of Geophysical Survey Report

Geophysical Survey Report

of

Swaffham Prior Heat Network

For

Oxford Archaeology East

On Behalf Of

Cambridgeshire County Council

Magnitude Surveys Ref: MSSTL657

HER Event Number: ECB6228

April 2020

Swaffham Prior Heat Network MSTL657 - Geophysical Survey Report DRAFT

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Unit 17, Commerce Court

Challenge Way

Bradford

BD4 8NW

01274 926020

[email protected]

Report By:

Julia Cantarano Ingénieur PCIfA

Report Approved By:

Finnegan Pope-Carter BSc (Hons) MSc FGS

Issue Date:

27 April 2020

Abstract Magnitude Surveys was commissioned to assess the subsurface archaeological potential of a c.13.25ha area of land at Swaffham Prior, East Cambridgeshire, Cambridgeshire. A fluxgate gradiometer survey was successfully completed across the survey area. The geophysical survey has detected anomalies of archaeological, agricultural and natural origin. Archaeological activity has been identified in the form of former land divisions not matching any recorded former field boundaries and likely to be furlong boundaries. Anomalies related to historical agricultural use have been detected and interpreted as ridge and furrow and modern ploughing. Natural variations have also been identified in the form of rill features and broader natural variations likely related to colluvial deposits at the base of slopes. Modern activity is present within the dataset but is limited to magnetic disturbance along field boundaries, and to a small concentration of dipolar anomalies in the northwest corner related to the adjacent farm.


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Contents Abstract ................................................................................................................................................... 2

List of Figures .......................................................................................................................................... 4

1. Introduction .................................................................................................................................... 5

2. Quality Assurance ........................................................................................................................... 5

3. Objectives........................................................................................................................................ 5

4. Geographic Background .................................................................................................................. 6

5. Archaeological Background ............................................................................................................. 6

6. Methodology ................................................................................................................................... 7

Data Collection ........................................................................................................................ 7

Data Processing ....................................................................................................................... 8

Data Visualisation and Interpretation ..................................................................................... 8

7. Results ............................................................................................................................................. 9

Qualification ............................................................................................................................ 9

Discussion ................................................................................................................................ 9

Interpretation ........................................................................................................................ 10

General Statements ...................................................................................................... 10

Magnetic Results - Specific Anomalies .......................................................................... 10

8. Conclusions ................................................................................................................................... 12

9. Archiving ....................................................................................................................................... 13

10. Copyright ....................................................................................................................................... 13

11. References .................................................................................................................................... 13

12. Project Metadata .......................................................................................................................... 14

13. Document History ......................................................................................................................... 14


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List of Figures Figure 1: Site Location 1:25,000 @ A4 Figure 2: Figure 3: Figure 4: Figure 5: Figure 6: Figure 7: Figure 8: Figure 9: Figure 10: Figure 11: Figure 12: Figure 13:

Location of Survey Areas Magnetic Total Field (Lower Sensor) (Overview) Magnetic Gradient (Overview) Magnetic Interpretation (Overview) Magnetic Interpretation over Satellite and Historic Maps Magnetic Gradient (North) Magnetic Interpretation (North) XY Trace Plot (North) Magnetic Gradient (South) Magnetic Interpretation (South) XY Trace Plot (South) Magnetic Interpretation with Contours

1:3,000 @ A3 1:2,000 @ A3 1:2,000 @ A3 1:2,000 @ A3 1:3,000 @ A3 1:1,500 @ A3 1:1,500 @ A3 1:1,500 @ A3 1:1,500 @ A3 1:1,500 @ A3 1:1,500 @ A3 1:2,000 @ A3


Magnitude Surveys Ltd 5 | P a g e

1. Introduction Magnitude Surveys Ltd (MS) was commissioned by Oxford Archaeology East on behalf of Cambridgeshire County Council to undertake a geophysical survey on a c. 13.25 ha area of land near Swaffham Prior, East Cambridgeshire, Cambridgeshire (TL 5757 6397).

The geophysical survey comprised hand-pulled, cart-mounted GNSS-positioned fluxgate gradiometer survey.

The survey was conducted in line with the current best practice guidelines produced by Historic England (David et al., 2008), the Chartered Institute for Archaeologists (CIfA, 2014) and the European Archaeological Council (Schmidt et al., 2015).

The survey was conducted in line with a WSI produced by MS (Magnitude Surveys, 2020).

The survey commenced on 17th of April 2020 and took two days to complete.

2. Quality Assurance Magnitude Surveys is a Registered Organisation of the Chartered Institute for Archaeologists (CIfA), the chartered UK body for archaeologists, and a corporate member of ISAP (International Society of Archaeological Prospection).

The directors of MS are involved in the cutting edge of research and the development of guidance/policy. Specifically, Dr. Chrys Harris is the Vice-Chair of the International Society for Archaeological Prospection (ISAP); Finnegan Pope-Carter is a Fellow of the London Geological Society, as well as a member of GeoSIG (CIfA Geophysics Special Interest Group); Dr. Kayt Armstrong is the Editor of ISAP News, and is the UK Management Committee representative for the COST Action SAGA; Dr. Paul Johnson has been a member of the ISAP Management Committee since 2015, and is currently the nominated representative for the EAA Archaeological Prospection Community to the board of the European Archaeological Association.

All MS managers have relevant degree qualifications to archaeology or geophysics. All MS field and office staff have relevant archaeology or geophysics degrees and/or field experience.

3. Objectives The objective of this geophysical survey was to assess the subsurface archaeological potential of the survey area.



4. Geographic Background The survey area was located c.200m west of the village of Swaffham Prior (Figure 1). Survey was undertaken across two fields, one under arable usage and the other under pasture. The survey area was bounded by Goodwin Farm to the northwest, by Heath Road to the west and southwest and by further fields to the north and to the east (Figure 2).

Survey considerations:

Survey Area

Ground Conditions Further Notes

1 Arable, young crop at 150mm. Sloping down from north-east to south-west.

Bounded by hedgerows to the north, by a ditch to the east. The western boundary comprised hedges and continuation on other fields and the southern boundary was a continuation of the same field.

2 Grassland-pasture. Sloping down from north-east to south-west.

The field was bounded to the north by hedgerows, however the survey area stopped short of this boundary. The field also continued to the south of the survey area. A ditch boundary to the west separated Areas 1 and 2. Overhead power cables were present along the northern boundary.

The underlying geology comprises part of the Grey Chalk Formation with no superficial deposits recorded across the survey area (British Geological Survey, 2020).

Soils consist of; freely draining lime-rich loamy soils over most of the survey area; and of shallow lime-rich soils over chalk or limestone in the northwest and south corner and along the western boundary. (Soilscapes, 2020).

5. Archaeological Background The following section summarises an archaeological assessment of known heritage assets within a 1km radius of the survey area, following an HER search provided by Oxford Archaeology East.

The survey area is located between two scheduled monuments. An Iron Age settlement and Roman villa site (with burial grounds) c.1km north-west of the survey area (NHLE 1006875). The second scheduled monument comprises a section of the linear rampart bank and ditch earthwork of Devil’s Dyke (NHLE 1003262) c.1km north-east of the survey area.

Approximately 450m to the west of the survey area, a Mesolithic flint findspot (CHER 06691) was recorded. This included 5 cores, blades and flakes, a scraper and a tranchet axe. A pebble mace head with hourglass perforations was also located within the area.

A polished flint axe (CHER 06420) was recovered in 1960, c.500m south-west of the survey area.

Probable Bronze Age ring ditches measuring up to c.27m in diameter have been identified to the south-east of the survey area (CHER 06690 and CHER06454).



An Iron Age pottery scatter was recorded at Gallow’s Hill c.250m north-east of the survey area (CHER 11537A).

Evidence of Roman activity in the search area includes a possible mausoleum, excavated in 1993 c.250m north-east of the survey area, containing a skeleton of an adolescent (CHER 11054). In this location an iron sickle, a bronze strap fitting and two sherds of Roman pottery were also identified. Evidence of a possible seven other inhumations were recorded at least two of which are associated with Anglo-Saxon grave goods.

Medieval activity within the search area is limited to a number of furlong boundaries including: CHER 10279, CHER 10280, and CHER 10281.

A number of other anomalies of unknown date or origin have been identified, including six skeletons c.500m west-north-west of the survey area. No associated finds or evidence was recorded during excavation in 1966. A number of crop marks identifying probable ditches (CHER06870, CHER06871 and CHER 10549), probable ring ditches (CHER 10134, CHER 10135 and CHER 06723) and probable field systems (CHER 06872) have been recorded in the surrounding area.

6. Methodology Data Collection

Geophysical prospection comprised the magnetic method as described in the following table.

Table of survey strategies:

Method Instrument Traverse Interval Sample Interval

Magnetic Bartington

Instruments Grad-13 Digital Three-Axis Gradiometer

1m 200Hz reprojected to 0.125m

The magnetic data were collected using MS’ bespoke hand-pulled cart system GNSS-positioned system].

6.1.3.1. MS’ cart system was comprised of Bartington Instruments Grad 13 Digital Three-Axis Gradiometers. Positional referencing was through a multi-channel, multi-constellation GNSS Smart Antenna RTK GPS outputting in NMEA mode to ensure high positional accuracy of collected measurements. The RTK GPS is accurate to 0.008m + 1ppm in the horizontal and 0.015m + 1ppm in the vertical.

6.1.3.2. Magnetic and GPS data were stored on an SD card within MS’ bespoke datalogger. The datalogger was continuously synced, via an in-field Wi-Fi unit, to servers within MS’ offices. This allowed for data collection, processing and visualisation to be monitored in real-time as fieldwork was ongoing.

6.1.3.3. A navigation system was integrated with the RTK GPS, which was used to guide the surveyor. Data were collected by traversing the survey area along the longest possible lines, ensuring efficient collection and processing.



Data Processing Magnetic data were processed in bespoke in-house software produced by MS. Processing steps conform to Historic England’s standards for “raw or minimally processed data” (see sect 4.2 in David et al., 2008: 11).

Sensor Calibration – The sensors were calibrated using a bespoke in-house algorithm, which conforms to Olsen et al. (2003).

Zero Median Traverse – The median of each sensor traverse is calculated within a specified range and subtracted from the collected data. This removes striping effects caused by small variations in sensor electronics.

Projection to a Regular Grid – Data collected using RTK GPS positioning requires a uniform grid projection to visualise data. Data are rotated to best fit an orthogonal grid projection and are resampled onto the grid using an inverse distance-weighting algorithm.

Interpolation to Square Pixels – Data are interpolated using a bicubic algorithm to increase the pixel density between sensor traverses. This produces images with square pixels for ease of visualisation.

Data Visualisation and Interpretation This report presents the gradient of the sensors’ total field data as greyscale images, as well as the total field data from the lower sensors. The gradient of the sensors minimises external interferences and reduces the blown-out responses from ferrous and other high contrast material. However, the contrast of weak or ephemeral anomalies can be reduced through the process of calculating the gradient. Consequently, some features can be clearer in the respective gradient or total field datasets. Multiple greyscale images at different plotting ranges have been used for data interpretation. Greyscale images should be viewed alongside the XY trace plot (Figures 9 & 12). XY trace plots visualise the magnitude and form of the geophysical response, aiding in anomaly interpretation.

Geophysical results have been interpreted using greyscale images and XY traces in a layered environment, overlaid against open street maps, satellite imagery, historic maps, LiDAR data, and soil and geology maps. Google Earth (2020) was consulted as well, to compare the results with recent land usages.

Geodetic position of results - All vector and raster data have been projected into OSGB36 (ESPG27700) and can be provided upon request in ESRI Shapefile (.SHP) and Geotiff (.TIF) respectively. Figures are provided with raster and vector data projected against OS Open Data.



7. Results Qualification

Geophysical results are not a map of the ground and are instead a direct measurement of subsurface properties. Detecting and mapping features requires that said features have properties that can be measured by the chosen technique(s) and that these properties have sufficient contrast with the background to be identifiable. The interpretation of any identified anomalies is inherently subjective. While the scrutiny of the results is undertaken by qualified, experienced individuals and rigorously checked for quality and consistency, it is often not possible to classify all anomaly sources. Where possible an anomaly source will be identified along with the certainty of the interpretation. The only way to improve the interpretation of results is through a process of comparing excavated results with the geophysical reports. MS actively seek feedback on their reports as well as reports of further work in order to constantly improve our knowledge and service.

Discussion The geophysical results are presented in consideration with satellite imagery and historic maps (Figure 6).

The fluxgate gradiometer survey has responded well to the environment of the survey area and a range of anomalies have been identified. The geophysical survey has primarily detected anomalies of archaeological, agricultural and natural origin. Natural variations have been identified in the northeast of the survey areas in the form of rills - small channels in the soils caused by water flowing and channelling out the soil layer. Broader natural variations have been detected in the southern half of the survey area, these likely relate to colluvial deposits accumulated at the base of the slope (Figure 13). Modern activity is limited to magnetic disturbance along field boundaries and to a small concentration of dipolar anomalies in the northwest corner related to the adjacent farm (Figure 3).

Archaeological activity has been identified as a series of linear ditch like features. These features do not match any former field boundaries recorded on available historic maps, which suggests they represent an earlier organisation of landscape divisions (Figure 5). These features also correspond with a HER record for furlong boundaries which gives an indication of a likely provenance (see Section 5.8). One of the linear features presents a distinct negative magnetic signal which has made its characterisation more difficult, however it is possible that this is related to an earthwork rather than a ditch-type boundary.

Agricultural activity has been identified as ridge and furrow in the west of the survey area. The orientation of this cultivation aligns with some of the previously discussed furlong boundaries; however, it is unlikely that they are contemporary as the ridge and furrow extends across the boundaries. Modern ploughing has also been identified across the entire survey area.



Three anomalies were categorised as undetermined. They are located at the edge of the survey area and due to the lack of context, it could not made clear whether they are of agricultural or possible archaeological origin.

Interpretation General Statements 7.3.1.1. Geophysical anomalies will be discussed broadly as classification types across

the survey area. Only anomalies that are distinctive or unusual will be discussed individually.

7.3.1.2. Magnetic Disturbance – The strong anomalies produced by extant metallic structures along the edges of the field have been classified as ‘Magnetic Disturbance’. These magnetic ‘haloes’ will obscure the response of any weaker underlying features, should they be present, often over a greater footprint than the structure they are being caused by.

7.3.1.3. Ferrous (Spike) – Discrete ferrous-like, dipolar anomalies are likely to be the result of isolated modern metallic debris on or near the ground surface.

7.3.1.4. Ferrous/Debris (Spread) – A ferrous/debris spread refers to a concentrated deposition of discrete, dipolar ferrous anomalies and other highly magnetic material.

7.3.1.5. Undetermined – Anomalies are classified as Undetermined when the anomaly origin is ambiguous through the geophysical results and there is no supporting or correlative evidence to warrant a more certain classification. These anomalies are likely to be the result of geological, pedological or agricultural processes, although an archaeological origin cannot be entirely ruled out. Undetermined anomalies are generally not ferrous in nature.

Magnetic Results - Specific Anomalies 7.3.2.1. Archaeology Possible (Strong/Weak) – Across the survey area, several positive,

strong and weak linear anomalies have been identified [1a, 1b & 2a], suggestive of ditch-like features (Figure 5). The morphology and orientation of these anomalies do not match any boundaries depicted on the available historic maps, which suggests that they represent an earlier organisation of landscape divisions; in this case a HER record for medieval furlong boundaries corresponds with the location of these anomalies (see section 5.8). The linear anomalies [1a & 2a] present a general northwest-southeast orientation, and a weaker positive linear anomaly [1b] runs perpendicular to them (Figures 8 and 11). These linear anomalies form strips of land through Areas 1 and 2 which is consistent with the probable furlong boundary origin. Another anomaly [1c] of similar magnetic signal has been identified in the south of Area 1 (Figure 8). It follows a similar orientation to [1a & 2a] and [1b] but does not continue through Area 2. [1c] does not correspond with any boundaries depicted on the available historic maps, however it is unclear how, or if, it interrelates with [1a, 1b & 2a] and may



suggest evidence of a non-contemporary landscape division, although its potential as an additional furlong boundary cannot be ruled out.

7.3.2.2. Archaeology Possible – Along the southern boundary a negative linear anomaly [1d & 2b] is running approximately parallel to [1a & 2a] (Figure 5). Although a furlong boundary producing a negative magnetic signal is possible - if it was an earthwork rather than ditch-type boundary - its distinct magnetic signal with the anomalies [1a, 1b & 1c] renders its classification ambiguous. A more recent agricultural origin cannot be ruled out, however, it is not visible on aerial imagery or available historic maps which suggests a possible archaeological origin.

7.3.2.3. Ridge and Furrows – A series of weak positive parallel linear anomalies most visible in the total field (Figure 3), and characteristic of ridge and furrow have been detected in the south of Area 1 (Figure 8). This cultivation appears to respect [1c], but not [1b], which would support that [1c] and [1b] are not contemporary (see section 7.3.2.1).

7.3.2.4. Agricultural (Trend) – Weak linear trends of straight regularly spaced parallel linear anomalies have been detected across the survey area running in a northeast to southwest orientation, which are typical of modern ploughing features. These linear trends also correspond to the modern ploughing direction visible on satellite imagery. Weak linear trends have also been identified in the perpendicular orientation. These trends are characteristic of agricultural activity; however, it is unclear whether these trends are of modern or historic origin, or whether drainage or cultivation related.

7.3.2.5. Natural – In the northern part of Area 1, weak positive linear anomalies have been detected (Figures 3 and 8). Although similar in appearance to ridge and furrow trends, they also correspond with topography changes and suggest possible rill features. Rills are small channels in the soils caused by water flowing and channelling out the soil layer which can produce these linear patterns in the magnetic data. The freely draining soil and topography create good conditions for rill features; additionally, the absence of recorded superficial geology and the chalk bedrock is a good environment for producing enough magnetic contrast and making these channels detectable with magnetic survey.

7.3.2.6. Undetermined – Three positive linear anomalies [1e & 2c] were detected across the survey area (Figures 8 and 11). They are all located at the edge of the survey area which hinders classification due to lack of context. Although they are characteristic of anthropogenic activity, they could either be of agricultural or archaeological origin.



8. Conclusions A fluxgate gradiometer survey has successfully been undertaken across the survey area. The geophysical survey has detected a range of different types of anomalies of archaeological, agricultural, natural and modern origin. The underlying geology and topography have allowed the identification of possible rill features in the northern part of the survey area and broader natural variations in the south. Modern activity is limited to magnetic disturbance along field boundaries and to a small concentration of dipolar anomalies in the northwest corner related to the adjacent farm.

Archaeological activity has been identified across the survey area in the form of possible former field divisions matching the location of a HER record of furlong boundaries which suggest a likely provenance consistent with the magnetic data.

Historic and modern agricultural activity has been detected as ridge and furrow cultivation and modern ploughing.

Three linear anomalies have been classified as undetermined due to their uncertain origin. These are located at the edge of the survey area and due to the lack of context, it is unclear whether they are of agricultural or possible archaeological origin.



9. Archiving MS maintains an in-house digital archive, which is based on Schmidt and Ernenwein (2013). This stores the collected measurements, minimally processed data, georeferenced and un-georeferenced images, XY traces and a copy of the final report.

MS contributes reports to the ADS Grey Literature Library upon permission from the client, subject to the any dictated time embargoes.

10. Copyright Copyright and the intellectual property pertaining to all reports, figures, and datasets produced by Magnitude Services Ltd. is retained by MS. The client is given full licence to use such material for their own purposes. Permission must be sought by any third party wishing to use or reproduce any IP owned by MS.

11. References British Geological Survey, 2020. Geology of Britain. [Swaffham Prior, Cambridgeshire]. [http://mapapps.bgs.ac.uk/geologyofbritain/home.html/]. [Accessed 22/04/2020].

Chartered Institute for Archaeologists, 2014. Standards and guidance for archaeological geophysical survey. CIfA.

David, A., Linford, N., Linford, P. and Martin, L., 2008. Geophysical survey in archaeological field evaluation: research and professional services guidelines (2nd edition). Historic England.

Google Earth, 2020. Google Earth Pro V 7.1.7.2606.

Olsen, N., Toffner-Clausen, L., Sabaka, T.J., Brauer, P., Merayo, J.M.G., Jorgensen, J.L., Leger, J.M., Nielsen, O.V., Primdahl, F., and Risbo, T., 2003. Calibration of the Orsted vector magnetometer. Earth Planets Space 55: 11-18.

Salmon. S., 2020. Magnitude Surveys. Written Scheme of Investigation for a Geophysical Survey of Swaffham Prior, East Cambridgeshire, Cambridgeshire.

Schmidt, A. and Ernenwein, E., 2013. Guide to good practice: geophysical data in archaeology. 2nd ed., Oxbow Books, Oxford.

Schmidt, A., Linford, P., Linford, N., David, A., Gaffney, C., Sarris, A. and Fassbinder, J., 2015. Guidelines for the use of geophysics in archaeology: questions to ask and points to consider. EAC Guidelines 2. European Archaeological Council: Belgium.

Soilscapes, 2020. [Swaffham Prior, Cambridgeshire]. Cranfield University, National Soil Resources Institute [http://landis.org.uk]. [Accessed 22/04/2020].



12. Project Metadata MS Job Code MSTL657 Project Name Swaffham Prior Heat Network Client Oxford Archaeology East Grid Reference TL 5757 6397 Survey Techniques Magnetometry Survey Size (ha) c.13.25ha Survey Dates 17/04/2020 – 18/04/2020 Project Manager Finnegan Pope-Carter BSc (Hons) MSc FGS

Project Officer Leanne Swinbank, BA ACIfA HER Event No ECB6228 OASIS No N/A S42 Licence No N/A Report Version 0.3

13. Document History Version Comments Author Checked By Date

0.1 Initial draft for Project Officer to Review

JC LS 23 April 2020

0.2 Draft for Project Manager to Review

JC FPC 24 April 2020

0.3 Draft for Client JC FPC 27 April 2020

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