Middle Pleistocene Hoxnian Stage interglacial deposits at Hitchin, Hertfordshire, England

Middle Pleistocene Hoxnian Stage interglacial deposits at Hitchin, Hertfordshire, England S. Boreham and P. L. Gibbard BOREHAM, S. & GIBBARD, P. L. 1995. Middle Pleistocene Hoxnian Stage interglacial deposits at Hitchin, Hertfordshire, England. Proceedings of the Geologists' Association, 106, 259-270. Interglacial lake deposits resting on till and glaciofluvial gravels, and overlain by 'brickearth' I krn south of Hitchin town centres are correlated with the Hoxnian Stage. These deposits are shown to be equivalent to those oreviously described by Clement Reid at the tum of the century. The sediments fill a basin-like depression or depressions that may have originated as a kettle-hole. The inorganic character of the sediments suggests that the basin may have been fed by a stream. The basal part of the deposit formed under late-glacial conditions indicated by Hippophae and Betula scrub. The upper part of the deposit represents the development of thermophilous woodland. Vertebrate remains and Palaeolithic artefacts associated with the sediments are discussed. Later infill of the basin by 'brickearth' probably took place under a periglacial climate. In common with many other Hoxnian sites in Hertfordshire, the sequence records only part of the interglacial. Such incomplete sequences have been attributed to climatic variation in water supply through the period. Subdepartment of Quaternary Research, Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2' 3EA

1. INTRODUCTION

2. LOCAL GEOLOGY

Interglacial deposits have been known to occur beneath the southern side of Hitchin town in northern Hertfordshire, since at least the mid-nineteenth century (Fig. I). They were originally described by a local geologist Hill (1891, 1896), from sections exposed in two brick pits. Subsequently, Reid (1897, 1901) also studied these sections and put down a number of boreholes in the area to establish the relationship of the interglacial deposits to the underlying till and glaciofluvial gravels (Fig. 1). The interglacial deposits consisted of fossiliferous silty clay overlain by a discontinuous shelly Chara marl. The silty clays were interpreted as lake deposits (Reid, 1901), while the marl was thought to represent a shallow pool. On the basis of the topographic situation, sedimentary sequence, plant remains and associated Palaeolithic artefacts and faunal material, Reid (1897) correlated the sequence with that at Hoxne, Suffolk. The results of this and associated studies were summarized by Bloom (1934). Sections at the original excavations of Jeeve's and Ransome's pits continued to be available until the 1960s, during which time R. G. West took a sample for analysis (West, 1955). By 1970 however, the pits were closed and built over. The location of Jeeve's and Ransome's pits is shown in Fig. 1. It should be stressed that these lacustrine sequences are distinct from interglacial tufa deposits originally reported by Kennard (1943) from Oughtonhead Lane, west of Hitchin and re-investigated by Kerney (1959).

Following early descriptions of the Hitchin sections (Hill 1891, 1896), Reid (1897, 1901) demonstrated that the lacustrine and associated sediments consist of a lower black or brown well-bedded, often laminated, silty clay with plant fragments, freshwater molluscs and some fish tooth fragments. This was locally overlain by a discontinuous upper white or brown well-bedded calcareous shelly Chara marl or loam. The latter apparently graded up from the underlying lake deposits in Jeeve's pit (Hill, 1896), whereas in one section in Ransome's pit it rested on a gravelly 'brickearth' (Reid, 1897; Chapman, 1903). The marl has yielded plant macrofossils, Mollusca, Ostracoda, fish bones and mammalian remains (Reid, 1897). Reid (1901) concluded that the lower silty clays were lake deposits, while the upper Chara marl accumulated in shallow pools; an environment which this sediment typifies (cf. Boreham & West, 1993). Both he and Hill (1891) noticed that the lake clays were locally decalcified beneath the marl, a feature that may suggest local drying out and possibly even weathering of the exposed surface of the sediment before emplacement of the marl. The deposits have been termed the Hitchin Lake Beds by Reid (1897,1901) and subsequent workers. In 1971 P. L. Gibbard put down two boreholes (Q1 and Q2) in an old brickpit on the south side of Charlton Lane, Hitchin, close to the position of Reid's CR4 and CR8 boreholes (Fig. I). These boreholes revealed organic sediment, overlying a blue-grey chalky sandy till, capped by orangebrown silty clays (Gibbard, 1974). The site is approximately 500 m north of Gosmore village [TL 1885 2774]. The original land surface was at c. 79.0 mOD, although

Current address: Godwin Institute of Quaternary Research, Dept of Geography, University of Cambridge, Downing Place, Cambridge CB23EN. Proceedings of the Geologists' Association, 106,259-270.

0016-7878/95 $07·00 © 1995 Geologists' Association

260

S. B O R E H A M

& P.

L. GIBBARD

675-725 em Below 725 em

Geolo gy

w'~"M ... Till

",",.. Glaciofluvial sands and gravels

:.:.'<

Midd le Cha lk

"BG SII

e...... G ~ Sutveyauge< hole

"al '

Oomonl"eldt>onll\OlO

"pe

PI10ry bypasa bo_

" QI

sooa borehole

a Al

ObwMtion by D. T. Aldlss

a Wl 11-

~tnoIl'IS

-11

Uno 01 S8dIon

Fig. 1. Map showing positions of the sites discussed in the text.

some 5 m thickness of 'brickearth' had been removed from the site (Fig. I). Borehole Ql penetrated the entire thickness of the deposit, from which a continuous series of samples was obtained (Fig. 2). Borehole Q2 was located 4.5 m southwest of Q 1, but provided a much shorter sediment sequence than the latter. The sequence of sediments in boreholes Ql and Q2 are described below using the notation of Troels-Smith (1955). Charlton La ne (Ql) Above 150 em Made ground . 150--200cm Red-brown clay with pebbles. 200--245 em Light brown silty clay. 245- 325 em Red mauled cla y. 325-370 em Dark brown silty clay. 370--400 em Brown silty clay with shell and plant fragments. Grey silty clay marl with shell 400--470 em fragments. Grey-brown organic silty clay. 470--500 em Grey silty clay with Rutilus 500--575 em tooth. Dark grey silty clay with shell 575-675 em and plant fragment s.

As3 Ggi Ag2 As2 Agl As2 AgI Ldl As2 Ag l Ldl As2 AgI LeI As2 Agl Ldl As2 Agl Ldl As2 AgI Ldl As2

Dark grey clayey silt. Blue-grey chalk y sandy diamicton (till).

As2 Ag2

Charlton Lane (Q2) Made ground. Above 240 em As2 Ag2 Banded orange-yellow silty 240--275 em clay. Some plant fragments. As2 Ag2 275-313 em Dark brown/grey silty clay with abundant shells. As2 Agi Ldl Orange-yellow bedded silty 313-350 em clay with plant fragments. Dark grey silty clay with plant As2 Ag2 350--363 em fragments and shells. Banded grey-green and brown As2 Agl Ldl 363-407 em silty clay with shell fragments. Asl Ag2 Ldl Grey silty clay 407-452 em Dark grey silty clay with plant Agl As2 Ldl 452-459 em fragments. Dark grey silty clay with layers Agl As2 Ldl 459-505 em of plant and shell fragments. Dark grey silty clay with many As2 Ag2 505-573 em fragments of chalk and flint. Blue grey chalky sandy Below 573 em diamicton (till).

The sediments were found to be very calcareous throughout, 2-3 on the Troels-Smith ( 1955) scale, except in decalcified horizons. In detail the sediments are very complex, comprising many small units which vary in thickness, colour, texture and composition. At 547 em in Q2, small-scale contorted bedding occurred, above which the beds dipped steeply for 20 em, returning to a more horizontal orientation above. Similar strongly dipping beds were seen between 363--407 em and 315-363 em in Q2. Between 240-350 em in the same core, discordant orange-yellow mottled grey coloration of the silty clay was seen. It appeared that this coloration resulted from irregular weathering and decalcification of the sediment, possibly before deposition of the overlying ' brickearth' since no disturbance of the bedding occurred across the junction. Other coloration of the sediments was noted in the cores from borehole Q2 (see above). This comprised a complex alternation of brown, grey, grey-green and cream units. These colour changes may be partially cyclic. Chemical analysis of the clay fractions (Gibbard, 1974) demonstrated that the colouring resulted from variation in the concentration of ferrous iron oxide; the grey-green sediment being richest in ferrous oxide (25%). The light grey sediment being next richest (21%), then the cream marly clay (17%), then the dark green sediment (16%) and finally the brown clay being poorest (12%). Pollen analyses through the different coloured bands showed no obvious correlation between pollen preservation and ferrous iron concentration (Gibbard, 1974). The colour variations can also be seen in the photograph of the section in Bloom (1934; fig. 3). The fine-grained stratified sedimentation indicates accumulation in a lacustrine environment. However, the high concentration of inorganic material suggests that the Charlton Lane sequence possibly originated near a stream inflow. The bedding in the sediment and particularly its

MIDDLE PLEISTOCENE HOXNIAN STAGE INTERGLACIAL DEPOSITS

coloration may be associated with fluctuating conditions of runoff, possibly resulting from vegetational or environmental variation. The steeply inclined and partially contorted bedding seen in borehole Q2 strongly suggests that small-scale slumping occurred on occasions. During the remapping of the area in 1992, the British Geological Survey (BGS) put down a series of boreholes. One of their boreholes, number 225 near Maydencroft Manor, Gosmore, Hitchin, penetrated an organic deposit, overlying a light grey, chalk-rich till, capped by an orangebrown silt, sand and clay 'brickearth'. The ground surface here is at 84.9 m OD and the borehole was located approximately 250 m north of Maydencroft Manor [TL 1828 2760] and 300 m southwest of the Charlton Lane boreholes (Fig. I). BGS borehole 225 penetrated the entire thickness of the organic deposit (Fig. 2), although the section between 395 em and 420 em was lost. However, a continuous series of samples was obtained from the remaining sediments. The sediment encountered in the borehole was as follows: Maydencroft Manor (BGS 225) Above 395 em 395--420 cm 420--456 ern 456--468 ern 468--480 cm Below 480 cm

Orange-brown sand with silt and clay. Missing. Grey-black organic clay. Grey-black organic silty clay. Grey sandy clayey silt. Light grey sandy chalk -rich diamicton (till).

As l Agl Ga2

Ldl Dgl As2 Agl Ldl As2 Asl Gal Ag2

Two sections A-A and B-B (Figs 1 and 2) have been constructed from the borehole evidence and show that the organic deposits at Charlton Lane (QI) and Maydencroft Manor (BGS 225) fill a depression in the glaciofluvial deposits, but are directly underlain by till. The organic sediments again suggest deposition in a shallow-water lacustrine environment. Throughout the area the lake deposits are overlain by sediment of variable thickness, referred to as 'brickearth'. This term is widely used in southern England to describe virtually any material used to make bricks. The Hitchin 'brickearth' deposits consist of indistinctly bedded, yellow, red or brown clayey or sandy silts with pebbles increasing towards the base (Hill, 1891; Reid 1897). The photograph in Bloom (1934) (Fig. 3), shows that the 'brickearth' fills vertically-orientated, 'pipe-like' structures that penetrate the underlying stratified deposits to a depth of 2-3 m. These structures superficially resemble solution pipes or frost cracks and if so are probably of periglacial origin. The 'brickearth' overlies the interglacial deposits, and where they are absent, the till and glaciofluvial deposits. The 'brickearth' was originally up to 10 m thick, and has been proved to this depth in borehole Q5 on the comer of St John's Road and Standhall Road [TL 18822855]. The deposits were observed in a partially overgrown cutting at TL 188284 and 187285 where they are 6.5 m thick. They show it to be a decalcified sandy clayey silt with narrow, irregular and impersistently pebbly and sandy horizons and bed-like lenses. The dried faces show vertical, polygonal,

261

columnar jointing typical of those associated with siltrich deposits of loessic origin. Subsequent workings and building over much of their outcrop has resulted in considerable disturbance of the deposits. Nevertheless, the greatest thickness of 'brickearth' appears to have been deposited within hollows in the ground surface, being thin or absent on higher ground (Aldiss, 1993). On the basis of these observations, the Hitchin 'brickearth' is thought to have been deposited by a combination of aeolian, colluvial and fluvial processes (Gibbard, 1974). From sections A-A and B-B (Figs 1,2) it would appear that the Charlton Lane boreholes (QI; Q2) are located near the deepest part of the basin, although Reid's neighbouring CR8 borehole showed an even greater thickness of lake sediment. In contrast, the sequence from Maydencroft Manor apparently occupies a more marginal position in the same basin. Although the extent of the lacustrine sediments described by Hill and Reid was not fully determined, they were thought to occupy a hollow in the surface of the glaciofluvial deposits. Section C-C constructed from boreholes in the vicinity of Ransome's Pit shows this to be the case. These lacustrine deposits have a similar lithology and thickness to those from Charlton Lane and Maydencroft Manor, and occur at a comparable elevation. However, the latter deposits are apparently not underlain by till. It has not been possible to demonstrate that the lacustrine sediments at Charlton Lane are continuous with those at Jeeve's Pit, some 700 m to the northeast, because excavation and building has removed or obscured much of the evidence, although the continuity is almost certain.

3. POLLEN ANALYSES AND VEGETATIONAL HISTORY A series of pollen analyses were made from the Charlton Lane (Q1) and the Maydencroft Manor (BGS 225) boreholes. The resulting diagrams are shown in Figs 4 and 5. Pollen preparation in both cases followed standard chemical methods (West, 1977). The pollen sum of total land pollen and spores excluding aquatic types has been used. Pollen nomenclature follows Clapham, Tutin & Moore (1987), and pollen type conventions follow Andrew (1970). The pollen sum was at least 300 for all levels. Pollen preservation was variable especially in the lowermost and uppermost sediments from each sequence, in which degraded, broken and crumpled pollen was found. However, no obvious non-sequences were identified from the pollen spectra, although some changes in pollen may result from variation of local conditions rather than regional ecological changes. The following detailed description of the characteristics of local pollen assemblages biozones at Charlton Lane and Maydencroft Manor is based on the pollen diagrams (Figs 4 and 5).

(a) Charlton Lane Biozone CL a (Charlton Lane (QI): 690--
262

S . BOREHAM 80S

.

&

beco~ing

: ~ =~ 70

". -

.

~

-. >:- :-..... . .•

eo

. .'.

. .« .

mO.D.

B

80SA

CR 12

CAe Ql /02

90

Q3

~3S

B

60 mO.D.

90

C

80S 24

as

CR CR CR 1 2 15

P 14

C 'Brickearth '

eo Lake sediments 70

60 mO.D.

P. L . GIBBARD

GlaCiofluvial sandsand gravels Middle Chalk

Fig. 2. Section constructed from borehole records from sites shown in Fig. I.

to silty clay with shell and plant fragments. This biozone is dominated by non-tree pollen , including Gramineae, Cyperaceae and a variety of open-ground herbs. Trees are represented only by Betula pollen , that increases markedly through the zone, and low uniform values of Pinus pollen . They are accompanied by low numbers of Salix and Hippophae pollen. The pollen of aquatic types is also present. Substantial numbers of reworked pre-Pleistocene pal ~nomorph s , dominated by Juras sic and Cretaceous types, indicate s that material was being washed into the basin. Biozone CL b (Charlton Lane (Q1): 630-390 em). This zone occurs within grey silty clay with mollusc shells becoming brown organic silty clay above. The assemblag~ IS domin ated by Betula pollen, which reaches 85%, before declining to 40% at the top of the sequence. The pollen of other arboreal taxa includes Pinus which is present at frequencies of 5-15%, and Quercus, Ulmus, Alnus, Corylus and Salix which are also present at low frequencies. The pollen of Gramineae continues at frequencies of 5-25% throughout the zone (Gibbard, 1974). Herb taxa indicating open-ground conditions include Rumex, Armeria, Chenopodiaceae and Compositae. Aquatics are recorded throughout the biozone; Menyanth es and Alisma are present near the base, with Potamogeton, and Typha latifolia and Sparganium type in the upper part. The marked decline of reworked pre-Pleistocene palynomorphs above 600 cm marks ~he stabili zati~n of local land surfaces presumably by the increased density of the vegetation cover.

(b) Maydencroft Manor Biozone MM a (Maydencroft Manor 472.5-456 em). This biozone is developed in grey silty clay with mollusc shells.

more organic above. The zone is dominated by Gramineae pollen at frequen cies of 50-70%. Arboreal pollen is represented by Betula and Pinus which are present at frequencies of 5-15 %. Other arboreal taxa including Quercus, Alnus and Corylus occur at low frequencies. The pollen of Salix is present throughout and Hippophae occurs at frequencies of 1-7 % declining towards the top of the zone. Biozone MM b (Maydencroft Manor 456-433 em ). The sediment in this biozone comprises grey-black organic silts and clays. Although dominated by Gramineae pollen, the zone has Betula at frequencie s of 20-30%, with a single count at 447.5 cm reaching 75%. In contrast. Pinus pollen is present at frequencies of 5-10%. The pollen of other arboreal taxa including Quercus, Alnus , Corylus, Salix and Hippophae also occur at low frequencies. Biozone MM e (Maydeneroft Manor borehole 433-420 em). The assemblage occurs in the grey-black organic clay. The zone is dominated by Filicales spores which reach frequencies of 60-70% at the top of the sequence. This increa se is accompanied by a marked decline in the marginal aquatic Sparganium type. The pollen of Alnu s reaches frequencies of 30%, with lower frequenci es of !'inu s and declining Betula. Other arboreal taxa present mclu~e Qu ~reus, Tilia, Corylus and Hedera. The pollen of Gramineae IS present at frequencie s of 15-25%. Low frequencies of reworked pre-Pleistocene palynomorphs occur throughout most of the sequence, but are almost absent above 433 em. Their occurrence suggests that some inwash, possibly of local bank material, took place during much of the time represented . The stratigraphy and pollen diagrams from both Charlton Lane and Maydencroft Manor (Figs 4 and 5) show a number of similarities. However, the profile from Charlton Lane exceeds 3 m in length , compared to 0.5 m for that from Maydencroft Manor. On the basis of the analytical results presented, the shorter sequence appears to have been deposited . at the margin of a lake, whereas the longer s~quence IS appare?t1y from the lake centre. Consequently different t.aphonomlc processes, such as sorting of pollen by wave action and sediment focusing, are likely to have affected the pollen assemblages. This has been considered when comparing the pollen profiles. Both sequences suggest the presen ce of Betula woodland, with thermo-philous trees increasing upwards in the profiles. However, the sequence from Maydencroft Manor contains moderate frequencies of Hippophae pollen at the base and a stronger signal of the pollen of thermophilous trees at the top. The basal part of the pollen profile appears to be poorly developed at Charlton Lane. It is also marked by a substantial influx of pre-Pleistocene reworked material; an indication of reworking of sediments from the local catchment by inflowing streams or surface wash. In addition, the upper part of the Charlton Lane sequence is oxidized, so that pollen in the later part has not been preserved. The high frequency of Gramineae pollen

MIDDLE PLEISTOCENE HOXNIAN STAGE INTERGLACIAL DEPOSITS

263

Fig. 3. Reproduction of a photograph in Bloom (1934) taken c. 1885 by T. B. Latchmore of an old clay pit at Hitchin in the 'Palaeolithic Brickearth', from which a number of artefacts were obtained. It appears that the first 5-6 ft (c. 2 m) below the top soil is composed of 'stony brickearth', which has been piped down into the stratified loam beneath. The white card gives the position where a flint implement was found, probably the one seen in the photograph at the feet of William Hill, the geologist.

observed from the Maydencroft Manor sequence may represent the presence of marginal Phragmites reed-swamp at the lake edge. In addition the high frequency of Filicales spores at the top of the sequence may indicate differential weathering of palynomorphs. Within weathered sediment many pollen types become degraded, leaving resistant spores as the most abundant group. However, the accompanying decline in aquatics and pre-Pleistocene palynomorphs at the point at which the Filicales rises probably reflects decreased inwash brought about by infilling of the basin and a colonization of the wet surface by ferns.

4. PLANT MACROFOSSILS The Chara marl has yielded a number of macroscopic plant remains from site CR15 (Reid, 1901) (Fig. 1): 1. seeds and fruits

Alisma plantago-aquatica L. Arctium lappa L. Carduus nutans L.

Cnicus sp. L. (Cirsium) Eleogiton (Scirpus) fluitans (L.) Link Heracleum sphondylium L. Lapsana communis L. Leontodon taraxacoides (Hirta) (Vill) Merat Mentha aquatica L. Nymphaea alba L. Prunella vulgaris L. Solanum dulcamara L. Sonchus arvensis L. Stachys palustris L. Stachys sylvatica L. Stella ria media L. Zannichella palustris L. 2. leaves Quercus sp. 3. stems and oogonia Chara

The remains can be placed into three main groups. Aquatic plants including Alisma plantago-aquatica, Eleogiton fluitans, Nymphaea alba and Zannichellia palustris grew

264

S . BOREH AM

& P . L. GI BBARD

D811lh

em

a

400

Ii 500

1 .1

a

,

1

.

1

I

I I··

o

L......L..I L .J

.

'0 20 30 40 50 60 70 80

a

10

0

I l w w w L.J L.." l-o LJ ~ . .. l l l l 0 a a a 10 20 30

a a a a a

LJG l..J L.." w

w ~

0

000

0000

L... L,w w

0

0

0

'Yo Total pollen. spores - aqua tics

Fig. 4. Pollen diagram from borehole QI, Charlton Lane, Hitchin.

Depth

c:on

440

450

~.

:

.

470

L _ 20

40

60

20

LL L

000000

L LI 0

0

0

L

20

Clf. Total poIlon + spores - aq uancs

Fig. 5. Pollen diagram from borehole BGS 225, Maydenc roft Manor, Gosmo re, Hitchin.

40

L

~

60000

LLLLL

000

0

0

__

L~

_ _

000000

265

M I DDLE PLEISTOC ENE H O X NI A N S T A GE I N T ER G L A CI A L DEPOS ITS

577

.,, 489

CL b

517

············ · ·1 ~

w

L ~ w L L J l ... u L..J L.. L L.J I . 0000000000000

II L

'

L

503 507

+, .

537 523

i LJ ~ l

o

10 20 30 ' 0 50 60 70 80

so

324 31g 3' 2 324

I

335 392 320 318 301 304

... \...

I

3Bi1 369 395

II I

. ..........!...L...

o

~ L 0 0

302 301 304 302 306 311 317 302 306

L L L L L L L L L L L L L 0

0

0

0

0

0

0

0

0

0

0

0

0

L!~J-~L-~~-0 20 40 60

20

40

60

80

MM c

266

S . BOREHAM

& P.

in the shallow water of the pool , whilst around the margins the marsh commun ity included Mentha aquatica , Sonchus arvensis and Stachys palustris. Evidence of dry, grassy, open ground around the pool is indicated by Arctium lappa, Carduus nutan s, Heracleum sphondylium , Lapsana communis, Leontodon taraxacoides, Prun ella vulgaris, Solanum dulcamara, Stachys sylvati ca and Stellaria media; of these Carduu s nutans, Leontodon taraxacoides and Prunella vulgaris also indicate base-rich soil condit ions. The leaves of Quercus indicate temperate forest in the immediate vicin ity. Temperate conditions are also indicated by Arctium lappa, Heracleum sphondylium, Sta chys palustris, S. sylvatica, Mentha aquatica, and possibly Solanum dulcamara . The association of the remains of large vertebrates with the marl (section 7, below) may, in part, account for the gra ssy, open-ground conditions around the site indicated by many of the plant macro fossils. Under temperate conditions fore st would usually occur (cf. West, 1980) and both the pollen and leaf finds indicate that it grew nearby. However, particularly where animal numbers are high, grazing, brow sing and trampling may commonly lead to a reduction in forest cover, especially near a water body (Phillips, 1974). Further, the occurrence of Heracl eum sphondylium is interesting in this respect since it is noted as having 'edible leaves much esteemed by herbivorous animals' (Clapham, Tutin & Warburg, 1962).

L. GIBBARD

temperate substage of an intergla cial (Turner & West, 1968). However, the Charlton Lane sequence only records the phase of Betula woodland rise to dominance. Thi s would suggest that , although a slightl y thinner sequence, the sediment s from Maydencroft span a greater period of time than those at Charlton Lane. It is possible to correlate the local pollen assemblage biozones identified at Charlton Lane and Maydencroft Manor with the Hoxnian regional pollen assemblage biozones established for Eastern England using the system des cribed by Turner & West (1968) and Turner (1970 ). On this basis, the organic deposits at Maydencroft Manor can be correlated with the late Angl ian and early Hoxnian. Likew ise, the Charlton Lane sequence which is of early Hoxnian age appear s to include latest Anglian age sediment at the base. The local assemblage biozones can be correlated as shown in Table I. Since the deposits at Charlton Lane appear to be the direct equivalents of the basal silty clays and overlying Chara marls described by Reid, it appears that these sediments also represent contemporary early Hoxnian deposition. In addition, a single sample of the Chara marl (calcare ous clay-mud) from Jeeve's pit collected by R. G. West in 1954 (West, 1955) contained a pollen spectrum characteristic of the thermophilous woodland , which has been assigned to Ho lIe (Table 2). The youngest pollen bearin g sediments preserved are, therefore , of Ho II age . It appear s that either sedimentation ceased at this point, or that later interglacial deposits were weathered (decalcified and oxidized) and eroded.

5. AGE OF THE INTERGLACIAL DEPOSITS The age of the organic deposits at Charlton Lane and Maydencroft Manor is indicated by their stratigraphical po sition and their contained plant remains. The se deposits directly overlie unweathered Anglian age Lowestoft Formation till and associated meltwater sediment. Log ically, therefore the basin formed immediately following the decay of the Anglian ice. Late Anglian sediments are characterized by moderate frequencies of Hippophae pollen (West, 1955; 1980; Turner, 1970) , similar to those encountered at the base of the Maydencroft Manor sequence, and possibly the lowermost part of that at Charlton Lane. The pollen spectra also indicate the development of Betula woodland, followed by the rise of thermophilous trees. Vegetation of this type is characteristic of the pre-

Tabl e I . Correlation of local to regional pollen assemblage biozone s Cha rlton Lane Ab sent Ab sent CLb ?CLa

Maydencroft Manor Absent

MM c MMb MMA

Anglian/Hoxnian substages Late temperate Earl y temperate Pre -temperate Late-glacial

(Ho Ill ) Ho 11 Ho I i An

Table 2. Pollen co unt data from a single sample of Chara marl (ca lcareo us clay-mud) collec ted from Jeeve' s pit by R. G. West in 1954 (West, 195 5) Taxa

Betula Pinus Ulmus Tilia Quercus Alnus Acer Picea Corylus Hed era Ilex Gramineae Matricaria type Cornpositae Ligulitlorae Umbe lliferae Cru cifera e Filicales undifferentiated Sparganium type Sum pollen + spores - aquatics

% 3. 1 4.9 5.2 0.3 3.8 35.7 0.3 0.7 38.8 1.0 1.0

2.5 0.3 0.3 \.0 0.3 0.3 0.3

286

MIDDLE PLEISTOCENE HOXNIAN STAGE INTERGLACIAL DEPOSITS

6. COMPARISONS WITH OTHER HOXNIAN POLLEN SEQUENCES The nearest sites with well developed Hoxnian palynological sequences available for comparison with Charlton Lane and Maydencroft Manor are those from Fishers Green (Gibbard & Aalto, 1977). 5 km to the southeast; Hatfield polytechnic (Sparks, West, William & Ransom, 1969), 20 km to the south, Slade Oak Lane (Gibbard, Bryant & Hall, 1986), 40 km to the southwest and the parastratotype succession at Marks Tey (Turner, 1970) 75 krn to the east. Although the deposits at Marks Tey represent continuous deposition from the late Anglian through to the early Wolstonian, those from Hatfield and Fishers Green have a more limited, punctuated timespan being restricted to the late Anglian, Ho I, Ho II and Ho III. Other Hoxnian sequences from Bell Lane (Gibbard, 1974, 1977) and Stanborough (Sparks et al., 1969) in Hertfordshire are also known, but represent even less time, on .the basis of their preserved palynological sequences (Table 2). With the exception of the doline filling at Slade Oak Lane, the sequences considered are all from shallow depressions developed on Anglian gravel or till, and are therefore comparable in terms of their origin, form and stratigraphy to the Hitchin deposits. It appears that deposition began at these sites in the late Anglian. characterized by high frequencies of Hippophae pollen. In contrast, deposition at Slade Oak Lane began in Ho IlIa. In substage Ho I, Betula dominated the vegetation at all the sites, although Juniperus scrub appears to have become developed locally. Substage Ho II is apparently absent from the deposits at Fishers Green and is incomplete at Hatfield, possibly due to fluctuating water levels. Similarly, Ho II is only poorly developed at Maydencroft Manor, although the record from Marks Tey is much more detailed, with three substages recording the development and decline of mixed oak woodland. In the Hertfordshire area, the Ho IlIa sequences are of minor thickness and are characterized by increasing Alnus and low frequencies of Corylus. In comparison, substage Ho IlIa at Marks Tey and Slade Oak Lane have higher frequencies of Corylus. Substage Ho I1Ib is distinguished by high frequencies of Abies , but Ho IV is apparently absent from the Hertfordshire sites. It is represented at Marks Tey and Slade Oak Lane by declining Abies and increasing frequencies of Gramineae.

7. WATER LEVEL FLUCTUATIONS AND BASIN SEDIMENTATION Gibbard & Aalto (1977) and Gibbard et al. (1986) considered the evidence for water level fluctuations from Hoxnian sites in eastern England as a means of explaining the discontinuous sedimentation seen in the shallow basins. On this basis it seems that water levels may have fallen in the middle of Ho I and risen at the end of Ho II, possibly resulting from increased precipitation. These fluctuations in water levels are clearly recorded at Marks Tey (Turner, 1970). They have also been recently noted further afield. For example, at Beetley in Norfolk, West (1991) found

267

evidence for a drop in water level in Ho I, represented by a phase of ' land slip' in pond sediments. There is, therefore, a remarkable consistency of the timing of this event, and potentially other water level fluctuations all across East Anglia. At Charlton Lane, deposition in the first part of Ho 1seems to have been rather rapid judging from the thickness of sediment that accumulated. This suggests that contemporary water level was relatively high in the basin and the high inorganic content of the sediments indicates the probable inflow of a stream. At Maydencroft Manor, the poor development of Ho II may represent evidence for a drop in water level at this time, but the lack of weathering suggests that the sediments remained moist throughout the intervening period. However, it is impossible to confirm the subsequent rise in water levels seen elsewhere, in the absence of Ho III bearing sediments at this site. These depositional changes may reflect local hydrological conditions, which are also suggested by the abundance of ~ertain taxa such as Alnus in the pollen record, particularly 10 the upper part of the profile. The position of the Maydencroft Manor sequence at the margin of the Hitchin basin shows that if water levels rose in Ho III, they were either not high enough to reach or overtop these sediments, that any subsequent deposition has been eroded or that the depositional environment had been modified in the intervening time. The cessation in sedimentation of the organic, presumably shallow-water sediments on the basin margin and the accompanying change from the accumulation of predominantly inorganic lacustrine sediments in the basin centre to shallow-water marl-dominated pond sediments in Substage Ho II, marks a significant evolution in the sedimentary environment. The restricted and isolated nature of these beds may have resulted from deposition in isolated pools or might indicate later erosion of this unit before deposition of the overlying 'brickearth'. Nevertheless, the marl and calcareous silt appear to have been restricted to the central part of the basin fill. The sedimentary change from lake to pond sediments was accompanied by some con~emporary wea~hering of the pre-existing deposits, observed in places by HIli (1891, 1896) and Reid (1987, 1901). This evidence suggests that the basin may have been partially infilled by this time. This may have taken place either because a stream or streams were entering the basin, as mentioned above, or because of regional water level fall or a combination of the two factors. The net effect was to reduce or possibly fragment the basin into a shallow depression infill with pools. It is not clear whether the stream ceased or continued to flow over the basin infill. The occurrence in borehole CR5 of what Reid (1897) calls a 's tony brickearth' underlying shell marl and a similar deposit in borehole CR3 may possibly suggest, where the deposits are distinct from the overlying ' brickearth' , that the stream continued to flow throughout the period. It would further suggest that the marl pools were present on the stream floodplain. The lack of sediment identifiably later than Ho II in the basin suggests that during the second half of the stage (Ho III-IV) the basin became part of the stream floodplain.

268

S. BOREHAM

&

8. FAUNA AND ARCHAEOLOGY (a) Vertebrates A number of vertebrate remains were collected from the Chara marl deposit during the last century. They are currently stored in Hitchin Museum, were originally listed by Reid (1897) and were re-examined by Dr A. J. Stuart (pers. comm.). They are as follows: • Rodentia Microtus sp. (identified by Newton, in Reid, 1901) • Carnivora Ursus sp. • Proboscidea Palaeoloxodon antiquus and Falconer and Cautley • Perissodactyla Dicerorhinus sp. • Artiodactyla cf. Megaloceros giganteus (Blumenbach) Dama dama (L.) Cervus elaphus L. Capreolus capreolus (L.) Bos sp. and or Bison sp.

a vole

a bear Straight tusked elephant a rhinoceros giant deer fallow deer red deer roe deer aurochs and or bison

The fauna is of typical interglacial temperate type but contains no remains that indicate a specific date. A Hoxnian age has already been assigned to the marl and the fauna certainly resembles those known from other Hoxnian sites (Stuart, 1974, 1980). Equus sp. has not been reliably reported from these deposits.

(b) Ostracoda Chapman (1903) recorded the following Ostracoda from the Chara marl: • Cyprididae Cyclocypris laevis (Miiller) Cyclocypris globosa (Sars) Scottia browniana (Jones)

P. L. GIBBARD

Cypris virens (Jurine) Erpetocypris reptans (Baird) Plonocypris vidua (Miiller) Ilyocypris gibba (Ramdohr) Ilyocypris bradii Sars Candona pubescens (Koch) Candona lactea Baird Candona candida (Miiller) Candona candida var. tumida, Brady and Robertson Candona candida var. claviformis, Brady and Norman • Darwinulidae Darwinula stevensoni, Brady and Robertson • Cytheridae Limnocythere inopinata (Baird)

The taxa indicate a temperate freshwater pool or small lake with still or very slowly flowing water, possibly partially overgrown. Many of the forms occur in Britain today (Chapman, 1903). Molluscan and insect remains are also known from the 'freshwater' deposits at Hitchin, but the horizon from which they were collected is not specified (Reid, 1897).

(c) Palaeolithic artefacts Palaeolithic flint implements have been found in the gravelly basal unit of the 'brickearth' (Reid, 1897) and a few were recovered from the marl beneath. Examples were illustrated by Evans (1896) and were described briefly by Oakley (1947). According to Roe (1891), the artefacts are 'mostly in fine sharp condition, quite appropriate to a provenance within the "brickearth"'. It is not known whether all the artefacts were collected from the same horizon, but 'the technology is very comparable to that of the other Group II sites'. The latter include Hoxne, the Middle Gravel bed at Swanscombe (Barnfield Pit), Dovercourt and Foxhall Road, Ipswich, from sediments which range from mid-Hoxnian to early Wolstonian age (Roe, 1981; Wymer, 1985).

9. STRATIGRAPHICAL IMPLICATIONS OF THE HITCHIN LAKE DEPOSITS The late Anglian and Hoxnian Hitchin Lake Beds beneath south Hitchin are important stratigraphical markers since they overlie Lowestoft Formation till and glaciofluvial

Table 3. Comparison of the timespan of the Hitchin deposits with nearby Hoxnian sequences Fishers Green eWo HolY Ho III Ho II HoI iAn

Hatfield

Slade Oak Lane

Marks Tey

Bell Lane

Stanborough

Hitchin

MIDDLE PLEISTOCENE HOXNIAN STAGE INTERGLACIAL DEPOSITS

gravels of Anglian age occupying the Hitchin-Stevenage Gap (Gibbard, 1974; Woodland, 1970; Aldiss, 1993). The latter fill a channel excavated into the bedrock to below -20 m OD and represent a thickness in excess of 100 m. Laterally equivalent deposits occur beneath the Hoxnian interglacial deposits at Fishers Green (Gibbard & Aalto, 1977) and at other sites in the area (Boreham & Gibbard, unpublished). The Fishers Green deposits are thought to have formed in a kettle-hole left by decaying stagnant ice. It is not clear whether the Hitchin lake deposits were laid down in a single water body or in a series of connected ponds. It is possible, however, that the Maydencroft Manor-Charlton Lane basin also represents a kettle-hole independent of that described by Hill, Reid and Bloom, although this seems unlikely. The evolution of the basin suggests that the occupation of the area by large mammals and Palaeolithic humans occurred on the land surface exposed during the middle to late Hoxnian (i.e. during and post-substage Ho lIe) and possibly into the earliest Wolstonian. This timing of the occupation parallels closely, that of the human occupation at Hoxne (cf. Wymer, 1985; Singer, Gladfelter & Wymer, 1993). The overlying 'brickearth' deposits are clearly later in origin and it is thought that they represent colluvial, aeolian and fluvial activity during an early part of the subsequent Wolstonian cold Stage (sensu Gibbard & Turner, 1990). The wedge- or tongue-like frost or periglacial features that penetrate the lake sediment and are infilled by the 'brickearth' sediment must be intermediate in age between the two accumulations. They presumably also date from the early Wolstonian. The deposits now underlie the interfluve between the Ippollitts Brook and the River Hiz and therefore they must pre-date the cutting of the modem river valleys. Recently, suggestions have been made that interglacial sequences filling enclosed lake basin-like hollows on the Lowestoft Formation might represent multiple interglacial events (cf. Bowen, Hughes, Sykes & Miller, 1989; Bowen, 1992; Ashton, Bowen, Holman, Hunt, Irving, Kemp, Lewis, McNabb, Parfitt and Seddon, 1994). On the basis of the evidence assembled here - the identical stratigraphical position of the deposits, the highly similar vegetational development, and the comparable coeval water level changes, it is highly improbable that the sequences discussed represent anything other than a single glacial-interglacial-glacial cycle of deposition.

10. CONCLUSIONS The Pleistocene deposits filling the basin beneath southern Hitchin sampled at Maydencroft Manor and Charlton Lane

269

appear to represent the infilling of a kettle-hole that formed at the end of the Anglian glaciation. The lake thus formed, was infilled by sedimentation during the first half of the temperate period, probably as a consequence of a stream flowing into the depression. The basin seems to have evolved from a lake into a shallow-water pond complex and possibly later into part of the stream floodplain during the second half of the stage. The area around the basin was colonized by temperate deciduous forest with an open treeless area around the waterbody, particularly during the shallow water phase. The possibility that these open areas were maintained by large mammals cannot be excluded since mammalian remains have been recovered from the marls. These richly fossiliferous sediments have also yielded ostracods, molluscs, fish, as well as plants. Palaeolithic humans also inhabited the area. Later infill of the basin, possibly under cold climate periglacial conditions, is indicated by the large thickness of colluvial and aeolian deposits that overlie the lake sediments. The former were derived predominantly by reworking of local glacial materials. The discontinuous sedimentary sequence and vegetational development recorded by the Hoxnian interglacial deposits from the Maydencroft Manor-Charlton Lane basin are rather similar to those from many of the other sequences from Hertfordshire. These basins almost certainly formed as kettle-hole like depressions, and contain sediments which record remarkably comparable changes in vegetational development. Breaks in deposition at these sites are interpreted as water table fluctuations resulting from changes in precipitation and/or evapo-transpiration. These changes have also been detected in more distant British Hoxnian sequences and are therefore thought to represent regional scale climatic changes.

ACKNOWLEDGEMENTS The authors are indebted to D. T. Aldiss, P. Hopson and R. Shepard-Thorn (BGS) for co-operation, kindly supplying the material from Maydencroft Manor and detailed evidence from geological mapping. They also thank K. D. Bennett for the provision of the Psimpoll program used to generate the pollen diagrams, and J. Dye for assistance with pollen preparation. PLG thanks NERC for a studentship during the tenure of which the boreholes and investigations at Charlton Lane were undertaken. The early work was carried out under the supervision of Professor R. G. West FRS. The authors are grateful to J. J. Wymer and R. Scaife for helpful reviews of the manuscript.

REFERENCES ALDISS, D. T. 1993. Geological notes and local details for 1:10 000 Sheet TLl2NE (Hitchin). Open-file technical report WA/92/61. British Geological Survey. ANDREW, R. 1970. The Cambridge reference collection. In (Walker, D. & West, R. G.; eds) Studies in the Vegetation History

of the British Isles. Cambridge University Press, Cambridge, 225-231. ASHTON, N. M., BOWEN, D. Q., HOLMAN, J. A., HUNT, C. 0., IRVING, B. G., KEMP, R. A., LEWIS, S. G. McNABB, J., PARFITT, S. & SEDDON, M. B. 1994. Excavations at the

270

S. BOREHAM

&

Lower Palaeolithic site at East Farm, Bamham, Suffolk 1989-92. Journal of the Geological Society, London, 151, 599---{i05. BLOOM, E. F. D. 1934. Geology. In (Hine, R. L.; ed.) The Natural History of the Hitchin Region. Hitchin and District Regional Survey Association, Hitchin. BOREHAM, S. & WEST, R G. 1993. Late Quaternary sediments with Chara encrustations in southern Fenland. Geological Magazine, 130543-544. BOWEN, D. Q. 1992. Aminostratigraphy of non-marine Pleistocene Mollusca in South Britain. Sveriges Geologiska Undersokning, 81, 65--67. - - HUGHES, S., SYKES, G. A. & MILLER, G. H. 1989. Land-sea correlation in the Pleistocene based on isoleucine epimerisation in non-marine molluscs. Nature, 340, 49-51. CHAPMAN, F. 1903. Ostracoda from the Chara-marl of Hitchin. Transactions of the Hertfordshire Natural History Society, 11, 6Q---{i2. CLAPHAM, A. R, TUTIN, T. G. & MOORE, D. M. 1987. Flora of the British Isles (3rd edition). Cambridge University Press, Cambridge. - - , - - & WARBURG, E. F. 1962. Flora of the British Isles (2nd edition). Cambridge University Press, Cambridge. EVANS, J. 1896. The Stone Age in Hertfordshire. Transactions of the Hertfordshire Natural History Society, 8, 167-187. GIBBARD, P. L. 1974. 'Pleistocene stratigraphy and vegetation history of Hertfordshire,' PhD thesis, University of Cambridge. - - 1977. Pleistocene history of the Vale of St. Albans. Philosophical Transactions of the Royal Society of London, B280, 445-483. - - & AALTO, M. M. 1977. A Hoxnian interglacial site at Fisher's Green, Stevenage, Hertfordshire. The New Phytologist, 81, 805-812. - - , BRYANT, I. D. & HALL, A. R 1986. A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England. Geological Magazine, 123, 27-43. - - & TURNER, C. 1990. Appendix: Definition of a boundary stratotype locality for the British Wolstonian Stage. Quaternaire, 1,40. HILL, W. 1891. Our forgotten lake. Journal of the Hitchin Natural History Club, 12,93-97. - - 1896. Excursion to Hitchin. Proceedings of the Geologists' Association, 14,415-419. KENNARD, A. S. 1943. The post-Pliocene non-marine Mollusca of Hertfordshire. Transactions of the Hertfordshire Natural History Society, 22, 1-18. KERNEY, M. P. 1959. An interglacial tufa near Hitchin,

P. L. GIBBARD

Hertfordshire. Proceedings of the Geologists' Association, 70, 322-337. OAKLEY, K. P. 1947. Early Man in Hertfordshire. Transactions of the Hertfordshire Natural History Society, 22, 247-257. PHILLIPS, L. M. 1974. Vegetational history of the Ipswichian/ Eemian interglacial in Britain and continental Europe. The New Phytologist, 73, 589--604. REID, C. 1897. The Palaeolithic deposits at Hitchin and their relation to the Glacial Epoch. Proceedings of the Royal Society ofLondon, B287, 535-570. - - 1901. A further note on the Palaeolithic deposits at Hitchin. Transactions of the Hertfordshire Natural History Society, 2, 63--64. ROE, D. A. 1981. The Lower and Middle Palaeolithic Periods in Britain. Routledge and Kegan Paul, London. SINGER, R., GLADFELTER, B. G. & WYMER, J. J. 1993. The Lower Palaeolithic Site at Hoxne, England. The Chicago University Press, Chicago. SPARKS, B. W., WEST, R. G., WILLIAMS. R B. G. & RANSOM, M. 1969. Hoxnian interglacial deposits at Hatfield, Hertfordshire. Proceedings of the Geologists' Association, 80, 243-267. STUART, A. J. 1974. Pleistocene history of the British vertebrate fauna. Biological Reviews, 49, 225-266. - - 1980. The vertebrate fauna from the interglacial deposits at Sugworth, near Oxford. Philosophical Transactions of the Royal Society of London, B289, 87-89. TROELS-SMITH, J. 1955. Karakterisering af lese jordarter. Danmarks Geologiske Undersogelse IV Raekke. 3, No 10. TURNER, C. 1970. The Middle Pleistocene deposits at Marks Tey, Essex. Philosophical Transactions of the Royal Society ofLondon,B257,373-440. - - & WEST, R G. 1968. The subdivision and zonation of interglacial periods. Eiszeitalter und Gegenwart, 19, 93-101. WEST, R G. 1955. 'The Pleistocene vegetation and geology of East Anglia.' PhD thesis, University of Cambridge. - - 1977. Pleistocene Geology and Biology (2nd edition). Longrnans, Green, London. - - 1980. Pleistocene forest history in East Anglia. The New Phytologist, 85, 571--622. - - 1991. Pleistocene palaeoecology of central Norfolk. Cambridge University Press, Cambridge. WOODLAND, A. W. 1970. The buried tunnel-valleys of East Anglia. Proceedings of the Yorkshire Geological Society, 37, 521-578. WYMER, J. J. 1985. Palaeolithic sites of East Anglia. Geobooks, Norwich.

Received 22 August 1994; revised typescript accepted 25 October 1994