CgMs Consulting Land North of High Street Walton Felixstowe

on behalf of
CgMs Consulting
Land North of High Street
Walton
Felixstowe
Suffolk
geophysical survey
report 2642
April 2011
Contents
1.
2.
3.
4.
5.
6.
7.
Summary
Project background
Historical and archaeological background
Landuse, topography and geology
Geophysical survey
Conclusions
Sources
1
2
2
3
3
6
7
Figures
Figure 1:
Figure 2:
Figure 3:
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Survey location
Geophysical survey, overview at 1:2000
Geophysical survey, unfiltered data
Geophysical survey, filtered data
Geophysical interpretation
Archaeological interpretation
Trace plot of geomagnetic data
© Archaeological Services Durham University 2011
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
1.
Summary
The project
1.1
This report presents the results of a geophysical survey conducted in advance of
proposed development of land north of High Street, Walton, Felixstowe, Suffolk. The
works comprised the geomagnetic survey of 12 hectares of arable land.
1.2
The works were commissioned by CgMs Consulting and conducted by Archaeological
Services Durham University.
1.3
A number of curvilinear anomalies were identified across the site which could
possibly reflect the remains of ring-ditches. One of the larger of these anomalies, on
the northern edge of the site, corresponds to a cropmark on aerial photographs,
which has been interpreted as a Bronze Age round barrow.
1.4
Probable ditches were detected across the site which could reflect medieval or
earlier, possibly prehistoric, boundaries.
1.5
Ground disturbance, probably associated with gravel extraction, was detected in the
south-east corner of the survey area.
1.6
Two known former field boundaries, a backfilled pond and modern services were
also detected.
Results
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
2.
Project background
2.1
The survey area was located on land north of High Street, Walton, Felixstowe,
Suffolk (NGR centre: TM 290 360). One survey totalling approximately 12ha was
conducted in one land parcel. The site is bounded by High Street to the south, the
A154 Candlet Road to the north, the A14 Port of Felixstowe Road to the west and
residential housing to the east. The south-west corner of the site was occupied by
disused paddocks and a rifle range.
2.2
The development proposal is for a mixed-use development comprising residential,
commercial and recreational facilities.
Location (Figure 1)
Development proposal
Objective
2.3
The principal aim of the survey was to assess the nature and extent of any subsurface features of potential archaeological significance within the proposed
development area, so that an informed decision may be made regarding the nature
and scope of any further scheme of archaeological works that may be required in
relation to the development.
Methods statement
2.4
The surveys have been undertaken in accordance with a Written Scheme of
Investigation provided by Archaeological Services Durham University and approved
by the Suffolk County Archaeologist.
Dates
2.5
Fieldwork was undertaken between 21st and 25th March 2011. This report was
prepared during April 2011.
Personnel
2.6
Fieldwork was conducted by Tom Fitton and Natalie Swann (Supervisor).
Geophysical data processing and report preparation were conducted by Natalie
Swann and Duncan Hale, the Project Manager, with illustrations by David Graham.
Archive/OASIS
2.7
3.
The site code is SFW11, for Suffolk, Felixstowe, Walton 2011. Archaeological Services
Durham University is registered with the Online AccesS to the Index of
archaeological investigationS project (OASIS). The OASIS ID number for this project is
archaeol3-98702.
Historical and archaeological background
Previous archaeological works
3.1
An archaeological desk-based assessment has been undertaken for this site
(Chadwick & Smith 2010). The results of that assessment are summarised below. No
other previous archaeological work has been undertaken within the proposed
development area.
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
The prehistoric period (up to AD 70)
3.2
The Suffolk Historic Environment Record (HER) notes three Bronze Age ring-ditches,
interpreted from aerial photography, within the proposed development area (PDA).
These ring-ditches are likely to be part of a wider prehistoric landscape as six further
Bronze Age ring-ditches and prehistoric field systems and recorded within 1km of
the PDA.
The Roman period (AD 70 to 5th century)
3.3
There is no direct evidence of Roman occupation within the PDA but evidence for
Roman occupation has been recorded 900m south-west of the site.
3.4
The PDA lies outside the medieval centre of Walton and was likely to have been farm
land during the medieval period.
The medieval period (5th century to 1540)
The post-medieval and modern period (1541 to present)
3.5
Historic and modern maps show that the south-west corner of the site and the south
edge of the site along High Street have been developed since the post-medieval
period; the rest of the site has remained as open farm land. Quarrying was also
noted on the west edge of the PDA.
4.
Landuse, topography and geology
4.1
At the time of survey the proposed development area comprised a single field of
arable land ploughed into ridges for potatoes. The ridges were aligned northwest/south-east, except around the field edges where they ran parallel to the field
edges. It was not possible to collect data in the south-west corner of site due to
overgrown vegetation within the paddocks.
4.2
The area was predominantly level with a mean elevation of approximately 22m OD.
The site lies approximately 3km from the Felixstowe coast.
4.3
The underlying solid geology of the area comprises Neogene Red Crag Formation
(sand), which is overlain by Quaternary drift geology of Kesgrave Catchment
Subgroup sand and gravel.
5.
Geophysical survey
Standards
5.1
The surveys and reporting were conducted in accordance with English Heritage
guidelines, Geophysical survey in archaeological field evaluation (David, Linford &
Linford 2008); the Institute for Archaeologists (IfA) Draft Standard and Guidance for
archaeological geophysical survey (2010); the IfA Technical Paper No.6, The use of
geophysical techniques in archaeological evaluations (Gaffney, Gater & Ovenden
2002); and the Archaeology Data Service Guide to Good Practice: Geophysical Data
in Archaeology (draft 2nd edition, Schmidt & Ernenwein 2010).
Technique selection
5.2
Geophysical survey enables the relatively rapid and non-invasive identification of
sub-surface features of potential archaeological significance and can involve a suite
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
of complementary techniques such as magnetometry, earth electrical resistance,
ground-penetrating radar, electromagnetic survey and topsoil magnetic
susceptibility survey. Some techniques are more suitable than others in particular
situations, depending on site-specific factors including the nature of likely targets;
depth of likely targets; ground conditions; proximity of buildings, fences or services
and the local geology and drift.
5.3
In this instance, based on aerial photographic cropmark evidence it was considered
likely that cut features such as ditches and pits might be present on the site, and that
other types of feature such as trackways, wall foundations and fired structures (for
example kilns and hearths) might also be present.
5.4
Given the anticipated shallowness of targets and the non-igneous geological
environment of the study area a geomagnetic technique, fluxgate gradiometry, was
considered appropriate for detecting the types of feature mentioned above. This
technique involves the use of hand-held magnetometers to detect and record
anomalies in the vertical component of the Earth’s magnetic field caused by
variations in soil magnetic susceptibility or permanent magnetisation; such
anomalies can reflect archaeological features.
Field methods
5.5
A 30m grid was established across the survey area and tied-in to known, mapped
Ordnance Survey points using a total station survey instrument.
5.6
Measurements of vertical geomagnetic field gradient were determined using
Bartington Grad601-2 dual fluxgate gradiometers. A zig-zag traverse scheme was
employed and data were logged in 30m grid units. The instrument sensitivity was
nominally 0.03nT, the sample interval was 0.25m and the traverse interval was 1m,
thus providing 3,600 sample measurements per 30m grid unit.
5.7
Data were downloaded on site into a laptop computer for initial processing and
storage and subsequently transferred to a desktop computer for processing,
interpretation and archiving.
Data processing
5.8
Geoplot v.3 software was used to process the geophysical data and to produce both
a continuous tone greyscale image and a trace plot of the raw (minimally processed)
data. A plot of filtered data is also provided. The greyscale images and
interpretations are presented in Figures 2-6; the trace plot is provided in Figure 7. In
the greyscale images, positive magnetic anomalies are displayed as dark grey and
negative magnetic anomalies as light grey. A palette bar relates the greyscale
intensities to anomaly values in nanoTesla.
5.9
The following basic processing functions have been applied to the geomagnetic data:
clip
clips data to specified maximum or minimum values; to
eliminate large noise spikes; also generally makes statistical
calculations more realistic
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
zero mean traverse
destagger
interpolate
5.10
sets the background mean of each traverse within a grid to
zero; for removing striping effects in the traverse direction
and removing grid edge discontinuities
corrects for displacement of geomagnetic anomalies caused
by alternate zig-zag traverses
increases the number of data points in a survey to match
sample and traverse intervals; in this instance the data have
been interpolated to 0.25m x 0.25m intervals
The following filter has been applied to the geomagnetic data (Figure 4):
low pass filter
applied with Gaussian weighting to remove high frequency,
small-scale spatial detail, in this case near-surface noise
caused by ferrous debris and potato ridges.
Interpretation: anomaly types
5.11
A colour-coded geophysical interpretation plan is provided. Three types of
geomagnetic anomaly have been distinguished in the data:
positive magnetic
regions of anomalously high or positive magnetic field
gradient, which may be associated with high magnetic
susceptibility soil-filled structures such as pits and ditches
negative magnetic
regions of anomalously low or negative magnetic field
gradient, which may correspond to features of low magnetic
susceptibility such as wall footings and other concentrations
of sedimentary rock or voids
dipolar magnetic
paired positive-negative magnetic anomalies, which typically
reflect ferrous or fired materials (including fences and
service pipes) and/or fired structures such as kilns or hearths
Interpretation: features
5.12
A colour-coded archaeological interpretation plan is provided. Except where stated
otherwise in the text below, positive magnetic anomalies are taken to reflect
relatively high magnetic susceptibility materials, typically sediments in cut
archaeological features (such as ditches or pits) whose magnetic susceptibility has
been enhanced by decomposed organic matter or by burning.
5.13
Several circular and curvilinear positive magnetic anomalies have been detected
across the survey area. These anomalies generally reflect very slight increases in
high magnetic susceptibility materials and could represent the remains of soil-filled
ditches. The circular anomaly on the northern edge of the survey, which measures
approximately 30m in diameter, is more intense than the others and corresponds to
a Bronze Age ring-ditch interpreted from aerial photographs.
5.14
It is possible the other curvilinear anomalies reflect the remains of further ringditches, however, their weaker nature suggests they may have been truncated by
deep ploughing on the site. One possible small ring-ditch extends into an area of
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probable former gravel extraction, and so would post-date the gravel pit (para. 5.16
below).
5.15
A number of linear positive magnetic anomalies have also been detected across the
survey area. These are likely to reflect soil-filled features such as ditches and may
reflect former field or land boundaries. One such feature in the east of the area
broadly corresponds to a known former field boundary. Since the others do not
appear to correspond to boundaries shown on the historic OS or tithe maps of the
area they could reflect medieval or earlier, possibly prehistoric, boundaries.
5.16
A very weak negative magnetic anomaly and a chain of occasional discrete dipolar
magnetic anomalies were detected aligned broadly north-east/south-west in the
western half of the survey area. These anomalies correspond to the location of a
field boundary shown on OS maps until 1976.
5.17
A large diffuse positive magnetic anomaly was detected in the south-eastern corner
of the survey area. This is likely to reflect past ground disturbance and may be the
result of gravel extraction. Gravel quarrying is recorded on the 1909 OS map
immediately west and south-west of the site. Two short rows of small dipolar
magnetic anomalies were detected within the area of the probable gravel pit. These
are presumed to post-date the gravel extraction and are unlikely to be of
archaeological significance.
5.18
The dipolar magnetic anomaly on the southern edge of the site probably reflects the
backfill of a pond, which was shown on early OS maps.
5.19
A chain of dipolar magnetic anomalies was detected aligned north-west/south-east
across the site; it turns south at a concrete manhole visible in the field. This anomaly
almost certainly reflects a ferrous pipe. A similar anomaly on the eastern limit of the
survey area may also reflect a modern service.
5.20
A linear negative magnetic anomaly, aligned east-west in the east of the survey area,
is likely to reflect a non-ferrous pipe, possibly attached to the main ferrous pipe
across the site.
5.21
The only other anomalies detected here are small, discrete dipolar magnetic
anomalies. These almost certainly reflect items of near-surface ferrous and/or fired
debris, such as horseshoes and brick fragments. Concentrations of such debris were
detected near the south-western limits of the survey.
6.
Conclusions
6.1
12ha of geomagnetic survey was undertaken on land north of High Street, Walton,
Felixstowe, Suffolk, in support of a proposed planning application.
6.2
A number of curvilinear anomalies were identified across the site which could
possibly reflect the remains of ring-ditches. One of the larger of these anomalies, on
the northern edge of the site, corresponds to a cropmark on aerial photographs,
which has been interpreted as a Bronze Age round barrow.
Archaeological Services Durham University
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Land North of High Street· Walton· Felixstowe· Suffolk· geophysical survey· report 2642· April 2011
6.3
Probable ditches were detected across the site which could reflect medieval or
earlier, possibly prehistoric, boundaries.
6.4
Ground disturbance, probably associated with gravel extraction, was detected in the
south-east corner of the survey area.
6.5
Two known former field boundaries, a backfilled pond and modern services were
also detected.
7.
Sources
Chadwick, P, & Smith, M, 2010 Land North of High Street, Walton, Felixstowe,
Suffolk: archaeological desk-based assessment. Unpublished report CgMs
Consulting
David, A, Linford, N, & Linford, P, 2008 Geophysical Survey in Archaeological Field
Evaluation. English Heritage
Gaffney, C, Gater, J, & Ovenden, S, 2002 The use of geophysical techniques in
archaeological evaluations. Technical Paper 6, Institute of Field
Archaeologists
IfA 2010 Draft Standard and Guidance for archaeological geophysical survey.
Institute for Archaeologists
Schmidt, A, & Ernenwein, E, 2010 (draft) Guide to Good Practice: Geophysical Data in
Archaeology. Archaeology Data Service
Archaeological Services Durham University
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Land North of High Street, Walton,
Felixstowe, Suffolk
on behalf of
CgMs Consulting
geophysical survey
report 2642
Figure 1: Survey location
Contains Ordnance Survey
Open Data © Crown copyright
and database right 2011
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survey location
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Walton
Felixstowe
Suffolk
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report 2642
Figure 2: Geophysical survey overview
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proposed development area
magnetic survey
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Felixstowe
Suffolk
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Figure 3: Geophysical survey, unfiltered
data
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Felixstowe
Suffolk
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Figure 4: Geophysical survey, filtered
data
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negative magnetic anomaly
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Walton
Felixstowe
Suffolk
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Figure 5: Geophysical interpretation
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soil-filled feature
possible backfilled pond
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possible quarrying
service pipe
former field boundary
inspection cover
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Walton
Felixstowe
Suffolk
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Figure 6: Archaeological interpretation
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Felixstowe
Suffolk
geophysical survey
report 2642
Figure 7:
Trace plot of geomagnetic data