species present are tolerant of a wide range of climatic conditions but many are known only from temperate episodes. The presence of Belgrandia marginata is particularly noteworthy because in Britain it is only known from interglacial periods. It is now extinct in Britain but still inhabits small areas of southern France and Spain (Germain, 1930). In Britain, B. marginata last occurred during the Ipswichian Stage and is abundant at the type site at Bobbitshole, where it occurred throughout most of Ip II (Sparks, 1957). It is also known from earlier interglacials (Kerney, 1977). The decline of B. marginata above 43.5 em probably reflects the changing local environment and its intolerance of the increasingly muddy and silty conditions brought about by the drying out of the pool, rather than a deterioration of the regional climate.

150

100

50

The uppennost sample (13-30 em: weight 200 g) contained no identifiable material. fr: fruit; c: capsule; b: bud; n: nut or nutlet; a: achene; e: embryo; frst: fruitstone; 0: oospore; cp: capsule; s: seed; sp: sporangium.

Ostracods were obtained from the same samples as those analysed for molluscs. In addition, the 250-500 Ilm fraction of samples at 68-71.5 em, 43.5-50 em and 28.5-33.5 em were also picked for smaller specimens (Table 3). The preservation of the ostracods is unusual as the valves have an iron-varnish coating. This facilitated identification of certain features and may have increased the preservation potential, especially of the larger (e.g. Herpetocypris) or fragile shells. The presence of the Eocene species Hemicyprideis montosa demonstrates derivation from nearby Tertiary outcrops. The faunal assemblage is limited and dominated by Candona neglecta and Herpetocypris brevicaudata. The latter species is regarded as a cool spring species where it occurs in soft substrates. Eucypris pigra is another species typical of cool springs. Limnocythere inopinata prefers lacustrine conditions (Absolon, 1973) and provides evidence for still or slowly flowing water. Candona neglecta is also common today on soft-bottom sediments at the margins of lakes or ponds and in springs and running water. Ilyocypris biplicata, which occurs towards the top of the profile, can withstand bodies of water which periodically dry up. Therefore, the overall ostracod assemblage reflects a still or slowly flowing water habitat. No brackish-water species were recovered. 4. PALAEOENVIRONMENT The sedimentological and palaeontological evidence are in close agreement concerning the depositional environment and demonstrate that the organic bed at Pennington accumulated in an abandoned channel or depression on the floodplain of a river. The fossil assemblages are consistent in indicating slow-flowing or still water conditions under a fully temperate climate. Surrounding the channel marshy

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Table 2. Mollusc numbers from the organic sediments in borehole X at Pennington Marshes Sample depth (em) Dry mass (g)

71.5-76 405

Valvata piscinalis MUlier Belgrandia marginata (Michaud) *Bithynia tentaculata (Linne) shells *Bithynia tentaculata (opercula) Carychium minimum Mtiller Lyminaea truncatula (MUlier) Lymnaea peregra (MUlier) PlanorbislGyraulus Anisus leucostoma (Millet) Armiger crista (Linne) Hippeutis complanatus (Linne) OxylomalSuccinea Cochlicopa lubrica (MUller) Vertigo sp. Vallonia pulchella (MUlier) Vallonia spp. Punctum pygmaeum (Drapamaud) Vitrina pellucida (MUlier) Zonitoides nitidus (MUlier) DeroceraslLimax Euconulus fulvus (MUlier) CepaealArianta tPisidium casertanum (PolO tPisidium obtusale (Lamarek) t Pisidium spp. Minimum number of shells

1 1 2 73

1

68-71.5 241 1

1 44

-

61.5--68 414 7 4 1 12 9 4

-

-

-

6 33 1 4 31 44 1 2 4

3 1 3

-

-

-

-

-

77

3 2

1 1 38 25

1 3 1 1 1 3 1 1 3

-

-

-

-

1 3

4 4

-

-

-

-

-

1 7 11

1

76 52 1

2 1 1 85 57

23.5-28.5 266

18.5-23.5 261

13.5-18.5 128

13 35

6 17

II

9

I

10

-

10 -

-

-

4 10

-

3 10

5 9 1 12 14 1

2

1

3

1

1

47

54

146

83

101

202

8 12 5 1 12 9

r

1 5

-

-

14

• Only the highest figure, in this case that of the opercula, is used for frequency calculations. t The numbers in the table refer to individual valves and are halved for frequency calculations.

28.5-33.5 275

-

-

-

-

-

-

-

4 3

33.5-38.3 247

-

-

-

38.5--43.5 253

1 3

1 4

1

11 18 1 8 13 10 1 1 1 3

1 6

1 4

-

-

43.5-50 317

-

-

55--61.5 449

8 19 3 2 6 4

I

5 1 15 41

Cl

3 1 -

2 2

9 35 2 1 1 1

91

73

12 2

223

97

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INTERGLACIAL DEPOSITS IN HAMPSHIRE, UK

47

Table 3. Ostracod numbers from the organic sediments in borehole X at Pennington Marshes Sample depth (cm)

61.5-68 Pleistocene taxa Candona neglecta Sars

Eucypris pigra (Fischer) Herpetocypris brevicaudata Kaufmann Herpetocypris sp. I1yocypris biplicata (Koch) Lilllnocythere inopinata (Baird)

IvA-III

Ic IcA-1II

2v

Ic

55-61.5

lc 2c'i'

9v

le'i' led

Ie

Iv

4e

4v

2e

f

Ie

Tertiary taxa Helllicyprideis lIlontosa (Jones & Sherborn) v: valve; c: carapace; f: fragments: A-I ... A-IV: moult stages.

conditions occurred and birch-dominated deciduous forest colonized the drier areas. The overall succession reflects the gradual drying out of the environment, possibly resulting from alluviation. No evidence of brackish conditions were detected, although the proximity of the coast is suggested by the occurrence of maritime plants (Plantago maritima and Glaux).

5. COMPARISON WITH NEIGHBOURING INTERGLACIAL DEPOSITS The dating of the gravel units of the Solent River system is difficult, largely because of the rarity of organic deposits of Pleistocene age. The few that are known have generally been attributed to the Ipswichian Stage. The evidence from each of these sites is now briefly reviewed. Interglacial deposits occur 15 km northeast of Pennington Marshes at Stone Point (West & Sparks, 1960; Brown, Gilbertson, Green & Keen, 1975). These occur in a similar context to the Pennington Organic Bed, that is within gravel, but they occur within the higher Lepe Gravel Member of the New Forest Formation, attributed to the Solen! River. They comprise interbedded peats and clays that occur between +1 to -2.7 mOD (Reid, 1893; West & Sparks, 1960; Brown et al., 1975; Green & Keen, 1987). The pollen and molluscan assemblages indicate accumulation in an estuarine situation, when the local forest was dominated by Quercus. At Selsey, Sussex (Reid, 1892; West & Sparks, 1960), 54 km to the east of Pennington, a more complete sequence is represented. Here, estuarine clays rest on fluvial organic deposits filling a channel excavated into the Bracklesham Beds. The sediments are rich in fossils and have yielded

pollen, plant macrofossils, shells and vertebrates. The pollen assemblages from Pennington are similar to the end of Ip Ib and beginning of Ip IIa at Selsey, but this is not necessarily indicative of time-equivalence. The deposits cannot be lithostratigraphically related to the Solent River sequence. At Selsey, Stone and probably also West Wittering, 10 km north-west of Selsey (Reid, 1892; Preece, Scourse, Houghton, Knudsen & Penney, 1990), a marine transgression occurs during the early temperate substage of an interglacial, usually interpreted as the Ipswichian (Ip IIb). Barber & Brown (1987) reported an organic deposit, underlying the gravels of Terrace 3 (Kubula, 1980) at Ibsley in the Avon Valley. An unusual fully temperate, herbdominated pollen assemblage, unlike any other from Britain, was recorded. Although they considered the possibility of a Middle Devensian Upton Warren Interstadial age, an Ipswichian age was favoured because of the presence of !lex and Hedera pollen. They therefore tentatively correlated the sediments with Ip IIb and ascribed the unusual character of the assemblage to extensive deforestation by large herbivores (cf. Phillips, 1974; Gibbard & Stuart, 1974). In Newtown estuary on the Isle of Wight, a fossiliferous deposit occurs at low water mark (Jackson, 1939; Munt & Burke, 1986). Although only accessible at extremely low tides, this site has yielded many vertebrate remains. It appears to be a composite deposit with a warm stage fauna including straight-tusked elephant (Palaeoloxodon antiquus) and Hippopotamus, a cold stage fauna including reindeer (Rangifer tarandus), wooliy rhinoceros (Coelodonta antiquitatus) and mammoth (Mammuthus primigenius), and a more recent redeposited Mesolithic

48

L. G. ALLEN ET AL.

assemblage including aurochs (Bos primigenius). The wann stage fauna includes elements typical of the Ipswichian Stage (Munt & Burke, 1986). The cold fauna probably relates to the Devensian. Attempts to obtain fossil pollen and spores from sediments adhering to these bones have so far failed. Organic silty clays associated with the raised beach gravels at Bembridge, Isle of Wight, have been described by Preece et al. (1990). These appear to have accumulated in a saltmarsh and, although yielding pollen assemblages dominated by herbs, were thought to span the transition from Ip lIb to Ip III with a Quercus-Corylus pollen assemblage zone giving rise to one with significant frequencies of Carpinus. The sediment cannot therefore be equivalent to the sequence at Pennington. The correlations described above are based predominantly on biostratigraphical evidence, but recently the attribution of some of the organic deposits to the Ipswichian (Last) Interglacial has been questioned (Miller et al., 1979; Bowen et al., 1989). The Selsey organic deposits were originally said to have yielded the bivalve Corbicula fluminalis (West & Sparks, 1960) and Hippopotamus sp. (Sutcliffe, 1960), but the fragmentary bone of the latter has subsequently been re-identified (A. I. Sutcliffe, pers. comm.). Some authors (e.g. Keen, 1990) believe that Corbicula was not present in Britain during the Ipswichian, and that its occurrence in Late and late Middle Pleistocene contexts characterizes an additional post-Hoxnian event, which has been equated with Oxygen Isotope Stage 7. Some support for this has come from aminostratigraphy. Deposits yielding Corbicula have given consistently higher amino acid ratios than those with Hippopotamus which are taken to represent 'true' Last Interglacial localities (Bowen et al., 1989; Bridgland, 1994). In the case of Selsey, ratios of 0.165 ± 0.019 and 0.154 ± 0.02 were obtained from Corbicula fluminalis and Valvata piscinalis respectively, although these shells did not come from the same channel as that investigated by West & Sparks (Stinton, 1985, p. 201). A similar ratio from Corbicula fluminalis (0.164 ± 0.014) was obtained from West Wittering (Bowen et al., 1989). Both groups of ratios were higher than equivalent ratios (0.09 ± 0.01) for the Ipswichian stratotype at Bobbitshole (Bowen et al., 1989). No ratios are available from Stone, the Bembridge Raised Beach or Pennington and the possibility that some of these sites might also belong to different stages cannot at present be tested. 6. LOCAL STRATIGRAPHICAL SIGNIFICANCE The organic deposits at Pennington Marshes occur within the Pennington Gravel. This unit has a very limited distribution on land, bordering the coast from Milford-on-Sea to Lymington (Allen, 1991; Allen & Gibbard, 1993). The type section is located in the quarry at Lower Pennington (SZ 309 927; 4 mOD), where up to 3.1 m of horizontally bedded gravel and sand is exposed (Fig. 1). The gravel shows both matrix-supported and clast-supported beds and contains interstratified sand lenses. Finer sediments are rare

but occasional thin lenses of clay, silt and fine sand are present. Everard (1954) formerly included the Pennington Gravel within his 15 ft (4.5 m) stage, a morphological unit recognized on the basis of altitude. It was later assigned by Keen (1975,1980) to his 'low terrace division', whereas the British Geological Survey included it as part of their 'Plateau Gravel level 2' (Mathers, 1982). Clast lithological analysis shows that the major constituents of the Pennington Gravel are angular, nodular and weathered flint (82.5-91.5%), rounded and broken rounded flint (3.0--10.5%), chert (1--4%) and quartz (0.5-3.5%), together with small quantities of other lithologies including quartzite, schorl and Greensand. The limited distribution on land limits determination of its precise gradient, but it appears to be similar to that of higher units (Allen, 1991; Allen & Gibbard, 1993). On the basis of sedimentology, the fluvial gravel aggradations in the Solent system are thought to have been deposited under cold climates (Allen, 1991; Allen & Gibbard, 1993). The sequence at Pennington also implies initial deposition under a cold climate (Pennington Lower Gravel), then organic sedimentation during an interglacial, followed by a reversion to fluvial aggradation during a cold episode (Pennington Upper Gravel). The organic deposits at Stone Point (Lepe Country Park), allow division of Lepe Gravel member into two subunits; the Lepe Lower Gravel and Lepe Upper Gravel (Allen, 1991; Allen & Gibbard, 1993). The organic deposits here occupy depressions in the dissected surface of the lower gravel unit. Reid (1893) described them as being overlain by subangular flint gravel which he correlated with that exposed in the present cliff section. Brown et al. (1975) found that the gravels above and below the organic deposit were lithologically and sedimentologically indistinguishable, the same situation as at Pennington. The Stone Point organic deposits occur on the surface of a higher and earlier gravel member and are therefore likely to be older than those at Pennington. However, they may have been emplaced later in the same interglacial by a transgression that aggraded to the surface of the higher terrace. Alternatively, they might represent the infill a tributary channel developed following abandonment of the Lepe Lower Gravel surface during downcutting to the lower, Pennington Gravel level. Reynolds (1987) identified two superimposed layers of brickearth above the Lepe Upper Gravel at Stone Point, the lower of which contained palaeoargillic features that were suggestive of interglacial pedogenesis. He interpreted this as demonstrating that the Lepe Upper Gravel was of preDevensian age. The presence of palaeoargillic features in the brickearth could be explained by the pedogenesis having taken place during Early or Middle Devensian interstadials, as suggested by Preece et al. (1990). This suggestion appears to have been confirmed by recent thermoluminescence dating (Parks & Rendell, 1992). Following the deposition of the Lepe Upper Gravel, the older brickearth may have been moved onto the gravel surface from higher ground by slope processes such as solifluction.

INTERGLACIAL DEPOSITS IN HAMPSHIRE. UK

7. CONCLUSIONS

The discovery of fossiliferous interglacial sediments at Pennington is important as it contributes new evidence that has a bearing on the interpretation of other interglacial deposits in the region. The Pennington Organic Bed is fully temperate and non-marine in character throughout. The palaeontological evidence does not allow attribution to any particular stage. This results from the fact that only a very small part of the interglacial is represented and the organic sediments have been partly oxidized, particularly in their upper levels, so biasing pollen preservation. Some idea about the age of the Pennington Organic Bed can be gained from its stratigraphical context in relation to neighbouring sites. It occurs between -3.9 and -5.3 m OD and rests on the Pennington Lower Gravel. It therefore occurs within a lower gravel succession than the estuarine interglacial deposits at Stone Point, which occur between about +1 m to -2.7 mOD (West & Sparks, 1960; Brown et ai., 1975). This would suggest that the Stone organic deposits are older than those at Pennington, and therefore casts doubt on their attribution to the Last (Ipswichian) interglacial. Two explanations could still accommodate the interglacial sediments at these two sites within a single stage. The first requires that the Pennington freshwater sediments accumulated during the early part of the interglacial, before the transgression responsible for the

49

deposition of the Stone sediments. In this case the transgression may have risen above the level of the higher terrace and deposited material on its surface. Alternatively, the terrace deposits at Stone may have been laid down by a tributary, rather than the main Solent River. This would explain their occurrence at higher elevations. However, there is no independent evidence that this was the case. Indeed the few palaeocurrent measurements that have been obtained from the gravels at Stone Point suggest a flow a little north of east (Brown et al., 1975). The simplest interpretation, and the one we would favour, is that the Pennington Organic bed accumulated during the early part of the Ipswichian Stage, whereas the estuarine sediments at Stone Point may belong to an earlier interglacial. If this interpretation is correct, it implies that the Pennington Lower Gravel may be Wolstonian and the Pennington Upper Gravel Devensian. The age of the gravels at Stone Point may therefore require reappraisal.

ACKNOWLEDGEMENTS L. G. Allen gratefully acknowledges receipt of a NERC Studentship and thanks the Southern Water Authority for providing the borehole records. S. Boreham modified the original illustrations.

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Received 19 December 1994; revised typescript accepted 19 April 1995