land at anchor road/deanery road bristol...

© Cotswold Archaeology

LAND AT ANCHOR ROAD/DEANERY ROAD BRISTOL

STRATIGRAPHY AND PALAEOENVIRONMENT

CA PROJECT: 1314 CA REPORT: 02104

Author: Keith Wilkinson

Approved:

Signed:

Mark Collard

…………………………………………………………….

Issue: 01 Date: 30 September 2002

This report is confidential to the client. Cotswold Archaeology accepts no responsibility or liability to any third party to whom this report, or any part of it, is made known. Any such party relies upon this report entirely at their own risk. No part of this report may be reproduced by any means without permission.


Headquarters Building, Kemble Business Park, Cirencester, Gloucestershire, GL7 6BQ Tel. 01285 771022 Fax. 01285 771033 E-mail: [email protected]

Land at Anchor Road/Deanery Road, Bristol

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CONTENTS

SUMMARY ..................................................................................................................... 4

1. INTRODUCTION ........................................................................................................ 5

2. GEOLOGICAL BACKGROUND .................................................................................. 7

3. METHODOLOGY ........................................................................................................ 7

4. LITHOSTRATIGRAPHY .............................................................................................. 9

5. FORAMINIFERA ......................................................................................................... 13

Introduction ........................................................................................................ 13 Methods ............................................................................................................. 13 Results ............................................................................................................... 14 Discussion .......................................................................................................... 15

6. DIATOM ANALYSIS .................................................................................................... 16

Methods ............................................................................................................. 16 Results ............................................................................................................... 17 Discussion .......................................................................................................... 18 Summary ............................................................................................................ 19

7. POLLEN ANALYSIS ................................................................................................... 19

Methods ............................................................................................................. 19 Results ............................................................................................................... 21 Interpretation ...................................................................................................... 23 Discussion .......................................................................................................... 26

8. PLANT MACROFOSSIL ANALYSIS ........................................................................... 28

Methods ............................................................................................................. 28 Results ............................................................................................................... 29 Discussion .......................................................................................................... 30


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9. DISCUSSION .............................................................................................................. 32

Palaeoenvironments ........................................................................................... 32 Addressing the research questions .................................................................... 35

10. CONCLUSIONS ........................................................................................................ 36

11. ACKNOWLEDGEMENTS ......................................................................................... 38

12. BIBLIOGRAPHY ....................................................................................................... 38

APPENDIX A: NOTES ON POLLEN TYPES .................................................................. 44

LIST OF ILLUSTRATIONS

Fig. 1. Location plan

Fig. 2. Location of geotechnical (GBH) and archaeological boreholes (BH) within the site

Fig. 3. Composite stratigraphy of the site obtained from geotechnical and archaeological

boreholes. For borehole locations see Fig. 2

Fig.4 Sedimentology of Units 2-5 in BH2

Fig. 5a Tree, shrub, climber and herb percentage pollen histogram from BH2

Fig. 5b. Marsh, aquatic, fern and algae percentage pollen diagram from BH2


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SUMMARY

A borehole survey carried out as part of a programme of archaeological investigations at

Anchor Road/Deanery Road, Bristol (centred on ST 58129 72626) has revealed important

new data regarding Neolithic and later prehistoric palaeoenvironments of what is now central

Bristol. Six lithostratigraphic units were recognised in two archaeological (BH2 and BH3) and

three geotechnical bores holes. The base of the sequence rests on Triassic sandstones,

formed between 250 and 200 million years ago. The top of this sandstone has weathered

and undergone soil formation, perhaps as recently as the late Quaternary. Overlying this

geological substrate is a thin mineral silt/clay that appears to have formed on the floodplain

of the river Avon between 9300 and 4000 cal. BC. As a result of movement of that river away

from the sampled site, or because deposition on the floodplain outstripped base level rise,

the area became marginalized from the river. This in turn led to the formation of organic mud

strata from 4220-3790 cal. BC onwards in which relatively well-preserved pollen grains were

found. The palynological data suggest that at this time the site consisted of freshwater pools

at the floodplain edge around which lay alder forest. Further away still, on the surrounding

high ground lime-dominated forests had developed. There is evidence in this part of the

sequence for burning and the influx of ash, perhaps as a result of localised forest clearance

by people. During the middle Neolithic sea-levels rose, and following a marine incursion of

the site around 3500 cal. BC the alder woodland declined. Following this episode a true peat

developed in freshwater conditions. Meanwhile human clearance of the lime forest

commenced. At around 3550-3050 cal. BC sea levels rose once more and the site became

part of the intertidal zone. Further mineral silts and clays were deposited in a saltmarsh

environment. However, some time later, perhaps in the Bronze Age the site was briefly

exposed once more to terrestrial processes, and there is short-lived evidence for human

exploitation of the marsh. Two subsequent episodes of similar saltmarsh exploitation are

recognised in the sedimentological data, but remain undated. In either the later medieval or

post-medieval periods the site was ‘reclaimed’ by the simple expedient of deliberately

depositing substantial quantities of sediment upon it and hence raising it above tidal level.


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1. INTRODUCTION

1.1 The site at Anchor Road/Deanery Road, Bristol (Fig.1; centred on ST 58129 72626)

was the subject of an archaeological evaluation and subsequent excavation by

Cotswold Archaeological Trust on behalf of Beaufort Western Limited (now Crosby

Homes (Special Projects) Limited) in 2000-2001 (CAT 2000; 2001). Both these

investigations concentrated on near-surface remains revealed in standard

archaeological trenches and dating to the medieval and post-medieval periods. To

investigate deeper parts of the site stratigraphy buried beneath recent ‘fill’ and

historic period estuarine/alluvial silts and clays, a borehole survey was also carried

out (Fig. 2), with a particular focus on peat strata that had been discovered as a

subcrop between +1m OD and +4m OD in a previous geotechnical survey, carried

out in March and April 2000 (Geotechnical Engineering Limited 2000) Peat forms is

a variety of depositional environments, but in lowland situations is typically

associated with floodplain edges or intertidal margins. These zones have been

frequently used by past human populations, while because of the waterlogged

conditions in which peat is commonly preserved, the potential for preservation of

archaeologically important material is high. Given that peat strata from central Bristol

have not previously been investigated, or indeed found during the course of

archaeological fieldwork (c.f. Bell 2001), the stratigraphy of the site is hence of both

local and regional significance.

1.2 Following completion of the archaeological fieldwork an assessment was carried out

of the borehole stratigraphy (Wilkinson 2001). This took the form of a detailed

description of the extracted cores and the plotting of the reconstructed stratigraphy,

against that revealed in the geotechnical boreholes (Fig. 3). As a result of this study

recommendations were made for further, more detailed analysis to address the

following questions:

• How old was the stratigraphy found below the ‘made ground’ in which the

medieval and post-medieval remains investigated by Cotswold

Archaeological Trust were found?


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• How did the peat revealed in the borehole studies form – in a brackish or

freshwater environment?

• Were the apparently weathered layers noted during the assessment in, and

at the surface of clays and silts, which occur below the ‘made ground’, the

result of terrestrial processes or merely localised discolourations of the

intertidal/alluvial stratigraphy?

• Was there any direct or indirect evidence of human activity in the terrestrial

sediments (i.e. peats or ‘surfaces’ in the silts and clays)?

• Did the clays and silts represent accumulation in brackish or freshwater

environments?

1.3 This document reports on the results of the analytical studies recommended in the

assessment report to address these questions (Wilkinson 2001). In the next section

the geological background to the area is discussed. Following this the stratigraphy of

the site, as revealed in the boreholes is discussed in relation to laboratory-based

sedimentological analyses and 14C dating, carried out on cores of archaeological

borehole BH2. Following the establishment of this lithological background, two

classes of microfossils that are used as aids to sea-level reconstruction are then

discussed; firstly Foraminifera and then diatoms. The last category of data

presented in the report relate to botanical remains found from the peat itself.

Palynology (i.e. the analysis of sub-fossil pollen) is discussed first, followed by a

study of plant macro-remains. The penultimate section of the report draws the data

of the previous sections together to both present an account of past environments of

the area surrounding the site in prehistory, and addressing the questions set out

above. The concluding pages of the report assess the importance of the new data

for future archaeological and palaeoenvironmental studies in Bristol. The editing

author wrote all sections unless indicated otherwise.

1.4 Throughout this report, except where otherwise indicated all depth measurements

are in relation to the ground surface. 14C dates are discussed in relation to

calibrations carried out using the INTCAL 98 curve of Stuiver et al. (1998). All

calibrated date ranges used have been calculated at 2σ (95.4%) probability using

the OcCAL 3.5 program (Bronk Ramsey 2000).


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2. GEOLOGICAL BACKGROUND

2.1 The site lies close to the modern channel of the River Avon on the edge of central

Bristol. The high ground of Brandon Hill rises behind the site and the Avon Gorge

lies just downstream The drift geology of the area is mapped by the British

Geological Survey (BGS) as ‘Quaternary Alluvium’, a catchall title used by the BGS

to describe all recent (usually Holocene) fine-grained deposits forming in riverine

and intertidal environments. In the case of this site ‘Quaternary alluvium’ includes all

Holocene intertidal and alluvial (senso stricto) silts/clay strata, as well as the

potentially archaeologically important peats. According to the BGS the solid geology

lying beneath the site comprises deposits of Triassic sandstones. These were found

as a subcrop by the geotechnical survey at depths of between 0m OD and –1m OD

(Fig. 3). The same survey demonstrated that 2-4m of fining upwards, red sands and

silts overlies the bedrock deposits, whilst on top of these ‘alluvium’ was found. The

red, fine-grained sediments are likely to be weathered and redeposited sediments

originally derived from the underlying Triassic sandstones.

3. METHODOLOGY

3.1 The original geotechnical investigation 10 boreholes and 15 test pits concentrated

on the northeast part of the site (Fig. 2). Given the equipment used (a cable

percussive drilling rig for boreholes and the rear actor of a mechanical excavator for

test pits) and the purpose of the investigation (aiding decisions regarding

foundation/pile configuration), stratigraphic resolution can be estimated at ±0.10-

0.20m at best. Such a low resolution is of only limited use in archaeological

investigations. Hence for the purposes of the archaeological investigation three

specifically archaeological boreholes were drilled in the south-eastern part of the site

(Fig. 2), where peat had been revealed by the geotechnical boreholes, and the

geotechnical boreholes were widely separated.

3.2 The boreholes were drilled by technicians from an external contractor, supervised by

Cotswold Archaeological Trust, over a period of three days in January 2001. The

author made a visit to the site for a single morning to monitor progress. The works


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were also visited by Bob Jones, Bristol City Council Archaeologist, and Vanessa

Straker, English Heritage South-west Regional Archaeological Scientific Advisor.

Prior to drilling, the uppermost 0.5m of demolition debris forming the top of the

‘made ground’ was removed by a mechanical excavator. A Copco Cobra pneumatic

auger fitted with a 1m core sampling device was used to drill the boreholes. The

procedure for each hole was to drill through ‘made ground’ that still remained using

a gouge head and then sample the underlying stratigraphy in Perspex tubes using

the core-sampler. In order to aid penetration the holes were tapered downwards by

using increasingly narrow diameter core samplers. Individual tubes were marked

with the borehole number, depth and direction of their base and were transported to

the laboratory for further study. This procedure worked reasonably well in the case

of BH2 and BH3, but the deposits in the vicinity of BH 1 were so compact that the

core sampler could not be used. Therefore for this borehole the entire stratigraphy

was drilled using the gouge head and sediment retrieved in the sample chamber

was opportunistically sampled. Because of these problems BH 1 is not considered

any further in this report.

3.3 Upon arrival at the laboratory the sample tubes (and bag samples from BH 1) were

placed in cold storage at 2-3oC. The tubes were then cut lengthways using an angle

grinder to reveal the stratigraphy. This was described according to the Troels-Smith

system commonly used on intertidal deposits (Troels-Smith 1955). However, for the

purposes of this report Troels-Smith nomenclature has been translated into standard

geological terminology (c.f. Tucker 1982). Following completion of the assessment

report (Wilkinson 2001) cores from BH2 were subsequently sub-sampled for

sedimentological, micro- and macro-biological analysis, and 14C dating. The focus

for these sub-samples was peat, encountered between 7.30m and 8.20m below

ground surface and associated deposits. ‘Blocks’ of sediment, each 20mm thick

were removed from the cores between 5.20m and 9.00m for sedimentological

analysis. Similar sub-samples, but of 10mm thickness were removed at varied

intervals for Foraminiferal analysis between 6.55m and 8.43m (12 samples), for

diatom analysis between 6.55m and 7.36m (8 samples), and for palynological

analysis between 7.20m and 8.20m (10 samples). Two samples, each a 100mm

thick block of sediment were removed for plant macro remain analysis between

7.35m and 7.85m, while two 20mm thick samples for 14C dating were taken from

near the top and base of the peat (Table 1). Methodologies used in each of these

analyses, with the exception of the sedimentary tests, are discussed in the relevant

sections below.


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3.4 The sedimentological tests performed on all 177 sub-samples obtained between

5.20m and 9.00m were designed to further characterise the deposit beyond that

obtained from the laboratory description, and to help reconstruct the environment in

which the sampled deposit formed. Four separate tests were carried out (Fig. 4).

Field moisture was determined by determining the percentage loss in mass of a

sample from its state when sampled and following air-drying at 40oC for 24 hours.

The moisture content of sediment relates to a combination of ground water table

height, grain size and organic content. Both low (χlf) and high (χhf) frequency

magnetic susceptibility were subsequently determined on the <2mm fraction of the

dried sub-samples according to the methodology of Gale and Hoare (1991, 221-

226). The results of each were then used to calculate percentage frequency

difference (χfd). Magnetic susceptibility variations are caused by a number of factors

including the type and quantity of iron minerals in the geological source material,

grain size, organic content, soil formation and burning. The last sedimentary test

determined the organic content. This was achieved by combusting sub-samples

previously used for magnetic susceptibility measurement at 550oC for four hours and

measuring the percentage loss in weight.

4. LITHOSTRATIGRAPHY

4.1 The stratigraphy revealed in BH 2 and BH 3, together with that detailed in the logs of

surrounding geotechnical boreholes (GBH) 3, 4 and 7, is broadly similar and can be

grouped into six strata (Units) (Fig. 3). Further details relating the accumulation of

four of these (Units 2-5) were further documented in the results of the sedimentary

tests detailed in the previous section (Fig. 4). The discussion below integrates

stratigraphic and sedimentary data, together with the results of 14C dating.

Unit 1 ‘Made ground’ - (c. +11 - +6.5m OD) 4.2 This stratum comprises a complex of diamicts, gravels and thin lenses of silt, all

(with the possible exception of the latter) deliberately deposited as demolition debris

and levelling material. Brick fragments of a relatively recent date occur throughout

the stratigraphy, suggesting that the whole layer cannot be older than the post-

medieval period. The silt lenses probably relate to localised flood events, from either

the adjacent river or a local stream. The lower boundary of the unit is sharp, while

differences in height between boreholes indicate that it was also undulating.


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Together these data suggest an unconformity between this and the underlying Unit

2.

Unit 2 Upper clay-silts (c. +6.5 - +3m OD) 4.3 In BH2 this unit comprises greyish brown (Munsell 2.5 Y 5/2) bedded clay silts with

occasional lenses of shell fragments and organic detritus, the latter typically

comprising 5-10% by mass of the total (Fig. 4). These characteristics suggest the

deposit formed in low energy intertidal or alluvial conditions. Nevertheless the upper

surface of the unit is a darker and of a more reddish hue (10 YR 3/2 Dark greyish

brown) than the underlying deposits, which may be suggestive of limited

pedogenesis or other weathering. If so, conditions must have been relatively dry

during the final phases of deposition. A similar dark band, some 0.10m thick also

exists in BH 2 at 5.67-5.77m and is accompanied by a minor peak of 15 –8 m3 kg-1

(from c. 10–8 m3 kg-1) in low frequency magnetic susceptibility (Fig. 4). These data

suggest that this part of the stratigraphy is also likely to have suffered minor

terrestrial weathering, possibly during a longer period of ground exposure than was

otherwise usual (e.g. short-lived tidal or flood pattern changes). More substantial

peaks in low frequency magnetic susceptibility occur at 6.10-6.14m and 6.48-6.50m,

although there are no visible changes in sediment morphology. The lower of these

peaks comprises just a single reading and is therefore should not be over

interpreted, but that between 6.10-6.14m is more likely to be a real phenomena. The

size of the latter peak (c. 85 –8 m3 kg-1), suggests more than simply short-lived

weathering, or indeed soil formation (both of which would be visible as a change in

stratigraphy, the latter also as a peak in χfd). It is more likely that burning, either on-

site or close by, and the deposition on site of ash have caused the peak.

Alternatively minute ceramic fragments, which as they consist of burnt clay have

distinctive magnetic properties, may form part of the stratigraphy at this point (c.f.

Stein and Teltser 1989). Either way the peak is an indication of human activity

during the formation of this part of the stratigraphy.

4.4 Between 6.60-6.80m organic carbon content of the sediment in the BH2 cores

almost doubles to around 20%, although there are no visible changes in the

appearance of the stratigraphy. It is therefore likely that during this phase of

deposition the sample site was at a more marginal location with respect to tidal or

river flooding processes than at other times during deposition of the sediments

comprising Unit 2.


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4.5 The lower boundary of Unit 2 with Unit 3 is visually sharp. However, the organic

carbon and field moisture data (Fig. 4) suggest a gradual change from the peat of

Unit 3 to the silt-clay of Unit 2. In other words the contact is conformable and hence

erosion of the peat surface is unlikely.

Unit 3 Peat (c. +4 - +1.5m OD) 4.6 This unit was found in both BH 2 and 3, while the geotechnical borehole descriptions

hint that its occurrence is widespread across the whole eastern part of the site. What

now remains is likely to be the highly compressed remnant of the original peat

accumulation, given its burial beneath 7m+ of overburden, and a relatively

unyielding geological subcrop below (c.f. Allen 1999). This peat probably formed at

the margins of either the intertidal zone or at the edge of a floodplain. A chronology

for peat formation was provided by two AMS 14C dates on samples submitted to the

University of Waikato Radiocarbon Laboratory, New Zealand (Table 1). The results

of these suggest that the peat of Unit 3 formed in the 4th millennium BC, i.e. the early

and middle Neolithic.

Lab. No. Depth (m) Result Calibration 2σ (95.4%)

Wk 10946 7.32-7.34 4594±63 BP 3550-3050 cal. yr. BC

Wk 10947 8.15-8.17 5174±61 BP 4220-3790 cal. yr. BC

Table 1. The results of AMS 14C dating of Unit 3

4.7 Laboratory description of BH 2 reveals the ‘peat’ to be a complex of highly humified

peats and organic muds separated by lenses of silts and clays. These variations are

also revealed by the peaks and troughs in the organic carbon and field moisture

content curves of BH2 (Fig. 4). The most organic-rich part of the sequence is

between 7.46m and 7.76m where organic contents commonly exceed 40%. Below

7.80m the deposits in BH2 are visibly recognisable as organic muds, where minor

changes in organic carbon content reflect variations in mineral in wash during

flood/tidal events versus in situ organic production. From this data a short-lived

period of mineral deposition, and hence flooding/high tidal events can be recognised

between 7.98m and 8.04m.

4.8 Undoubtedly the sedimentological characteristic of greatest interest within Unit 3 in

BH2 is the sustained peak in low frequency magnetic susceptibility at 8.00-8.12m

(Fig. 4). The peak thus occurs in organic mud deposits, but overlaps the lower


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portion of the mineral in wash event noted above. Although the magnitude of low

frequency magnetic susceptibility enhancement is only half that of the peak in Unit 2

(c. 35-45 –8 m3 kg-1 c.f. 85–8 m3 kg-1), it too, probably results from the influx of ash

from local burning. Possible alternative causes, such as soil formation and changes

in sediment source material are not indicated by either the morphology or

percentage frequency difference data (χfd).

4.9 The lower boundary of Unit 3 is diffuse, i.e. conformable where Unit 4 is present as

in BH2 (further confirmed by gradual decreases in organic carbon content across the

boundary), suggesting continual deposition during the associated environment

change, but sharp where Unit 5 underlies it (GBH7). In the case of the latter, the

peat probably formed directly on the surface of the weathered geological substrate.

Unit 4 Lower clay-silt (c. +3.5 - +2.5m OD) 4.10 This is a further mineral deposit of broadly similar character to Unit 2, except that it

is a great deal thinner (c. 0.3-0.7m). Its distribution may be restricted to the northerly

part of the composite borehole transect (Fig. 3), although this could be a factor of

the poor stratigraphic resolution of GBH 7, where it was not found. Unit 4 is likely to

have formed in a similar low energy fluvial/intertidal environment as Unit 2. Indeed

low frequency magnetic susceptibility measurements and organic carbon

percentages are both lower that those noted in Unit 2.

4.11 The lower boundary of Unit 4 is uniformly sharp, indicating an unconformity at the

top of Unit 5. Further evidence of the nature of this contact is the upper surface of

Unit 5 investigated in BH2. Both low frequency magnetic susceptibility

measurements, which show enhancement, and stratigraphic description, indicates

that the top of Unit 5 has been weathered and therefore that a significant time gap

exists between its emplacement and the deposition of Unit 4. In BH3 a thin layer of

fine gravel was found at the top of Unit 5, which is probably a lag deposit, resulting

from the preferential removal of fine-grained sediment by water (Fig. 3).

Unit 5 Red brown bedded silty sands 4.12 This unit consists of a fining upward (normally bedded) sequence of medium sands

to silts, with a minimal (2-3%) organic content. Its base was not found in the

archaeological boreholes, but the geotechnical boreholes suggest that it varies in

thickness from 2-4m. As has already been stated these deposits are likely to have

been produced through erosion and re-working (possibly in a fluvial environment) of


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the underlying Triassic sandstone bedrock. There is no indication from the available

data as to when this process took place. Nevertheless there is possible evidence

from magnetic susceptibility measurements of BH2 (χlf and χfd) that soil formation

has occurred within these deposits prior to their burial by Unit 4. This evidence is

from the minor peak in χlf at the surface of the deposit previously discussed, but

also in the generally high χfd readings throughout. The latter are likely to be the

result of a reduction in the grain size of paramagnetic particles by poorly understood

processes occurring during soil formation (Thompson and Oldfield 1986). However,

it is impossible to determine on present evidence, when these soil-forming

processes may have been active.

4.13 The nature of the boundary between Units 5 and 6 is uncertain as this part of the

stratigraphy was revealed only in the geotechnical boreholes.

Unit 6 Triassic sandstone 4.14 The bedrock geology was only reached in the geotechnical boreholes. The

geotechnical borehole logs suggest that Unit 6 is largely comprised of lithified sands,

although thin bands of conglomerate also occur.

5. FORAMINIFERA

Annette Kreiser

26 Gertrude Road, Norwich

Introduction 5.1 Twelve sediment samples from borehole BH2 were analysed for Foraminifera. The

aim was to assess whether Foraminifera are present and if so, to what extent they

could be used to identify the palaeoenvironment represented by the clay-silts in Unit

2 and the peats of Unit 3.

Methods 5.2 8 cm3 of wet sediment from each sample was wet sieved through 500µm, 125µm

and 63µm mesh sieves. Any foraminifera retained on the 125µm sieve were picked

out and identified at 30-40 x magnification under transmitted or incident light using a


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Brunel BMZ zoom stereo microscope. The 63µm fraction was also examined for the

presence of juveniles but it is generally not possible to confidently identify juvenile

tests to species level. Identification follows Murray (1973; 1979) and interpretation of

their ecology follows Murray (1991) and Haslett et al. (1997).

Depth (m.) No. of tests

>125µm in 8cm3

wet sediment

Species present and total Ecology* Forams

<125µm present

6.55-6..56 3 Jadammina macrescens

3 Brackish,

high-mid marsh

no

6.63-6.64 9 Jadammina macrescens

9 Brackish,

high-mid marsh

yes

6.75-6.76

0 yes

6.87-6.88 2 Jadammina macrescens

2 Brackish,

high-mid marsh

yes

7.01-7.02

0 yes

7.11-7.12

0 yes

7.21-7.22

6 Haynesina germanica

Jadammina macrescens

5

1

Brackish,

mid/low marsh

Brackish

high-mid marsh

no

7.29-7.30

0 no

8.13-8.15

0 no

8.21-8.23

0 no

8.33-8.35

0 no

8.41-8.43 0 no

* ‘Ecology’ refers to a literature-derived classification for the individual species and should not be taken as an inferred habitat

for a particular sample.

Table 2. Summary of the results of Foraminiferal analysis of BH2

Results

5.3 The results are presented in Table 2. Generally, foram tests were sparse, not

exceeding nine individuals in any one sample.


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5.4 Unit 2 (samples 6.55 -6.56m, 6.63-6.64m, 6.75-6.76m and 6.87-6.88m) yielded just

one species; Jadammina macrescens. This foram has an organic, agglutinated test

and is typically found living in vegetated high-middle salt marsh habitats. With the

exception of 6.55-6.56m, the 63µm fraction of the samples in Unit 2 also contained

occasional juvenile tests of J. macrescens plus fragments of adult tests.

5.5 In Unit 3 (samples from 7.01m to 8.43m), samples 7.01-7.02m and 7.11-7.12m

contained no adult forams although the 63µm fractions contained small forms of

both brackish and marine species. It is not unusual for small tests of fully marine

species to be found in estuarine habitats, particularly following storms (Murray

1991). Sample 7.21-7.22m contained five tests of Haynesina germanica plus one of

Jadammina macrescens. H. germanica is a brackish to marine species with a wide

salinity tolerance, though it is most commonly found in middle-low salt marsh

habitats. Below this level, from sample 7.29-7.30m to 8.41-8.43m, there are no

forams present.

Discussion 5.6 The presence of J. macrescens in the clay-silts of Unit 2 indicates that these

sediments have derived, in part at least, from a vegetated salt marsh environment.

However, with so few forams, and the lack of any other supporting evidence (such

as a high proportion of plant detritus usually found in association with these forams),

it is impossible to say whether the clay-silts were formed in a salt marsh, or were

derived from an eroding salt marsh surface elsewhere.

5.7 The presence of forams in the top three samples of Unit 3 suggests the sediment did

form in a brackish environment. However, most of the juveniles in the top two

samples were probably transported some distance. Also, H. germanica found in

7.21-7.22m has a wide salinity tolerance. Therefore the forams in this section of the

sequence are of limited use in reconstructing the environment of deposition.


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6. DIATOM ANALYSIS

Nigel Cameron

Department of Geography, University College London

Methods

6.1 Diatom preparation followed standard techniques: the oxidation of organic sediment,

removal of carbonate and some clay, concentration of diatom valves and washing

with distilled water. Two coverslips, each of a differing concentration of the cleaned

solution, were prepared from each sample and fixed in a mountant of suitable

refractive index for diatoms (Naphrax). Slides were scanned under phase contrast

illumination at magnifications of x400 and x1000. Where it was possible to carry out

percentage diatom analysis, diatom counts are derived from one or more traverses

of a coverslip with a suitable diatom concentration. Several diatom floras and

taxonomic publications were consulted to assist with diatom identification, including

Hendey (1964), Hustedt (1930-1966), Krammer and Lange-Bertalot (1986-1991).

Diatom species' salinity preferences were classified using the halobian groups of

Hustedt (1953, 1957: 199), these are summarised below:

1. Polyhalobian: >30 g l-1 salt

2. Mesohalobian: 0.2-30 g l-1 salt

3. Oligohalobian - Halophilous: optimum in slightly brackish water

4. Oligohalobian - Indifferent: optimum in freshwater but tolerant of slightly

brackish water

5. Halophobous: exclusively freshwater

6. Unknown: taxa of unknown salinity preference.

The principal source used for diatom ecological data was Denys (1992).


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Results

6.2 The eight slides prepared were scanned for their diatom content (Table 3). The two

deepest samples from 7.29-7.30m and 7.35-7.36m were suitable for detailed diatom

counting, but on account of the low valve concentrations the remaining six samples

are not. Diatoms were absent altogether from samples 2 and 3 (6.73-6.74 m BGL

and 6.85-6.86m) and the slide preparations have a high content of silt and clay.

6.3 The lowest sample from Unit 2, 7.23-7.24m contains two valves of the marine-

brackish species Pseudopodosira westii and a fragment of the marine species

Paralia sulcata. The sample from 7.13-7.14m has a single valve of Paralia sulcata

present. A small number of valves of marine, marine-brackish and brackish water

taxa are present at 7.03-7.04m. The marine diatoms are Paralia sulcata, Podosira

stelligera and Rhaphoneis sp.; the marine-brackish species Pseudopodosira westii

is present; and the brackish water species Nitzschia punctata is present. In the

topmost sample, 6.55-6.56m, marine (Podosira stelligera), probable brackish (cf.

Chaetoceros cysts) and freshwater, aerophilous taxa (Pinnularia major and

Pinnularia microstauron) are all present, as are Chrysophyte cysts.

Table 3. BH2: results of slide scanning (m - marine; m-b marine-brackish; b - brackish; f -

fresh)

6.4 Diatom analysis of the two basal samples reveals a diverse diatom flora. The marine

component of the flora increases from about 20% at 7.35m to 40% at 7.29m whilst

there is a decline of similar magnitude in the cumulative abundance of

mesohalobous and other brackish water halobian groups. In the bottom sample the

most abundant diatom is a benthic, brackish water taxon Nitzschia cf. vasta (15%)

Diatom Lab.

Sample No.

Depth (m) Diatoms

present

Quality of

Preservation

Valve

Conc.

Species

Diversity

Assemblage

Type

1 6.55-6.56 + poor low low m, b, f

2 6.73-6.74 - - - - -

3 6.85-6.86 - - - - -

4 7.03-7.04 + poor low low m, m-b, b

5 7.13-7.14 + poor poor low m

6 7.23-7.24 + poor poor low m, m-b

7 7.29-7.30 + good high high see count

8 7.35-7.36 + good high high see count


18


and the second most abundant taxon is the planktonic marine diatom Paralia sulcata

(11%). In the sample from 7.29m the planktonic marine diatoms Paralia sulcata

(18%) and Cymatosira belgica (11%) are most abundant, although brackish water

taxa such as Nitzschia hungarica, Nitzschia cf. vasta and salt tolerant freshwater

taxa such as Navicula rhyncocephala are also common. This shift to a more saline

flora appears to continue in the three succeeding samples where the surviving

valves are mainly marine and marine-brackish taxa. However, the survival of these

taxa is probably also a reflection of their robust structure and the poor conditions for

preservation.

Discussion

6.5 The uppermost sample, 6.55-6.56m from Unit 2 contains a mixture of poorly

preserved diatoms from marine (Podosira stelligera), brackish (cf. Chaetoceros

cysts) and freshwater habitats. However, the two freshwater Pinnularia spp.

identified from this sample are aerophilous diatoms and this observation, along with

the presence of chrysophyte cysts, is consistent with an input of terrestrial material

into a tidal, aquatic environment.

6.6 The transitional and organic sediments of Units 2 and 3 are represented by diatom

samples 5 to 8. All of these samples show that the site of deposition was tidal with a

high proportion of allochthonous marine diatoms and brackish water diatoms from

estuarine habitats. Benthic, mud-surface, habitats are well represented in the

autochthonous diatom assemblages from 7.29m and 7.35m depth. These benthic

species include a number of mesohalobous Nitzschia spp. and naviculoid species

such as the halophilous Navicula cincta and oligohalobous indifferent Navicula

rhyncocephala. Other diatoms represent attached (epontic) habitats such as the

surfaces of aquatic macrophytes. These species include oligohalobous indifferent

Gomphonema spp., Achnanthes spp. and Cocconeis spp. Other epontic diatoms

include oligohalobous to halophobous Eunotia spp which are likely to be epiphytic.

These halophobous, acidophilous diatoms do not grow in brackish water and their

presence reflects the input of freshwater of relatively low pH which may for example

originate from acidic bedrock such as sandstone.


19


Summary

6.7 Diatoms are present and well preserved in two samples from 7.35m and 7.29m.

Fragmentary diatom assemblages are present at 7.23m, 7.13m, 7.03m and 6.55m

below ground level. Diatoms are absent at 6.85m and 6.73m.

6.8 All the diatom assemblages that are preserved show evidence for brackish water,

tidal conditions with a significant component of allochthonous, marine plankton.

Autochthonous, brackish to freshwater, diatom floras are represented in the two

basal samples for which it was possible to make percentage counts. Allochthonous

diatom inputs from terrestrial and acidic freshwater sources have also been

identified.

6.9 There is an apparent increase in salinity from the base of the sequence to around

7.00m below ground level.

6.10 The uppermost diatom sample, from Unit 2, shows evidence for the input of

terrestrial material.

7. POLLEN ANALYSIS

Heather M. Tinsley

Department of Geography, University of Bristol

Methods

7.1 Prior to processing at the Environmental Archaeology Laboratory at Bristol

University, the samples were examined and the sediment described (Table 4)


20


Sample Number Depth from top of BH2 in metres Sediment type

1 7.20-.21 clay with organic remains

2 7.30-.31 clay with organic remains

3 7.38-.39 peat

4 7.50-.51 peat

5 7.62-.63 peat

6 7.74-.75 peat

7 7.86-.87 peat with some clay

8 7.97-.98 peat

9 8.09-.10 peat with silty sand

10 8.21-.22 peat with silty sand

Table 4. Description of stratigraphy as revealed in the palynological sub-samples

7.2 All samples were prepared using standard techniques (Moore, Webb and Collinson

1991). Initial digestion in dilute potassium hydroxide was followed by sieving, then

treatment with cold hydrofluoric acid for a week. Samples were washed with hot

10% hydrochloric acid and acetolysed, stained with safranin and mounted in

glycerol. Two tablets of Lycopodium spores were added to each sample at the start

of the preparation to allow pollen concentration to be assessed (Stockmarr 1971).

Samples were counted at a magnification of x400 with x1000 magnification used for

critical determinations. The pollen sum aimed for was >500 land pollen grains with

true aquatics counted outside this total. In addition to pollen, spores of ferns and of

the filamentous green algae Spirogyra and Mougeotia were counted. An unknown

spore, van Geel Type 128, (van Hoeve and Hendriske 1998) occurred in all

samples, and was also counted. The abundance of charcoal was estimated by

counting the numbers of charcoal particles greater than 40µm long on two traverses

of each slide.

7.3 Plant nomenclature follows Stace (1991), which was also used as a source for

ecological information. Pollen types generally follow Bennett (1994). A note about all

pollen types is to be found in the Appendix at the end of this report.

7.4 Pollen preservation was variable: in samples 1 and 2 it was moderate with some

grains fragmented and shrunken, though still identifiable; it was good in samples 3-

5, and moderate in samples 6-10 where corrosion of grains was common, but again

this was not a major problem in identification. The percentage of unidentifiable,


21


degraded pollen grains for each sample is shown in Fig. 5b. Pollen concentrations

were also variable, high in the central peat band, but much lower in the more mineral

rich samples. The pollen concentration in sample 10 was so low that a total of only

311 land pollen grains were counted.

Results

7.5 The results are presented in the form of a pollen diagram drawn using Tilia software

(Grimm 1990) (Fig.s 4a and 4b). Pollen data are expressed as percentages of total

land pollen (TLP). All pollen taxa included in the pollen sum are shown in the

diagram as solid bars. Aquatic taxa were counted outside the pollen sum and

expressed as percentages of total pollen plus aquatics (TPA). Fern spores were

counted outside the pollen sum and expressed as percentages of total pollen plus

ferns (TPF). Algal spores and charcoal fragments are also included in the pollen

diagram, but are shown as actual counts, not percentages. All taxa excluded from

the pollen sum are shown on the diagram as open bars. At the start of the diagram

there is a summary of the changing proportions of pollen of woody and herbaceous

species. The pollen types have been organised into three groups to aid in

interpretation – trees, shrubs and climbers, herbs (various environments), and

marsh and aquatic plants. The groupings of taxa are flexible; some pollen taxa have

members which inhabit a variety of ecological niches. For instance, Ranunculus

acris-type is a large taxon, which includes species typical of grassland (e.g. R. acris,

meadow buttercup, R. bulbosus, bulbous buttercup), but also some wetland species,

such as R. flammula and R. lingua (greater and lesser spearwort). The

Brassicaceae (cabbage family) include many herbs of disturbed ground but also

Rorippa - the water-cresses. Thus some of the flowering herbs included in the ‘herbs

(various environments)’ group may well have grown in marshland.

7.6 In this report, pollen of Corylus-type includes pollen of Corylus avellana (hazel) and

of Myrica gale (bog myrtle). The distinction between pollen of Corylus and Myrica is

not easy to make, Andrew (1984) noted that Myrica pollen can be identified on the

basis of the sloping ‘shoulders’ leading to each pore. All Corylus-type pollen grains

counted in this assessment have been assumed to be Corylus, but it is possible that

some are Myrica. Pollen of Cereal-type was distinguished from that of other grasses

on the basis of size, with all grains >40µm in diameter considered to be in this

group. Some (few) wild grasses also have grains of this size, they are Spartina

anglica (common cord grass), Ammophila arenaria (marram grass), Leymus


22


arenarius (lyme grass) which are all coastal species, also Glyceria (sweet grass),

which grows in mud by ponds and rivers, and Aira caryophyllea (hair grass) and

some Elytrigia species (couch grasses) which grow in dry sandy places (Moore,

Webb and Collinson 1991). In view of the topographic position of these peats, close

to the River Avon, it is possible that the Cereal-type pollen is that of Glyceria.

7.7 The pollen diagram has been zoned on the basis of changes in the tree pollen

curves. Two assemblages are recognised, Deanery Road 1 (DR1) and Deanery

Road 2 (DR2). The characteristics of these two assemblages are described below.

DR 1 (8.22m – 7.90m) Alnus-Tilia assemblage

7.8 Tree pollen forms between 71% and 82% TLP in this assemblage. Alnus (alder) is

the dominant tree taxon, forming 48% TLP in the basal sample, declining to around

32% TLP at the top of the zone. Tilia (lime) forms 14-16% TLP. Quercus (oak) and

Corylus-type (hazel) fluctuate between 6% and 14% TLP and 7% and 16% TLP

respectively. Pinus (pine), Ulmus (elm) and Betula (birch) are all represented at <2%

TLP. Occasional pollen grains of Fraxinus (ash), Salix (willow) and Viscum album

(mistletoe) occur. The principal herbaceous pollen types are Cyperaceae (sedges)

(7-15% TLP) and Poaceae (grasses) (3-12% TLP). A few Cereal-type grass pollen

grains occur. A range of flowering herbs is present, all at values of <1% TLP

including Plantago lanceolata (ribwort plantain), P. major (greater plantain),

Ranunculus acris-type (buttercup), Silene vulgaris (bladder campion), S. dioica (red

campion), Brassicaceae (cabbage family), Fabaceae (pea family), Lactuceae

(dandelion and related Asteraceae), Solidago virgaurea-type (daisy, aster and

related Asteraceae) and Artemisia (mugwort). A single grain of Erica pollen was

found. Chenopodiaceae (goosefoot family), a taxon associated with both disturbed

ground and halophytic environments, is represented at values of up to 3% TLP.

Occasional pollen grains of the wetland types Lythrum salicaria (purple loosestrife),

and Mentha-type (mints) occur. True aquatics such as Typha latifolia (bulrush),

Sparganium emersum-type (bur reeds, lesser bulrush), Sparganium erectum

(branched bur reed), Alisma plantago-aquatica (water plantain), and Samolus

valerandi (brookweed) are found at frequencies of <1% TPA. Fern spores, including

Pteridium (bracken), Polypodiaceae (polypody fern) and undifferentiated ferns, form

between 15% and 32% TPF; they are most frequent in the basal sample. Occasional

spores of the filamentous green algae Spirogyra occur along with a few spores of

van Geel Type 128. Pollen concentrations are very low in the basal sample (8.21m)


23


but then rise to around 100,000 grains/cc. Microscopic charcoal fragments >40µm in

length occur throughout the zone at low frequency.

DR 2 (7.90m – 7.20m) Quercus-Corylus assemblage

7.9 In this zone tree pollen is somewhat reduced compared with DR1, fluctuating

between 55% and 74% TLP, this is due to a decline in both Alnus and Tilia, though

increases in Quercus and Corylus-type partly compensate. Around 7.90m Alnus

declines markedly, and forms 9-15% TLP throughout the zone. Tilia declines more

gradually to around 5% TLP above 7.85m. Quercus and Corylus-type both increase

gradually. Frequencies fluctuate between 12 and 27% TLP for Quercus, and

between 14 and 37% TLP for Corylus-type. Pinus and Ulmus are represented

throughout the zone at values of <3% TLP. Occasional grains of Betula, Fraxinus

excelsior, Salix, Sambucus nigra (elder), Sorbus-type (includes whitebeam), Prunus

(cherry), and Hedera helix (ivy) occur. Herbaceous pollen increases slightly,

principally Poaceae which peak at 25% TLP in the middle of the zone (7.62m), and

Cyperaceae which peak at 18% TLP at 7.50m. A few Cereal-type pollen grains

continue to be represented at low frequencies. The range and frequency of most

flowering herbs is very similar to DR1, however, the start of the zone is marked by a

peak in Chenopodiaceae which reach 11% TLP at 7.86m and then fall to values of

<3% TLP before rising to 5-6% TLP above 7.40m towards the top of the zone.

Wetland types are represented mainly by Typha latifolia and Sparganium emersum-

type, the latter reaches 10% TPA at 7.50m. Fern spores are present throughout the

zone at values slightly lower than those of DR1. There is quite a marked increase in

the spores of Spirogyra and of van Geel Type 128 in DR2, and around 7.38m a

number of the distinctive spores of the algae Mougeotia occur. Pollen concentrations

rise markedly in the middle of the zone to values of over 200,000 grains/cc, but fall

back to only 20,000 grains/cc in the upper most sample. Microscopic charcoal is

found throughout DR2 but at very variable frequencies, becoming abundant at

7.76m, then declining markedly above this.

Interpretation

7.10 The peat bed at the site appears to have accumulated in a damp hollow close to the

River Avon and surrounded by trees. The deepest sediments examined for this

report contain a significant inorganic component, which possibly suggests some

active water movement at the site prior to the development of the marsh, and this


24


also accounts for the low pollen concentration in the basal sample. Peat

accumulation began around 5174 BP, (4220-3790 cal BC) in the early Neolithic

period.

7.11 In zone DR1 the pollen evidence suggests that the marsh was fringed by wet alder

woodland and that beyond this, probably on higher ground, there was extensive dry-

land woodland dominated by lime with some oak and hazel and occasional elm and

ash. Tilia (lime) pollen percentages underestimate the importance of lime in the

vegetation, Tilia is insect pollinated and the large sticky grains are shed close to the

tree. Work by Andersen (1970) suggested that pollen percentages for Tilia needed

to be corrected by a factor of x8, in order to be compared directly with Quercus.

Greig (1982) summarized evidence for the past importance of Tilia in the mid-

Holocene forests of Britain and Europe, using Andersen’s correction factor. The

pollen sum Greig used excluded Alnus and Corylus; if the DR1 data is recalculated

in this way, corrected values for Tilia are between 80% and 90%. According to Greig

(1982), corrected values of Tilia pollen > 60% indicate lime as the major forest

component. It is clear, therefore, that the dry-land woodland around the area at this

time was lime-dominated.

7.12 The marsh itself supported a vegetation of grasses (possibly Phragmites, common

reed) and Cyperaceae (sedges), with some Sparganium (bur reed). There appear to

have been some pools of open water, which supported aquatic communities

including Alisma plantago-aquatica (water plantain), Lemna (duckweed) and

Samolus valerandi (brookweed) as well as the filamentous green algae Spirogyra.

The range of other herbaceous pollen taxa found at low frequency in DR1 include

types which might have grown on the marsh, such as Brassicaceae (cabbage family

– includes water-cresses and cuckoo flower) and Ranunculus acris-type (buttercup,

spearwort). Other taxa may have been associated with the woodland edge such as

the Caryophyllaceae (pink family- including red campion), the Apiaceae (carrot

family- hedge parsley etc) and the Fabaceae (pea family – vetches etc).

7.13 Another group of herbs, which are represented at low frequency, are associated with

disturbed ground, these include Plantago lanceolata and P. major (ribwort and

greater plantain), Rumex (sorrels and docks), Lactuceae (dandelion family and

related Asteraceae), Solidago virgaurea-type (daisy and related Asteraceae) and

Artemisia (mugwort). These herbs are often associated with anthropogenic activity,

however in the case of waterside or estuarine situations disturbance might be the


25


result of natural processes of erosion resulting in unstable soils. The occurrence of

occasional grains of Cereal-type pollen at low frequencies in the pollen record could

indicate cultivation in the area, but the grasses in this taxon include Glyceria fluitans

(floating sweet grass), a freshwater aquatic grass which may have been growing in

the marsh, or at the edge of the river.

7.14 The presence of pollen of Chenopodiaceae (goosefoot family) in all samples from

zone DR1 is of interest. This is a large family with members growing in a wide range

of habitats, some anthropogenic, others influenced by brackish or salt water. It is

very difficult to distinguish pollen of different species and this has not been

attempted here. This pollen taxon could therefore be representative of plants

growing on salt marshes, strandlines, or anthropogenically disturbed ground. In view

of the situation of the site, close to the tidal River Avon, the presence of

Chenopodiaceae in DR1 may well indicate salt marsh communities in the vicinity.

Solidago virgaurea-type is another taxon which includes some plant species

associated with salt marshes such as Aster tripolium (sea aster). The increase in

pollen of Chenopodiaceae at the start of zone DR2 to 10% TLP occurs at the level

where clay was noted in the pollen sample. This suggests that the site was

influenced by an incursion of seawater bringing mineral material and resulting in

brackish conditions. This did not result in elimination of all the freshwater marsh

plants, Alisma plantago-aquatica (water plantain), for example, continued to be

present, but it may well have been the factor which triggered the decline in Alnus at

the start of DR2. At this stage the fringing alder woods were reduced and, as a

result, increasing pollen of oak and hazel reached the marsh surface. The high

Chenopodiaceae values are not maintained throughout DR2, suggesting that the

incursion was relatively brief, however the alder woods did not recover their former

extent. Between 7.75m and 7.31m the deposits are entirely organic and this also

supports the view that salt water flooding of the site ceased. Above 7.65m there are

increased frequencies of pollen of freshwater aquatics such as Typha latifolia

(bulrush) and Sparganium emersum-type (bur reeds, lesser bulrush) and peaks in

the counts for Spirogyra and Mougeotia, filamentous green algae associated largely

with freshwater conditions (van Geel 1986). The frequency of spores of van Geel

Type 128 also increases. This is a small circular spore with spines, often with a

curved furrow or suture. According to van Hoeve and Hendrikse (1998), it is

indicative of shallow, eutrophic fresh water. At the top of the pollen diagram, above

7.3m, there is a renewed increase in the frequency of Chenopodiaceae pollen (Fig.

5a) and this again correlates with increasing clay in the sediment, suggesting


26


another episode of flooding by salt or brackish water dated to just after 4594 BP

(3550-3050 cal BC).

7.15 Another significant change that occurs soon after the start of Zone DR2 is the

gradual decline in lime pollen. This is the result of changes in the dry-land

woodlands and does not appear to be related to the decline of alder around the

marsh. It is a much more gradual reduction than that of alder, with lime declining

from 14% TLP at the end of DR1 to 5% TLP by the middle of DR2. Using Greig’s

correction calculations (referred to above), this gives a corrected value for Tilia

pollen in DR2 of between 40% and 60% suggesting (according to Greig 1982) that

lime formed about half the tree cover. Compared with the woods of DR1, lime had

therefore undergone a significant decline. It is possible that this had an

anthropogenic cause; a lime decline is a widely recorded feature in post-Elm Decline

pollen diagrams from England and Wales, though the dates at which it occurs vary.

In mid-Wales, it appears to be rather later than at this site and it has been linked

with Bronze Age human activity (Walker 1993). However, on the Somerset Levels

(Caseldine 1988) a decline in lime follows the initial elm decline, which is dated c.

4800 BP. The date for the start of the decline in lime at the site must be rather

similar to this, as it lies between the two dated horizons of 5174 BP (4220-3790 cal

BC) and 4594 BP (3550-3050 cal BC). Despite the decline, lime remained an

important forest element along with oak in the dry-land woods in the Avon valley.

7.16 The presence of charcoal throughout both zones of the pollen diagram indicates that

anthropogenic activity was present in the area. However, relative charcoal

frequencies are fairly low except at 7.76m where they reach a peak, it may be

significant that this occurs when lime is declining, supporting the view that this

change in forest composition may have been a result of increasing anthropogenic

activity.

Discussion

7.17 The pollen record from the site is extremely important in the context of the Bristol

area. This is the first pollen diagram recording the pre-settlement vegetation of the

Avon valley around Bristol. It provides a record of the history of the Avon woodlands

during the Neolithic period.


27


7.18 The pollen evidence suggests that the site was freshwater marshland, which was

subject to an incursion of brackish water sometime between 4220 and 3050 cal BC.

This appears to have influenced the alder woods around the marsh, causing them to

be reduced in area or density. This ecological response to brackish/salt water

flooding is similar to that recorded in the pollen diagram from The Sweet Track F site

on the Somerset Levels (Coles, Hibbert and Orme 1973) where a decline in Alnus

pollen occurred prior to the flooding event which resulted in the deposition of the

Lower Wentlooge clay around 5210-4800 cal. BC (Tinsley 2002). The peat at this

site is very compressed, the 14C dates indicate that it formed over a minimum of 240

years but a maximum of 1170 years. It is therefore possible, given the resolution of

the pollen sampling, that other short-lived incursions may have taken place which

have not been picked up.

7.19 The pollen evidence demonstrates the importance of lime in the dry-land woodlands

of the Avon valley between 4220 and 3050 cal. BC. The significance of lime as a

forest component in the mid-Holocene woodlands of England and Wales has been

underestimated in the past owing to the problems of lime pollen representation

discussed above. However, evidence suggests it was probably widespread in the

South Gloucestershire and North Somerset area. A pollen diagram from Walton

Moor in the Gordano valley, North Somerset, has values for lime of around 10% total

pollen (uncorrected) in the mid-Holocene (Jeffries, Willis and Yem 1968). A buried

land surface on the Avon Levels of South Gloucestershire, which is dated between

3300-2200 cal. BC and 2930-2460 cal. BC, has lime frequencies of 5-10% TLP

(uncorrected) (Tinsley 1999). It is therefore likely that lime was the major forest

dominant around both of these sites. The decline in Tilia pollen, which starts around

the DR1/DR2 boundary at the site, reflects the gradual opening up of the lime-

dominated woodlands as a result of human activity during the Neolithic. In view of

the local importance of the site, it would be very useful to have a 14C date for the

lime decline around the DR1/DR2 zone boundary.

7.20 It is clear from the pollen evidence that elm was only a minor forest component in

the late Mesolithic/Neolithic woodlands of the Avon valley. Values for Ulmus pollen

are low in DR1 and remain low at the start of DR2. There is no obvious Elm Decline

horizon, a feature that is recognised at sites throughout Britain and dated to around

5000 BP (c. 3700 cal. BC). On the Somerset Levels, to the south, the first major

decline in elm has been dated at a number of sites to around 4800 BP, though some

dates are as early as 5300 BP (Caseldine 1988). The low values for Ulmus pollen in


28


the earliest peat which formed at the site around 5174 BP (4220-3790 cal BC)

suggest that the initial Elm Decline had either already taken place by this time, or

that elm was an insignificant contributor to the local forests compared to lime. There

is a temporary increase in Ulmus pollen in the middle of DR2, this occurs after the

decline in Tilia pollen, and it may simply indicate that the few elm trees present in

the woods were able to flower more freely as lime declined in frequency.

7.21 During the later prehistoric period, and during historic times the lime-dominated

woodlands of the Avon valley were reduced even further, but ancient woodland

remains today on the steep slopes of the Avon Gorge on its western side. Tilia

cordata (small leaved lime) is still an important species in some of the areas of

ancient coppice on the side of the gorge and in Leigh Woods. Rackham (1982), in

an account of the ecology and historical features of Leigh Woods, suggested that

lime was likely to have been the dominant species in the wildwood of this area. The

pollen diagram from this site supplies the evidence to support this assertion. The

present Avon Gorge woodlands include Prunus avium (wild cherry) and rare species

of whitebeam (Sorbus); it is interesting to note that both Sorbus and Prunus pollen

are recorded at very low frequency in the pollen diagram. Thus this site has

demonstrated a link between the woodlands which persist in the Gorge today and

those which grew in the valley of the Avon in Neolithic times.

8. PLANT MACROFOSSIL ANALYSIS

Julie Jones

Department of Geography, University of Bristol

Methods

8.1 As the samples were recovered from borehole tubes, they were of limited size,

100ml/95g (7.35-7.45m) and 75ml/80g (7.75-7.85m). They were soaked in warm

water and then sieved through a nest of sieves to a minimum mesh size of 250µm.

The bulk of both samples were mineral with limited organic preservation and in both

samples much of the organic matter was composed of small unidentifiable

fragments with dimensions of <1mm. However a limited assemblage of seeds was


29


recovered from each sample, the uppermost sample, 7.35-7.45m, showing the

better preservation. Occasional fragments of beetle and cladoceran ephyppia

(water-flea egg cases) were also noted. Full details are shown in Table 5.

Nomenclature and habitat information is based on Stace (1991).

Results

7.35-7.45m below ground surface

8.2 The organic matter in this sample was very fragmented, of small dimensions from

500µm - 1mm, with many of the seeds present also fragmented. Some remains had

lost the outer seed coat, e.g Chenopodiaceae indet (Goosefoot family), which meant

that not all seeds could be identified to species level. However the limited

assemblage suggests a predominantly wet environment with areas of open water

suggested by the presence of aquatic species such as pondweed (Potamogeton sp),

horned pondweed (Zanichellia palustris) and water crowfoot (Ranunculus subg.

Batrachium). Horned pondweed is a species of still to moderate flow in streams and

lakes in depths of up to 2m and includes slightly brackish water conditions. Both

water crowfoot and pondweed include some species, which can tolerate brackish

water conditions, but are primarily fresh water plants. Glaucous Bulrush

(Schoenoplectus tabernaemontani) can also tolerate brackish soils in lakes, pools

and ditches in depths of up to 0.5m, with common clubrush (Schoenoplectus

lacustris), a species of still to moderate flow in freshwater lakes, ponds and streams.

This can occur in association with water plantain (Alisma sp), a freshwater plant of

shallow margins. Overall the assemblage suggests a freshwater environment with

still to moderate water flow, but with a suggestion of brackish water influence.

7.75-7.85m below ground surface

8.3 The assemblage from this sample was more limited and preservation was poor.

Fragments of organic matter were mostly <1mm and largely unidentifiable.

Macrofossils were fragmented, or particularly in the case of the Alisma species, had

lost their outer casing. The most commonly occurring species were water plantain

and water crowfoot, as in the previous sample, plus single bulrush (Typha sp.) and

rush (Juncus sp.) seeds suggesting a predominantly freshwater environment. The

presence of greater plantain (Plantago major) also suggests a drier grassy or rough

ground habitat.


30


Discussion

8.4 The evidence provided by the samples examined, although limited, points to these

sediments being laid down in a predominantly freshwater environment, with some

indication of brackish water influence in the sample from 7.35-7.45m. The

occurrence of a thin lens of organic mud within Unit 3, suggests an episode of

slightly wetter conditions which temporarily halts the development of peat growth.

The presence of brackish water indicators in the peat above this organic mud may

suggest that this was a marginal area, perhaps in a transitional zone at the limit of

an upper saltmarsh, but that at this location the environment was largely freshwater.

There is no evidence for the age of these deposits and no suggestion of human

activity associated with them.

8.5 It is difficult to compare the results with the evidence provided by pollen and diatom

analyses (from 10mm spits), due to the thickness of the plant macro sample

(100mm). However, it may be possible to correlate the sample from 7.35-7.45m with

the deepest diatom sample from 7.35-7.36m, which suggests that the sediments

accumulated under brackish conditions. Here the most abundant diatoms are

species of benthic brackish water taxa and planktonic marine species. The pollen

record from 7.38-7.39m shows a small increase in Chenopodiaceae also suggesting

a possible increase in brackish conditions towards the top of the sequence

examined.

8.6 There are no diatom samples from an equivalent depth to the lowest sample

examined (7.75-7.85m bgs) and the pollen sample occurs just below this level at

7.86-7.87m bgs, although again a peak in Chenopodiaceae pollen occurs.


31


Depth below surface 7.35-7.45m 7.75-7.85m

Sample size (ml/g) 100/95 75/80 Habitat

CHARACEAE

Chara spp Stonewort 2 A

RANUNCULACEAE

Ranunculus subg. Batrachium

(DC.)A.Gray

Water Crowfoot 20 10 APR

CHENOPODIACEAE

Chenopodium rubrum/glaucum Red/Oak-leaved Goosefoot 2 CDs

Chenopodiaceae indet 3

POLYGONACEAE

Rumex sp Dock 1 frag DG

PLANTAGINACEAE

Plantago major L. Greater Plantain 1 CDG-o

ALISMATACEAE

Alisma sp Water Plantain 4 11 APR

POTAMOGETONACEAE

Potamogeton sp Pondweed 1 APR

ZANICHELLIACEAE

Zanichellia palustris L. Horned Pondweed 7 APR-fresh

&brackish

JUNCACEAE

Juncus sp Rush 1 GMRw

CYPERACEAE

Carex spp Sedge 1 +2f GMPRW

Schoenoplectus lacustris (L.)Palla Common Club-rush 5 BPR-

shallow

Schoenoplectus tabernaemontani

(C.Gmelin)Palla

Grey Club-rush 5 BPRs-

marshes,

duneslacks

& wet peaty

places

mostly near

sea

Schoenoplectus/Scirpus spp Club-rush/ 9f

POACEAE

Poaceae indet Grass 1

TYPHACEAE

Typha sp Bulrush 1 PR-reed,

swamp

Other remains

Beetle fragments <20 <5

Cladoceran ephyppia 4 2

Key: Habitats A: Aquatic. B. Bankside. C: Cultivated/Arable. D: Disturbed. G: Grassland. M: Marsh. P: Ponds, ditches -

stagnant/slow flowing water. R: Rivers, streams. o: open habitats. s: coastal. w: wet/damp soils

Table 5: Waterlogged plant remains.


32


9. DISCUSSION

9.1 This section is divided into two parts. In the first a palaeoenvironmental history of the

site is proposed, based on the data presented in Sections 2-8 above. Following this,

the questions set by the assessment report (Wilkinson 2001), and with which this

document began are addressed.

Palaeoenvironments

9.2 The rocks (Unit 6) on which the sequence sits are of Triassic date and formed in

riverine and alluvial fan environments at 250-200 million years ago. Since that date

the upper 2-4m of these sandstones have weathered forming unconsolidated

medium and fine sands (Unit 5). Within at least the top 0.5m of these deposits soil

formation has taken place, as demonstrated by magnetic susceptibility evidence

(χfd), although exactly when this occurred is open to question. The upper surface of

the unconsolidated sands has undergone more intense and probably more recent

(?late Quaternary) weathering as demonstrated by a minor peak in low frequency

magnetic susceptibility and morphological changes. Nevertheless an unconformity

and consequently a temporal hiatus exist at the top of the unit. The overlying clays

and silts (Unit 4) would appear, on the basis of their morphology to have formed in

an aquatic environment, probably during the first half of the Holocene (c. 9300-4000

BC). The complete absence of Foraminifera in a series of samples from these

deposits can tentatively be suggested as indicating that accumulation was in a

freshwater environment (Foraminifera do not inhabit freshwater environments). If so

this is likely to have been on the floodplain of the early Holocene river Avon, which

was not subject to tidal influences in this part of its reach before the mid Holocene.

9.3 Sometime around 4220-3790 cal. BC the site became marginalized from the river

Avon. This may have been caused by deposition on the floodplain outstripping the

rate of base level rise, or a migration of the river channel away from the site; the

present data do not allow a firm interpretation. However, the result was that organic

muds began to develop on the floodplain margin (Unit 3). Pollen preserved in the

muds suggests that pools of open water existed on the site, fringed by alder

woodland. On higher ground, perhaps even the slopes of Brandon Hill, lime


33


woodland existed with lesser forest components of oak, hazel and elm. Although

there is very limited evidence for human impact during the accumulation of the

organic muds from the palynological evidence (i.e. pollen grains that could be arable

weeds, cereals and indicators of ground disturbance were all found, but could also

be a result of increasing tidal influence in the region), there is a suggestion from the

magnetic data which indicate that people were active in the landscape. A sustained

peak in low frequency magnetic susceptibility from the middle of the organic muds is

most likely to have been caused by the deposition of ash on the floodplain margin.

This is unlikely to have come from the immediate vicinity of the sample site, but

could be the result of Landnam-type clearances (sensu Iversen 1941) further away,

which are typical of the early Neolithic period in southern England. These

clearances, being of relatively short duration are also notoriously difficult to

recognise in pollen diagrams, particularly where close interval sampling has not

been undertaken (as here). During the period of the early Neolithic represented by

the accumulation of the organic muds, the surrounding alder woodland gradually

declined. This was probably the result of minor incursions by tidal waters – to which

alder is not tolerant. One of these events is recognised in the organic carbon record

as a trough between 7.98-8.04m.

9.4 At approximately the contact between the organic mud and the peat (c. 7.85m) there

is a significant change in the pollen assemblage. The alder woodland declines still

further, while a peak occurs in Chenopodiaceae. As many species of the latter family

are characteristic of intertidal environments, a tidal incursion is the most likely

explanation of the changes. However, this would seem to have been short-lived as

the pollen of taxa of freshwater preference increase in overlying samples, while only

freshwater plants are found in the plant macrofossil sample from this part of the

stratigraphy. It would therefore seem that although the site was located close to the

high tide mark (probably Highest Astronomical Tide [HAT]), peat growth rapidly

outstripped sea level rise. Further vegetation changes also occurred during the

accumulation of the peat that have nothing to do with tidal processes. A decline in

lime pollen, accompanied by a peak in microscopic charcoal suggests that people

were impacting on the forest of the surrounding areas by about 3500 BC. However,

as would be expected in a marginal, boggy area the plant macrofossil evidence

suggests that there was no local impact.

9.5 Sometime between 3550-3050 cal. BC the peat ceased forming and instead

accumulation became dominated by mineral silts and clays (Unit 2). Several lines of


34


evidence suggest that this was the result of further sea level rise with which peat

growth could not keep pace. Diatom assemblages indicate the rapid increase in

importance of marine planktonic forms across this stratigraphic boundary and

generally suggest increasingly saline conditions. Similarly Cyperaceae dominates

pollen assemblages towards the top of the peat, which are here likely to be salt

marsh forms. The plant macro remains found in this part of the sequence, although

of freshwater preference can also tolerate brackish conditions. The transition to salt

marsh was nevertheless gradual as is shown by the slow reduction in organic

carbon content between the peat and overlying mineral sediments. By the time the

lowest mineral sediments of Unit 2 were being deposited, Foraminiferal evidence

suggests that the site lay on either the middle or low saltmarsh zone.

9.6 There are further indications of sea level fluctuation higher in the mineral clays and

silts of Unit 2. Between 6.50-6.80m organic carbon contents rise to levels

comparable with the organic muds of Unit 4, suggesting a more marginal situation

with respect to the foreshore. Foraminiferal evidence also suggests that during this

episode the site was on either the high or mid saltmarsh zone (c.f. low-mid

saltmarsh before), while the diatom assemblages include freshwater, aerophilous

elements and chrysophyte cysts as well as brackish and poorly preserved marine

forms. Thus it would appear that the position of the site had risen in relation to

relative sea level. Indeed the aerophilous diatoms and chrysophyte cysts indicate

the input of terrestrial material, a suggestion that is further supported by a large peak

in low frequency magnetic susceptibility which may be indicative of local burning,

and thus possible human use of the intertidal zone. Although it is impossible to date

this sea-level event, it can very tentatively be assigned to the Bronze Age, when the

rate of sea level rise had slowed in several sea level curves in southern Britain (e.g.

Long, Scaife and Edwards 2000).

9.7 A further episode of ash input into the salt marsh that is suggestive of burning

occurs at about 6.20m, but this time there is no other supporting evidence for a fall

in relative sea level. Subsequently the area would appear to have remained a

saltmarsh until ‘reclaimed’ in either the later medieval or post-medieval period, when

a substantial thickness of sediment was deliberately deposited (Unit 1) on the site to

raise it above the zone of tidal influence.


35


Addressing the research questions

How old was the stratigraphy found below the ‘made ground’ in which the medieval

and post-medieval remains investigated by Cotswold Archaeological Trust were

found?

9.8 This question can be definitively answered only in relation to the peat as no artefacts

or organic material suitable for 14C dating were found from any other deposit. The

peat formed in the early to middle Neolithic some time between 4220-3050 cal. BC.

It is very likely that the mineral silts and clays (Unit 4) beneath the peat date to the

middle or early Holocene, while it is possible that the relatively organic-rich zone of

the upper mineral silts and clays (Unit 2), between 6.50-6.80m was accreting in the

Bronze Age.

How did the peat revealed in the borehole studies form – in a brackish or freshwater

environment?

9.9 The peat is actually a complex of organic muds and true peat. These began to

accumulate in a freshwater environment at the margin of the Avon floodplain in the

early Neolithic period. At around 3500 cal. BC the peat was subject to marine

incursion. Subsequently its growth outstripped that of sea-level rise and it continued

to form in a freshwater environment. Soon before it stopped developing around

3550-3050 cal. BC it became part of the intertidal zone and thus subject to marine

influence.

Were the apparently weathered layers noted during the assessment in, and at the

surface of clays and silts, which occur below the ‘made ground’, the result of

terrestrial processes or merely localised discolourations of the intertidal/alluvial

stratigraphy?

9.10 Three of these zones were examined in the laboratory studies, one at c. 5.70m, a

second at 6.14-6.18m and the last at c. 6.50-6.80m. The last two at least appear to

have been caused by the short-lived operation of terrestrial processes in the

saltmarsh. Both contain possible evidence for human manipulation of the marsh in

the form of burning as expressed by peaks in the low frequency magnetic

susceptibility curve.


36


Was there any direct or indirect evidence of human activity in the terrestrial

sediments (i.e. peats or ‘surfaces’ in the silts and clays)?

9.11 As stated in the answer to the previous question there is indirect evidence for human

use of the saltmarsh, or at least burning in the local environment during accretion of

Unit 2. There is also evidence of further burning during the early Neolithic period,

causing a sustained peak in low frequency magnetic susceptibility between c. 8.00-

8.14m. The magnitude of this is such that it is likely that burning was at some

distance from the sample site. Palynological evidence may support this assertion as

possible grains of cereals, weeds and other indicators of ground disturbance were

also found at this level. A more definitive palynological indicator of human activity –

although again away from the sample site – is provided by the decline in lime pollen

from 7.90m upwards and the concomitant increase in microscopic charcoal and

Chenopodiaceae pollen. These last are likely to have been the due to clearance of

lime woodland for agriculture during the middle Neolithic period.

Did the clays and silts represent accumulation in brackish or freshwater

environments?

9.12 The lower silts and clays of Unit 4 are likely to have formed in a freshwater

environment at the margins of the Avon floodplain. However, the evidence for this

hypothesis is only slight, i.e. the absence of Foraminiferal tests from samples of this

deposit and the fact that there are no brackish water indicators in the lower portion

of the peats of Unit 3, with which Unit 4 has a conformable (i.e. time continuous)

boundary. In contrast the clays and silts of Unit 2 formed in a brackish, intertidal

environment, albeit with episodic exposure to terrestrial processes.

10. CONCLUSIONS

10.1 The investigation of the sequence at the site has provided important new

palaeoenvironmental data, particularly given that there are no previous pollen

diagrams from central Bristol, while those from surrounding areas (e.g. Tinsley

1999) are from stratigraphy of a later date. These data are at least of regional


37


significance. As has already been pointed out in Section 7, the origins of the

nationally famous Leigh Wood, argued by Rackham (1982) to be one of the few

remaining areas of ‘wildwood’ in England are now firmly established as a result of

palynological analysis here. The broad-leaved lime that characterises Leigh Wood

today was the dominant forest tree of the middle Holocene at this site, comprising

perhaps 80% of trees in the wood. However, even these temporally distant

woodlands were subject to modification through the actions of people. Both

palynological and sedimentological evidence suggests that clearance in, and

burning of the surrounding lime woods took place in the Neolithic period.

10.2 A second aspect of the investigation that has proved significant is the data relating

to sea level. The sea level history of the Severn Estuary and tributary rivers is

presently a subject of intense academic interest (e.g. Bell, Caseldine and Neumann

2000; Haslett et al 2001) given recent archaeological discoveries in the intertidal

zone and future worries with regard to the affects of global sea level rise. Evidence

from the site suggests that rising sea levels first directly impacted upon the site

around 3500 cal. BC, although the initial affects were short-lived. Subsequently,

between 3550-3050 cal. BC the site was incorporated within the intertidal zone. The

fact that it was possible to obtain a 14C AMS date of relatively high precision from the

demonstrably conformable contact between the terrestrial/freshwater peat of Unit 3

and the brackish silts and clays of Unit 2 means that the project has added a further,

high resolution sea-level index point (sensu Shennan 1989) to the Severn Estuary

sea level database (see Haslett et al. 2001) (Fig. 5).

10.3 The methodology employed by the project has been demonstrated to be robust by

the results obtained. Archaeologists in particular are often reluctant to employ

borehole surveys because of the limited window that a 100mm diameter (or less)

core provides into the past (e.g. Locock 2001). It is true that boreholes are unlikely

to record the presence of structural remains. However, when stratigraphic records

from boreholes are combined with relatively high resolution sedimentological and

micro-biological analyses of the kind carried out on these samples, information

relating to other aspects of human activity, not normally recovered in ‘standard’

archaeological excavations are revealed (e.g. evidence of sub-regional forest

modification and saltmarsh exploitation). In the case of sites where thick

stratigraphic sequences and a high water table are expected, borehole surveys are

the only realistic, safe and cost effective means of investigation.


38


10.4 The one major problem area experienced in the investigation of the sequence has

been in the establishment of a chronology. While it was relatively straightforward to

obtain 14C dates for the organic part of the sequence, the mineral strata remain

undated, except by reference to the former. This is particularly unfortunate given that

several episodes of human saltmarsh interference have tentatively been identified in

the sequence. Dating techniques other than 14C have been successfully applied to

mineral-dominated intertidal strata including Lead-210 and optically stimulated

luminescence (OSL). However, the former is only applicable to the last few hundred

years and the latter is impractical from borehole samples. Future studies should

investigate the possibility of using temporal markers to date such strata similar to

geochemical evidence for the rise in lead recognised in intertidal stratigraphy of the

Somerset Levels as a result of the Roman led extraction industry (Haslett et al.

1998).

11. ACKNOWLEDGEMENTS

The authors would like to thank Mark Collard, who managed the project for Cotswold

Archaeological Trust (CAT), Franco Vartuca (CAT), who supervised the drilling of

the archaeological boreholes and Peter Moore (CAT) for drawing Figs. 1 and 2.

Vanessa Straker (English Heritage) and Bob Jones (Bristol City Council) are

thanked for advice both during and after fieldwork, and Myra Wilkinson-van Hoek for

undertaking much of the laboratory-based sedimentological analysis.

12. BIBLIOGRAPHY

Allen, J.R.L. (1999) ‘Geological impacts on coastal wetland landscapes: some general

affects of sediment autocompaction in the Holocene of northwest Europe.’ The

Holocene 9, 1-12.

Andersen, S.Th. (1970) ‘The relative pollen productivity and pollen representation of North

European trees, and correction factors for tree pollen spectra.’ Danmarks geologiske

Undersøgelse. Series 2. 96, 1-99.


39


Andrew, R. 1984 A Practical Pollen Guide to the British Flora. Technical Guide 1.

Quaternary Research Association, Cambridge.

Bell, M.G. 2001 ‘Environmental archaeology in the Severn Estuary: progress and prospects.’

In Rippon, S. (Ed.) Estuarine Archaeology: the Severn and beyond. Archaeology in

the Severn Estuary 11. Severn Estuary Levels Research Committee, Exeter, 69-

104.

Bell, M.G., Caseldine, A. and Neumann, H. 2000 Prehistoric intertidal archaeology in the

Welsh Severn Estuary. CBA Research Report 120, Council for British Archaeology,

York.

Bennett, K. D. 1994 Annotated catalogue of pollen and pteridophyte spores of the British

Isles. Department of Plant Sciences, University of Cambridge, Cambridge.

Bronk Ramsey, C. 2000 OxCAL 3.5. Research Laboratory for Archaeology and the History

of Art, Oxford.

Caseldine, A. E. 1988 ‘A wetland resource: the evidence for environmental exploitation in the

Somerset Levels during the prehistoric period.’ In Murphy P. and French C. (Eds.)

The Exploitation of Wetlands. BAR British Series 186, British Archaeological

Reports, Oxford, 239-265.

CAT 2000 Land to the north of Anchor Road, Bristol. Archaeological Evaluation. CAT

typescript report 001198

CAT 2001 Land to the north of Anchor Road, Bristol. Archaeological Excavation and

Recoprding. CAT typescript report 01066

Coles, J.M., Hibbert, F.A. and Orme, B.J. 1973 ‘Prehistoric Roads and Tracks in Somerset,

England: 3. The Sweet Track.’ Proceedings of the Prehistoric Society 39, 256-293.

Denys, L. 1992 A check list of the diatoms in the Holocene deposits of the Western Belgian

Coastal Plain with a survey of their apparent ecological requirements: I. Introduction,

ecological code and complete list. Service Geologique de Belgique. Professional

Paper No. 246.


40


Gale, S.J. and Hoare, P.G. 1991 Quaternary sediments: petrographic methods for the study

of unlithified rocks. Belhaven, London.

van Geel, B. 1986 ‘Application of fungal and algal remains and other microfossils in

palynological analyses.’ In Berglund, B. (Ed.) Handbook of Holocene Palaeoecology

and Palaeohydrology. John Wiley, Chichester, 497-500.

Geotechnical Engineering Limited 2000 Beaufort House, Bristol. Ground Investigation. GEL

report 10971

Greig J. 1982 ‘Past and present lime woods of Europe.’ In Bell, M.G. and Limbrey, S. (Eds.)

Archaeological Aspects of Woodland Ecology. BAR International Series 146, British

Archaeological Reports, Oxford, 23-57.

Grimm, E. 1990 Tilia and Tilia.graph: PC spreadsheet and graphics software for pollen data.

INQUA COMM. Stud. Holocene Working Groups on Data-handling Methods

Newsletter 4, 5-7.

Haslett, S.K., Davies, P., Curr, R.H.F., Davies, C.F.C., Kennington, K., King, C.P. and

Margretts, A.J. 1998 ‘Evaluating late-Holocene relative sea-level change in the

Somerset Levels, southwest Britain.’ The Holocene 8, 197-207.

Haslett, S.K, Davies, P. and Strawbridge F. 1997 ‘Reconstructing Holocene Sea-level

Change in the Severn Estuary and Somerset Levels: The Foraminifera Connection.’

Archaeology in the Severn Estuary 8, 29-40.

Haslett, S.K., Davies, P., Davies, C.F.C., Margetts, A.J., Scotney, K.H., Thorpe, D.J. and

Williams, H.O. 2001 ‘The changing estuarine environment in relation to Holocene

sea-level and the archaeological implications.’ In Rippon, S. (Ed.) Estuarine

Archaeology: the Severn and beyond. Archaeology in the Severn Estuary 11.

Severn Estuary Levels Research Committee, Exeter, 35-54.

Hendey, N.I. 1964 An Introductory Account of the Smaller Algae of British Coastal Waters.

Part V. Bacillariophyceae (Diatoms). Ministry of Agriculture Fisheries and Food,

Series IV.


41


Heyworth, A. and Kidson, C. 1982 ‘Sea-level changes in southwest England and Wales.’

Proceedings of the Geologist’s Association 93, 91-112.

van Hoeve, M.L. and Hendrikse M. 1998 A study of non-pollen objects in pollen slides: the

Types as described by Dr Bas van Geel and colleagues. Utrecht.

Hustedt, F. 1930-1966 Die Kieselalgen Deutschlands, Oesterreichs und der Schweiz unter

Berucksichtigung der ubrigen Lander Europas sowie der angrenzenden

Meeresgebeite. In Dr. L. Rabenhorsts Kryptogamen-Flora von Deutschland,

Oesterrech und der Schweiz 7, Parts 1-3.

Hustedt, F. 1953 ‘Die Systematik der Diatomeen in ihren Beziehungen zur Geologie und

Okologie nebst einer Revision des Halobien-systems.’ Sv. Bot. Tidskr. 47, 509-519.

Hustedt, F. 1957 ‘Die Diatomeenflora des Fluss-systems der Weser im Gebiet der

Hansestadt Bremen.’ Ab. naturw. Ver. Bremen 34, 181-440.

Iversen, J. 1941 ‘Landnam i Danmarks stenalder.’ Danmarks Geologishe Undersøgelse

Series II 66, 1-67.

Jefferies, R.L., Willis, A.J. and Yem, E.W. 1968 ‘The Late- and Post-glacial history of the

Gordano valley, North Somerset.’ New Phytologist 67, 335-348.

Krammer, K. and Lange-Bertalot, H. 1986-1991 Bacillariophyceae. Gustav Fisher Verlag,

Stuttgart.

Locock, M. 2001 ‘Archaeological prospection and evaluation in the Severn Levels, 1989-

1999.’ In Rippon, S. (Ed.) Estuarine Archaeology: the Severn and beyond.

Archaeology in the Severn Estuary 11. Severn Estuary Levels Research Committee,

Exeter, 131-144.

Long, A.J., Scaife, R.G. and Edwards, R.J. 2000 ‘Stratigraphic architecture, relative sea-

level, and models of estuary development in southern England: new data from

Southamton Water.’ In Pye, K. and Allen, J.R.L. (Eds.) Coastal and estuarine

environments: sedimentology, geomorphology and geoarchaeology. Special

Publication 175, Geological Society, London, 253-279.


42


Moore, P.D., Webb, J.A., and Collinson, M.E. 1991 Pollen Analysis. 2nd edition. Blackwell

Scientific Publications, Oxford.

Murray, J.W. 1973 Distribution and Ecology of Living Benthic Foraminiferids. Heinemann,

London

Murray, J.W. 1979 British Nearshore Foraminiferids. Academic Press, London

Murray, J.W. 1991 Ecology and Palaeoecology of Benthic Foraminifera. Longman Scientific,

London.

Rackham, O. 1982 ‘The Avon Gorge and Leigh Woods.’ In Bell, M.G. and Limbrey, S. (Eds.)

Archaeological Aspects of Woodland Ecology. BAR International Series 146, British

Archaeological Reports, Oxford, 23-57.

Shennan, I. 1989 ‘Holocene crustal movements and sea-level changes in Great Britain.’

Journal of Quaternary Science 4, 77-89.

Stace, C. 1991 New Flora of the British Isles. Cambridge University Press, Cambridge

Stein, J.K. and Teltser, P.A. 1989 ‘Size distributions in artefact classes: combining macro-

and micro-fractions.’ Geoarchaeology 1, 1-30.

Stockmarr, J. 1971 ‘Tablets with spores used in absolute pollen analysis.’ Pollen et Spores

13, 615.

Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B.,

McCormac, F.G., van der Plicht, J. and Spurk, M. 1998 ‘INTCAL98 radiocarbon age

calibration 24,000-0 cal. BP.’ Radiocarbon 40, 1041-1084.

Thompson, R. and Oldfield, F. 1986 Environmental magnetism. Allen and Unwin, London.

Tinsley, H.M. 1999 ‘Seabank to Pucklechurch Pipeline: Pollen Analysis of upper and lower

buried land surface horizons from Farm Lane Field 136, Trench B east (west facing

section).’ Unpublished Report for Entrepose Laing.


43


Tinsley, H.M. 2002 ‘Pollen analysis of peat samples from Borehole A Shapwick Heath,

Somerset.’ Unpublished Report for the Shapwick Project.

Troels-Smith, J. 1955 ‘Characterisation of unconsolidated sediments.‘ Danmarks Geologishe

Undersøgelse, Raekke IV 3, 38-73.

Tucker, M.E. 1982 Sedimentary rocks in the field. Wiley, Chichester.

Walker, M.J.C. 1993 ‘Holocene (Flandrian) vegetation change and human activity in the

Carneddau area of upland mid-Wales.’ In Chambers, F.M. (Ed.) Climate Change

and Human Impact on the Landscape. Chapman and Hall, London.

Wilkinson, K.N. 2001 ‘Land to the North of Anchor Road, Bristol: Preliminary Stratigraphic

Report’. in CAT 2001


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APPENDIX A: NOTES ON POLLEN TYPES

The taxonomic level to which pollen grains can be identified varies; some can be identified to species level,

others to family and others to group. This report mainly follows theconventions used by Bennett (1994). The

following notes may be helpful:

Corylus-type includes Corylus avellana and Myrica gale.

Lactuceae includes Cichorium intybus, Lapsana communis, Hypochaeris, Leontodon, Picris, Lactuca (some spp)

Cicerbita alpina, Taraxacum, Crepis, Pilosella, Hieracium.

Solidago virgaurea-type includes Filago, Antennaria dioica, Gnaphalium, Inula, Pulicaria, Solidago virgaurea,

Aster, Erigeron, Bellis perennis, Senecio, Tephroseries, Tussilago farfara, Petasites hybridus, Bidens,

Eupatorium cannabinum.

Achillea-type includes Tanacetum vulgare, Otanthus maritimus, Achillea, Anthemis, Leucanthemum vulgare,

Matricaria, Tripleurospermum.

Ranunculus acris-type includes Ranunculus undifferentiated, Clematis vitalba, Pulsatilla vulgaris, Anenome

nemerosa.

Mentha-type includes Clinopodium, Origanum vulgare, Thymus, Lycopsus europaeus, Mentha, Salvia

.

Peucedanum palustre-type includes Bunium bulbocastanum Selinium carvifolia, Angelica sylvestris,

Peucedanum.

Sparganium emersum-type includes Sparganium undifferentiated, Typha angustifolia.

Cereal-type includes all grass pollen grains greater than 40 microns in diameter; this includes the cultivated

grasses (cereals) and Glyceria, Aira caryophyllea, Ammophila arenaria, Leymus arenarius, Elytrigia.

land at anchor road/deanery road bristol...

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