Late Pleistocene terrace deposits at Beckford, Worcestershire, England

Late Pleistocene terrace deposits at Beckford, Worcestershire, England

D. J. Briggs, G. R. Coope and D. D. Gilbertson

The Pleistocene deposits in the area of the village of Beckford. in the Carrant Valley, Worcestershire are described. A radiocarbon date of 27.650&250 years B.P. has been obtained from plant material in the terrace deposits in the valley. Detailed studies have been made of the terrace sediments and of the remains of Mollusca and Coleoptera in fossiliferous layers. Frost structures are described and their stratigraphy considered. The nature of the local environment and regional climate in the final stages of the Upton Warren Interstadial period is reconstructed in detail. The terrace appears to have aggraded in a periglacial environment, dominated initially by solifluction prou+s. Extensive solifluction may have led to local diversions of drainage. In time, the climate deteriorated, becoming cooler and somewhat less continental with the onset of the main Devensian glaciation. This latter stage was characterised by extensive ice-wedge growth, aeolian activity and, subsequently, by river incision.

1. Introduction

In recent years, studies of sedimentary, faunal and floral evidence have built up a detailed knowledge of both the historical and environmental significanceof Pleistocene deposits. Particular attention has been focused upon the deposits of Mid- and Late- Devensian age in Britain (Mitchell et al. 1973). These fall within the range of radiocarbon dating techniques which allow depositional sequences to be dated in absolute terms.

These combined methods have Fermitted the definition and detailed description of the Upton Warren Interstadial Complex (Coope et al. 1971), a long period of sub- arctic to temperate boreal conditions during which most of Britain was probably ice- free. This paper deals with events at the end of the interstadial, immediately prior to the onset of the Late Devensian glacial advance. It attempts to describe the nature of the transition from interstadial to stadia1 conditions in the Carrant Valley, a tributary of the Warwickshire Avon (Fig. 1).

2. Regional setting (Fig. 1 )

The Carrant Valley is cut into the Lower Lias Clay, between outliers of the Cotswold scarp. To the south of the valley stands Alderton Hill and the limestonecapped Oxenton and Crane Hills; to the north is Bredon Hill. The surface of Bredon Hill is Geol. J. Vol. 10, P t 1,1975 1

2 D. J. BRIGGS, G. R. COOPE AND D. D. GILBERTSON

intensely crenellated, and the slumped, broken limestone forms a series of flaps extending below the level of its regional outcrop into the Carrant Valley-evidence of the cambering which is widespread in the north Cotswolds (Kellaway er al. 1971 ; Briggs and Courtney 1972; Whittaker 1972).

Carrant Brook rises in the Lower Lias Clay col between Alderton and Bredon Hills, and flows westwards to join the Avon near its confluence with the Seven at Tewkesbury. It is a small, underfit stream, but is associated with extensive terrace deposits. These deposits can be related to the terraces of the Avon Valley where Tomlinson (1925) and Shotton (1953, 1968) have described the following sequence:

Avon 1 Terrace Late Devensian Glaciation

Avon 2 Terrace Mid-Devensian (Upton Warren Interstadial complex)

Avon 4 Terrace Early Devensian

Avon 3 Terrace Ipswichian Interglacial

Avon 5 Terrace Late Wolstonian Glaciation

The present course of the River Avon was initiated in the Wolstonian period by meltwaters from the combined ice-sheets of northwestern and northeastern provenance, which advanced into the Midlands (Shotton 1953; Bishop 1957). The terraces are composed mainly of erratic materials-quartz and flint-either supplied directly by outwash from ice upstream, or by reworking of earlier drifts. Some doubts remain about the detailed relationship of the earliest terraces (Shotton 1968), but the age and origin of the Second Terrace is better understood (Shotton 1953, 1968; Coope 1962,

LATE PLEISTOCENE TERRACE DEPOSITS AT BECKFORD 3

1968). It is with this terrace that the main deposits of the Carrant Valley can be compared.

The Avon 2 Terrace has been studied at several localities and a number of radiocarbon dates obtained from organic material in the terrace. These dates range from 38,700f700 years B.P. at Fladbury (Coope 1962) to 28,200f500 years B.P. at Brandon (Shotton 1968). Throughout much of this period the area was apparently subjected to a cold climate reaching, towards the end of the interstadial, tundra-like conditions (Coope et al. 1971).

3. The Carrant Valley

Within the Carrant Valley there occur extensive terrace deposits which have been mapped, in part, by the Geological Survey (Richardson 1929). These deposits are best exposed at Beckford Priory (Grid Reference: SO 982363, site 1 on Fig. l), but a number of other pits are visible at Aston Mill (SO 941352, site 2), Conderton (SO 962363, site 3), Beckford Court Farm (SO 977360, site 4), Beckford Playing Fields (SO 976360, site 5), Maddons Barn (SO 962358, site 6) and Ashton Bridge (SP 002367, site 7).

Older deposits of till and thin, weathered gravel are also present. Near Sedgeberrow is a small outcrop of clayey till, containing erratics of quartz, quartzite, sandstone and igneous rocks-all probably from the Bunter Pebble Beds-and Cretaceous flints and Liassic ironstone. The abundance of flint in the deposit, which accounts for 30 % of the stone content, suggests that it is a correlative of the Chalky Boulder Clay of the Midlands. Sedimentologically similar tills occur in the Vale of Moreton and are generally considered to be Wolstonian in age (Bishop 1957).

‘Boulder Clay’, partially underlying river gravels, was also mapped by Richardson (1929) in the Beckford area. However, this deposit cannot be traced by augering and is not exposed in any of the local excavations. The lack of till elsewhere in the valley suggests that considerable erosion has taken place since the incursion of Wolstonian ice.

Two small patches of high level gravel occur south of Carrant Brook. The main , outcrop is 0.8 km south of Sedgeberrow (SP 021380), where limestone gravel containing rare Bunter pebbles, caps a hill at 58 m O.D. A similar, less well preserved deposit exists on Washbourne Hill (SO 978343) at 60 m O.D., where coarse Bunter sandstones are common in the surface soil. No flints have been observed in either deposit, and it must be assumed that the gravels predate the till at Sedgeberrow. An early Wolstonian or Anglian glacial age is therefore suggested.

These deposits stand up to 20m above the more widely distributed BeckfordTerrace, indicating that considerable erosion took place during the interval between their aggradation. It was this activity which incised the valley almost to its present depth and removed the tills and terrace gravels, of which only remnants now survive. Most of this erosion probably occurred in the late Wolstonian glaciation and early Devensian period.

To the south of the Carrant Valley, the main drift deposits are the Cheltenham Sands-quartzose, well-sorted sands-and their associated limestone gravels. These deposits are banked against the foot of the western scarp of the Cotswolds, to a height of 85 m O.D. Their main outcrop is in the immediate vicinity of Cheltenham, but smaller patches have been mapped along the Cotswold footslope.as far north as Alderton(Richards0n 1929).Thus theyextend towithin 3 km of Carrant Brook, and in many respects they resemble the Beckford deposits.

4 D. J. BRIGGS, G. R. COOPE AND D. D. GILBERTSON F

I"

Fig. 2. Section in the Beckford Terrace at Beckford Priory, exposed May 1971.

4. The Beckford Terrace

4a. Stratigraphy The main terrace of Carrant Brook stands 0-15 m above the present floodplain. It

extends almost continuously down the right bank from near Grafton (SO 987371) to Kinsham (SO 931353) (Fig. 1). On the left bank, it is widespread near Aston-on- Carrant, and south of Sedgeberrow it can be traced across the col between the Carrant and Isbourne valleys, at 47 m O.D. The terrace is thus more extensive than shown on the existing Geological Survey maps.

Upstream of Sedgeberrow, the terrace thins out and is locally overlapped by alluvium. To the west, however, it falls gently and, near Aston-on-Carrant, at 27 m O.D., stands 5 m above the floodplain. In transverse section, the terrace shows a similar height range. Away from the river it rises 10 m and its surface becomes irregular. At the same time, the character of the deposit changes, from a well-bedded, sandy, waterworn gravel, overlain by cross-bedded sands, to a roughly stratified gravel composed entirely of local rocks. This appears to mark a transition from fluvially deposited materials in the valley-bottom, to soliflucted debris on the footslope of Breden Hill.

In the pits near Beckford, the nature of this transition can be demonstrated.

Beckford Priory Gravel Pit. SO 982363. 39.6 m O.D. (site 1, Figs 1, 2) 1.0 m Brown, silty clay loam, unbedded, containing flint and quartz pebbles with

occasional gravelly seams; locally intruding into the sands below. 2-0 m Quartzose sand; cross-bedded; well sorted; sub-rounded to rounded

grains; local seams of limestone gravel; disturbed at the top by involutions involving the overlying loam, and by ice-wedge casts originating at the surface of the terrace. Ice-wedge casts also occur at depth, extending into the gravel below. Roughly bedded limestone gravel, containing scattered quartz and quartzite pebbles, flint, marlstone and sandstone; locally disturbed by ice-wedge casts originating in the sands above.

1-5 m

Beckford Playing Fields. SO 977360. 42.7 m O.D. (site 5 , Fig.' 1) 1.0 m Brown, silty clay loam, unbedded, with rare quartz and flint pebbles. 2.0 m Coarse, roughly bedded limestone gravel; sub-angular; no erratics,

abundant Liassic Gryphaea and belemnites; locally sandy; gravel up to 10 cm diameter (average 3 4 cm); base not visible.

LATE PLEISTOCENE TERRACE DEPOSITS AT BECKFORD

Beckford Court Farm. SO 975366. 54.9 m O.D. (site 4, Fig. 1) Brown, silt loam with limestone fragments.

Coarse, unbedded limestone gravel; angular; very rare ironstone and marlstone, few Gryphaea; no erratics; pebbles up to 20 cm diameter (average 4-5 cm); base not seen.

5

0-4 m

1.6 m

4b. Periglacial structures There is evidence'in the terrace of several phases of periglacial activity (Fig. 2). A

large number of ice wedge casts occur at Beckford Priory (SO 982363) and Aston Mill Farm (SO 943152). They are predominantly deep, thin structures, terminating below the terrace surface. Thus they seem to have formed during terrace aggradation. Pkwk (1966) has shown that ice-wedges only develop where mean annual temperatures are -6°C to -8°C or lower; these features are thus evidence of severely cold conditions at the time of aggradation.

A second set of shallower, broader wedges exists at the top of the terrace, often filled with pale, bleached sand and silt. These apparently developed from a stable surface after terrace formation had ceased.

The terrace is everywhere overlain by a brown silty clay loam, reaching a thickness of 1 m, and consisting mainly of locally derived Lias Clay and weathering products of the terrace. This deposit appears to be 'head', derived by a combination of solifluction and aeolian activity; the high silt content is characteristic of loessic materia1;while the distribution of the head suggests that it has been derived from the Lias Clay at the foot of Bredon Hill.

The head has been intensively convoluted, and it extends into the sands below as deep pipes and wedges. The involutions truncate, and therefore are younger than both sets of ice-wedge casts. Formation of these involutions appears to have been the final phase in a prolonged period of periglacial activity.

4c. Biogenic deposits In 1971, in an excavation at the north end of the Beckford Gravel pit, several beds of

silt were exposed within the terrace sands (Fig. 3). The silts contained abundant plant, coleopteran and molluscan remains, plus, rarely, bones of vertebrates. Woody plant fragments were collected from the length of the most continuous and organically rich seam of silt in the west face of the section (Fig. 3, sample A) and were dated by radio-carbon assay in the Department of Geology at Birmingham University, by courtesy of Professor F. W. Shotton. They gave a date of 27,650f250 years B.P. (Bim 293.).

The silts apparently represent still-water sedimentation in small cut-off channels and ponds on the floodplain, away from the main stream. Two distinct periods of siltation occurred. The earliest is marked by the lower silt beds which, in the east face of the exposure,' comprise dark, peaty layers with scattered plant remains and abundant molluscs (sample M4, see Fig. 3). To the north, however, these beds grade into a grey-green silt and fine sand (sample M3). A younger, higher silt layer occurs in the west of the section. This is continuous over a distance of nearly 9 m, and contains the most prolific floral and faunal assemblages (samples MI and M2). This seam is possibly represented in the east face by discontinuous peaty beds 1-5 m below the surface of the section. There is a height difference of almost 1 m between the two beds, but this may reflect the cross-sectional form of the channel or pond in which the sediments aggraded or the effects of later cryoturbation. The higher silt bed in the east face contains little plant or molluscan material.


WEST FACE m R r H FACE

EAST FACE

Fig. 3. Fossiiiferous deposits in the Beckford Terrace at k k f o r d Priory (SO 982363).

Analyses were carried out of the non-marine Mollusca and Coleoptera in the biogenic deposits.

5. The Beckford Terrace sediments

Ten samples were collected from four excavations in the Beckford Terrace (Fig. 1). Analyses of particle size distribution, stone content, CaCO, content and heavy mineralogy were carried out (Table 1).

The terrrace sediments form two distinct groups. The gravels are coarse, poorly sorted.and almost wholly of local origin. The main elements of their stone and heavy mineral contents can be related to the local bedrocks. On the footslope of Bredon Hill the gravels are coarse, unbedded and angular, but nearer the present river, where they form the basal terrace deposits, they are waterworn, locally well-bedded, finer grained


Table 1. Sedimentology of The Beckford Terrace and Cheltenham Sands

I l a Ib 2a 2b 3a 3b CS lc 2c 3c 4 CG

I1 % cacoa 9.5 9.4 6.9 6.9 5.2 4.7 3.9 12.5 18.4 20.4 64.3 65.0

111 Mean Size (9) 1.70 1.54 1.55 1.73 1.80 1.90 1.69 -0.61 0.68 -1.40 003 -1.13

Size Sorting (p) 0.40 0.47 047 037 0.48 0-50 0.47 244 2.98 1.99 2 99 2.22

Kurtosis (q) 1.05 1.11 1.11 1.16 1.32 253 1.24 075 1.06 0.82 1.59 0.99

Skewness (p) 020 0.02 4 0 4 0.08 0.25 030 0.08 -0.09 0.04 -0.63 0.30 023

1v 1 %HeavyMinerals 0.21 0.21 0 1 6 0.14 0.16 011 0.31 0.47 1.73 0-71 4.66 . 3.10 I V

% Limonite 38 22 38 32 20 19 18 60 43 33 92 80

%Black&RcdIron 27 29 33 31 43 43 43 24 40 45 5 5

% Non-opaque 22 24 17 I8 21 20 22 6 8 9 1 5

YI % Zircon

% Pink Garnet

% Other Garnet % Brown

Tourmaline % Green

Tourmaline

% Red Rutile

% Other Rutile

% Staurolite

% Kyanite

% Epidote

% Sphene

% Other

5 6 7 1 4 7 8 8 A A A

5 2 3 4 6 4 8 - - -

33 44 42 45 40 44 45 T T R 50 32 31 20 32 30 25 - R A 3 1 1 - 2 - I - - -

- 1 2 - 1 - 1 - - - - 5 2 1 1 4 2 T - T

2 3 6 6 9 7 6 - - R T 1 3 - 1 2 1 1 - -

- - - 2 1 2 - - I -

1 2 2 2 1 1 2 - - -

- - 1 2 1 2 1 - T -

A -

- T

I Arabic numerals refer to randomly selected grab samples a t the localities shown on Fig. 1. the letters to horizons indicatedbelow: la, sand, 2.2 m from the surface; Ib, sand, 2.3 m: Ic. gravel, 3-1 m: Beckford Priory. 2a, sand, 1.7 m; Zb, sand, 1 4 m; Zc, gravel, 25 m; Aston Mill. 3a, sand, 1.3 m; 3b, sand, 1.3 m; 3c, gravel, 1.9 m; Conderton. 4, gravel, 2.3 m; Beckford Court. CS. average of 1 I samples of Cheltenham Sand. CG, Cheltenham Gravel.

11 Percentage CaCO. calculated from 45-1000p fraction using Collins Calcimeter.

111 Size parameters from 2-16,OOOp fraction by sieving and sedimentation; based on formulae modified from Folk (1966).

1V Percentage Heavy Minerals as % of 45-SOOp residue after treatment with HC1.

V Percentage limonite etc. as frequency percentage of the total heavy minerals. VI Percentage zircon etc. as frequency percentage of the non-opaque heavy fraction. A . . . abundant.

R.. .rare. T . . .trace. - . . . not recorded. ,


- BECKFORD TERRACE SANDS

- - - - - CHELTENHAM SANDS Of 11 samples)

-0.5 0.0 0.5 1.0 i5 2.0 2.5 3.0 PHI SIZE

Fig. 4. Particle size distributions of the Beckford Terrace sands and the Cheltenham Sands.

and often intercalated with well-sorted sands. The gravels therefore seem to have been derived from Bredon Hill and to have been carried by solifluction into the valley bottom, where they were subsequently reworked by fluvial action.

In contrast, the sands which overlie the gravels at Beckford and Aston Mill are almost wholly of foreign derivation. They are predominantly quartzose, well-rounded, well-sorted and frequently finely cross-bedded. Their heavy mineral content is uniform and markedly different from that of both the gravels and the local bedrocks (see, for example, Skerl 1925, 1927a). Many aspects of this heavy mineral assemblage are indicative of a Triassic influence. The range of minerals present, and, more significantly, the varieties of pink garnet (almandite), rounded brown tourmaline and ragged staurolite are closely akin to the minerals described from the Bunter and Keuper rocks by Bosworth (1912); Burton (1917); and Fleet (1923, 1927, 1929). Nevertheless, there


are serious anomalies. Garnet, one of the least persistent minerals in this residue, is far more abundant than in the Bunter rocks, apparently due to enrichment of the Beckford Sands with much colourless almandite. Conversely, zircon, an extremely durable mineral, occurs in only small amounts. The sands also contain significant proportions of epidote, a mineral not recorded in the Triassic rocks, but relatively abundant in the Lias of the Midlands (Skerl 1927b) and common in the Chalky Boulder Clay in Gloucestershire.

It is therefore difficult to identify clearly the source of these materials; they seem to represent a mixing of several different sediments. However, further understanding is gained by comparison of the Beckford Terrace deposits with the nearby Cheltenham Sands which are widespread in the area south of the Carrant Valley. In view of their distribution and sedimentology they are generally considered to be windblown deposits of Late Pleistocene age (Gray 1912; Richardson 1929; Briggs 1973). Their distribution along the western scarpfoot of the Cotswolds is suggestive of a western or northwestern provenance.

The similarity between the Cheltenham and Beckford Sands is striking (Table 1). The deposits have almost identical particle size distributions (Fig. 4) and are both composed of well-rounded quartz grains with minor admixtures of calcite. The two deposits also have very similar heavy mineral residues, characterised by the abundance of garnet and relative dearth of zircon. It appears that the two assemblages have a common source.

A further similarity between the Cheltenharn and Beckford deposits exists. At the base of the Cheltenham Sands there occur coarse, angular limestone gravels, the Cheltenham Gravels. Like the basal gravels of the Beckford Terrace, they appear to have been derived by solifluction of the local Jurassic bedrocks. At one locality, near Bishops Cleeve, these gravels contain a large ice-wedge cast.

Both the Cheltenham and' Beckford deposits therefore show a transition from coarse soliflucted materials to sands. It has previously been suggested that the Cheltenham Sands were deposited by aeolian activity (Richardson 1929); in view of their similarity, it seems likely that the Beckford Sands were supplied by the same process. However, just as the soliflucted gravels have undergone fluvial reworking in the Carrant Valley, so have these sands. It is probable that this fluvial activity accounts for the minor differences between the sedimentology of the Cheltenham and Beckford sands (e.g. % CaCO,, % heavy minerals).

The distribution of the Cheltenham Sands indicates a westerly provenance for the windblown material, and it is possible that most were supplied by deflation of the Severn and Avon terraces. During much of the Late Pleistocene period these terraces were probably bare of vegetation and unconsolidated. This derivation would account for the mineralogy of the sands, for the terraces contain a mixture of Bunter and Liassic-Cretaceous materials; it is also consistent with the almost continuous distribution of the sands between Cheltenham and Bredon Hill.

6. Non-marine Mollusca

Five samples were investigated. Shells from sample A were picked out during sieving prior to the plant material being radiocarbon dated in the Department of Geology at the University of Birmingham. The remaining molluscs were separated from samples Ml-M4 (Fig. 3) by washing the sediment through a 250p aperture sieve. Every mollusc apex or valve has been counted as one individual. Four thousand and sixty specimens have been identified and referred to eight species.

10 D. J. BRIGGS. G. R. COOPE AND D. D. GILBERTSON

Table 2. Non-Marine Mollusca at Beckford Priory

SAMPLE No. A M1 M2 M3 M4 M1-4

DRY WEIGHT 23 kg 17 kg 31 kg 35kg 106kg

Lymnaea truncatula

Lymnaea eregra

PIanorbis Ieucostoma

PIanorbis laevis Alder Succineapfeifferi var.

Pupilla muscorum (I...)

(Miiller)

(MiilleJ

(Millet)

schumacheri Andreae

Agriolimax cf. (L.1

, agrestis

Pisidium nitidum Jenyns

Pisidium sp.

117

38

69

48

220

98

15

- 1

19.3 118

6.3 38

11.4 69

7.9 48

36.3 227

16-2 3420

2.5 88

1

0 2 1

-

TOTAL 82 1954 - 973 - 527 - 606 - 4060

The Location of samples M1-M4 is shown on Fig. 3; sample A from the material used for CI4 analysis.

6a. Notes on identification Some difficulty was experienced in identifying the specimens. Many shells, parti-

cularly the succineids are damaged. All the specimens of Lymnaeu truncatulu (Miiller), Lymnaeu peregra (Miiller), Succinea and Pisidium are juveniles. The smallest planor- bids could not be identified with certainty. They have been attributed to Planorbis IaevL Alder and Planorbis leucostamu Millet in the same ratio as that in which the more adult shells of the two taxa occurred. The shells of Lymnaea peregru are all of the peregru rather than the ovatu form. There is a complete size range of Pupilla muscorum (L.) present. The adults are recognisable as the taller form, whose whorls are rather broader in relation to their height than is usually the case. This form has been described by Kerney (1963), Kemey, Brown and Chandler (1964) and Large and Sparks (1961) .from Devensian deposits in England. In the Beckford specimens, the parietal tooth is either missing or usually poorly developed. Shells of Agriolimux cannot be attributed with certainty to a particular species, but appear to be those of Agriolimax agrestis

' (L.). The variety of forms exhibited by the small succeneids is closely comparable with those of Succinea pfeireri var. schumacheri Andreae. This particular form is well illustrated by van Regteren Altena (1957) with specimens from the Tubantian deposits at Velsen in the Netherlands. The taxonomic status of this form has been discussed by van Regteren Altena (1957) and Sparks (in Shotton 1968). It is similar to Succinea pfeifferi groenlandicu, which has been found in Iceland and Greenland (Van Regteren Altena 1957 table W). Succinea pfeifferi var schumacheri has been regarded by Sparks (in Shotton 1968) as the characteristic form in cold Pleistocene deposits.

LATE PLEISTOCENE TERRACE DEPOSITS AT BECKFORD

6b. Local conditions and regional climate The lowest sample in the sequence is number M4 from the organic silts exposed on

the east face of the section. It differs significantly, in both the number of species present and their percentage representation, from the remaining samples.

In M4 Lymnaea truncatula, Succinea pfeifferi var. schumacheri and Agriolimax together comprise 58 % of the fauna. These species are characteristic of particularly damp localities, such as the moist banks of a stream or more probably, in this case, an area of open ground with many pools of water, perhaps fringed with marsh. The freshwater species present, Lymnaea peregra, Planorbis Iaevis and Pisidium nitidum Jenyns are tolerant of a wide variety of environments, but Planorbis leucostom tends to be found in habitats which may be liable to dry up. This may suggest that the pools were temporary in character. Significantly there are no species present which indicate moving water. The great majority of specimens (84%) quite clearly point to an open area, possibly with temporary ponds, through which a few small streams may have flowed. The presence of the xerophilous land snail Pupilla muscorwn in large numbers is quite common in British cold Pleistocene floodplain and. hillslope deposits, and suggests rather open, exposed conditions. It has probably been incorporated in sample M4 through derivation during floods from intervening sandy ridges on the valley floor. These ridges would have provided more suitable, drier habitats above the surrounding wetter ground. This particular species, however, has occasionally been observed living in marshy sites (Boycott 1934), and Kerney (1963); and Kerney et al. (1964) thought the present form was a member of marsh communities at various Late Devensian sites in southeast England.

The superabundance of Pupilla muscorum in the remaining samples is of interest. It is completely dominant in those horizons which, in the exposures, appeared particularly rich in plant remains. Much of this plant debris has clearly been transported to the site, as is shown by the broken and worn nature of the material. Succinea pfeifferi var. schwnacheri and Lymnaea truncatula-both in life more commonly associated with vegetation than P. muscorum-are most scarce where, in these plant rich samples, they might have been anticipated to be frequent. The reasons for this are not clear.

The stratigraphy and extensive derivation indicate markedly unstable ground conditions. The absence of specimens of Vallonia, which would have favoured a grass- bound surface, probably indicates the lack of a continuous grass cover, as suggested by Kerney (1971) in his study of the interstadial deposits at Halling, Kent.

The remarkably low diversity of the fauna points to a particularly harsh environment. Most of the species present can be found living today in the British Isles, but they are tolerant of cold climates and either almost reach, or occur within, the Arctic Circle in Scandinavia. Though extinct, Succinea pfeifferi var. schumacheri has been commonly found in Pleistocene deposits in association with species favouring harsh, open environments, and it is thus an indication of climatic severity. The temperatures represented by the molluscan fauna cannot be quantified precisely. However, in view of the correlation of this terrace with the Avon 2 Terrace, and the radio-carbon date obtained from the organic material at Beckford, this very restricted fauna can be interpreted as pointing to a particularly cold part of the Upton Warren Jnterstadial period.

The Beckford molluscan faunas have close parallels with those of early Devensian sites at Wretton (West et al. 1974) and the mid-Devensian sites at Bourton-on-the- Water (unpublished data), at Upton Warren (Coope et al. 1961), the Avon 2 Terrace generally (Todinson 1925), and especially at the base of this terrace at Brandon (Shotton 1968). Unfortunately, it is not possible from the molluscan evidence to

11


decide with conviction whether the slight differences between the Brandon and Beckford faunas indicate deteriorating climatic conditions prior to the main Devensian glaciation of the Midlands, or whether they only reflect differences in derivation and local environments.

7. The Coleoptera

The following Coleoptera were obtained from sample “A” during the concentration of the plant debris for C14 dating. The numbers in brackets after each entry indicate the minimum number of individuals present in the sample.

Carabidae:

Hydrophilidae :

Silphidae: Staphylinidae:

Byrrhidae: Scarabaeidae : Curculionidae :

Notiophilus aquaticus L. (1 5 ) . *Diacheila polita Fald. (2). Amara quenseli Schoen. (4). *Pterostichus blandulus Mill. (2). Helophorus aquaticus L. orland H. grandis Ill. (5) *Helophorus sibiricur Motsch. (1). *Helophorns obscurellus Popp. (6). Helophorus sp. (4). Thanatophilus dispar Hbst. (1). Olophrwn sp. (1). Stenus sp. (2). Philonthus sp. (1). *Tochinus SP- (1). Byrrhus fasciatus Forst. (2). Aphodius sp. (2). Otiorrhynchus nodosus Muell. (1). Otiorrhynchus rugifronsGyl1. (6).

Although this is a rather short faunal list, the relatively high proportion of taxa represented by more than one individual suggests that it is a fair sample of an impoverished fauna. Nevertheless the species present provide clear evidence of the local environment and of the climate of the times.

The local environment indicated by the Coleoptera can, for convenience, be considered under two headings. Firstly there is evidence, provided by the species of Helophorn, of a shallow pool, though the total absence of Dytiscidae suggests that these pools may have been very temporary. In this context, it is interesting to note that the most common species, H. obscurellus, is also one of the least dependent on water. Secondly the habitats round these pools were open, with but a sparse vegetation cover. The relative abundance of such xerophilous species as Notiophilus aquaticus and Amara quenseli suggests that this ground was well drained. The Coleoptera provide little evidence of the flora of the times. Byrrhus fasciatus lives on moss and the two species of Otiorrhynchus are polyphagous. The presence of the carcase beetle Thunutophilus dispur may be related to the large number of bones of rodents in the sample.

Some estimate of the climate of the times can be gained from the present day geo- graphical ranges of the species in this assemblage. With the exception of the single specimen of Tachinus all the species in the list may be still found living in Europe. The Tachinus was sent to Wolfgang Ullrich, currently carrying out a revision of the Palaeartic Tachinus. He comments (in lift. 1972) that it is no European or asiatic species known to him. This may be an extinct species but, in the light of past experience, it is more probable that it represents a yet undescribed living species. With this single exception, all the species from Beckford have been recorded previously from upper Pleistocene deposits. Of the 11 named species and species groups, four are no .longer

Those species indicated by are no longer to be found living in the British Isles.


to be found living in the British Isles. Diacheila polita lives today no nearer than the eastern part of the Kola peninsula Pterostichus blandulus and Helphom obscurellus occur no nearer than the Kanin peninsula and the range of Helophorus sibiricur extends as far west as the mountains of Fennoscandia. Taken as a whole, the assemblage suggests that the climate at the time was similar to that of the tundras of the present day, with average July temperatures below 10°C. This assemblage, however, lacks the exclusively eastern species so characteristic of the somewhat earlier period typified by the fauna from the deposits at Brandon (Coope 1968) and may represent an episode of less extreme continentality of climate. Nevertheless the climatic rtgime at this time was probably more continental than it is in Britain today.

8. The environment of deposition

The material from the organic silts in the terrace at Beckford Priory supplied a radiocarbon date of 27,650k250 years B.P. (BIRM. 293). This figure matches closely the dates obtained from Brandon (Shotton 1968) and can be compared with the dates from Fladbury (Coope 1962) and Upton Warren (Coope el al. 1961). It indicates that the deposits formed during the later part of the Upton Warren Interstadial Complex.

This date, the implications of the fauna, the evidence of the periglacial structures and the interpretation of the sediments all point to an environment of extreme seventy during terrace aggradation. The lack of diversity of both the coleopteran and molluscan faunas, and the intensity of soliflual action and frost-heaving are probably a result of this harsh climate. In the early stages of terrace formation, the dominant process was solifluction of the

local bedrocks. At this time aeolian activity was apparently negligible, for no foreign windblown material is discernible in the gravels. Within the Carrant Valley, these gravels were fluvially reworked and abraded. However, in view of the small catchment area of the Carrant and the calibre of the material, it is necessary to visualise a “nival” stream regime; during periods of thaw the valley probably carried large quantities of water, while throughout the rest of the year fluvial activity was slight. These conditions of periodic flood, combined with the sporadic incursion into the valley of coarse, soliflucted debris must have produced an unstable land surface.

At about 27,000 years B.P., significant changes occurred in the depositional environment. The intensity of solifluction diminished, and windblown sands entered the valley, apparently from the west. This suggests that, at this stage, a westerly air-stream must have developed, which may have resulted in the less continental climate of the period, as shown by both the coleopteran fauna and the reduction of stream com- petence.

Early in this phase, the fossiliferous organic silts were laid down. These deposits imply a local environment of open, dry land, crossed by small stream and containing small, temporary, ponds. These probably developed during the spring thaw, but dried up in the summer and winter periods. The climate represented by the fauna and sediments is one of general aridity, with a larger annual temperature range than in the area today (though less than earlier in the interstadial), and prevailing westerly winds. Mean summer temperatures were below 10°C, but, at times at least, the mean annual temperature, as indicated by the presence of approximately contemporaneous ice- wedge casts, must have been at least -6 to -8°C. This period seems to mark the climatic deterioration prior to the build up of the main Devensian ice-sheets, which are generally thought to have reached their maximum extent at between about 26,000 and 15,000 years B.P. (Coope et al. 1971).


Subsequently, aeolian activity ceased and sedimentation in the Carrant Valley gave way to a period of ice-wedge growth from a stable terrace surface, followed by fonna- tion of the overlying head and convolution of the deposits. This final phase of periglacial activity may be associated with the Late Devensian glaciation generally, but since the head lacks any organic material, it is difficult to precisely date its formation.

It was suggested by Shotton (1968) that the most recent phase of widespread terrace formation took place in the late Devensian (Zone III) period. Between the Upton Warren and late Devensian interstadials the rivers had incised their valleys significantly. It is therefore likely that the main periods of fluvial downcutting in the area were associated with full-glacial, rather than interstadial, conditions; clearly downcutting in the lower Avon did not commence until well after 27,000 years B.P.

The distribution of the Beckford Terrace led Richardson (1929) to suggest that Carrant Brook originally extended some way east of its present headwaters, and received waters from the Broadway, Childs Wickham and Winchcombe areas of the Cotswold scarp. Though based upon the assumption that the gravels were supplied solely by fluvial activity, this hypothesis is supported by the distribution of the terrace deposits. Their extension across the col near Sedgeberrow indicates that, at some stage, the Carrant and Isbourne systems were linked. It would seem reasonable to argue that the extensive solifluction, which supplied the coarse gravel from Bredon Hill, was responsible for choking of the Isbourne-Carrant confluence, resulting in diversion of the Isbourne northwards, directly to the Avon. This diversion presumably took place within the Upton Warren Tnterstadial.

It is finally worthwhile to relate the active supply of gravels from Bredon Hill to the cambering and slumping which has affected this area of the Cotswolds. Solifluction of these coarse materials requires the prior breakdown of the bedrocks by periglacial activity. As Whittaker (1972) has shown, Bredon Hill has been severely affected by intense cambering and gulling, and this activity may well have been responsible for at least part of the distintegration of the bedrocks.

9. Conclusions

The evidence from the Beckford Terrace suggests that by 27,000 years B.P. the climate of the south Midlands was becoming cooler and somewhat less continental than in the earlier part of the Upton Warren Interstadial. As a result, molluscan and coleopteran faunas became impoverished. There was a reduction in the influence of solifluction and the initiation of a westerly air-stream, causing deflation of the Severn-Avon terraces and introducing wind-blown sands to the Carrant Valley. This climatic change may have been related to the growth in Britain of the main Devensian ice-sheets, marking the termination of the Upton Warren Interstadial period and the establishment of full- glacial conditions.

Acknowledgments. The authors wish to thank Professor F. W. Shotton for providing facilities for Cl4 analysis of the material from Beckford. Dr. D. J. Briggs acknowledges receipt of an N.E.R.C. research studentship, held in the Department of Geography at Bristol University, during the period of this research. Dr. D. D. Gilbertson thanks Mr. B. W. Sparks for commenting on an earlier draft of the section on Mollusca and for providing specimens of Succineapfeiyeri var. schumacheri from the late Pleistocene deposits at Wretton. The authors also gratefully acknowledge the co-operation of the owners and workers of the gravel pits at Beckford and Aston Mill.


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D. J . Briggs, G. R . Coope, D. D. Gilbertson, Department of Geography, Department of Geology, Department of Geography, University, University, University of Adelaide, Sheffield S1 3JD P.O. Box 363, G.P:O. Box 498, Adelaide,

Birmingham B15 2lT S. Australia 5001

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Late Pleistocene terrace deposits at Beckford, Worcestershire, England