geophysical survey report land at beaufort drive, bourne

Geophysical Survey Report

of

Land at Beaufort Drive, Bourne, Lincolnshire

For

CgMs Heritage (Part of RPS Group PLC)

On Behalf Of

Longhurst Group & Alysia Care

Magnitude Surveys Ref: MSTF424

HER Event Number: ELI497

January 2019

Land at Beaufort Drive, Bourne, Lincolnshire MSTF424 - Geophysical Survey Report DRAFT

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Unit 17, Commerce Court

Challenge Way

Bradford

BD4 8NW

01274 926020

[email protected]

Version Purpose/Revision Author Interpretation/Figures Approved By Date Issued

Draft 1.0

Initial Draft to line manager

Amedeo Viccari BCs MSc PCIfA

Amedeo Viccari BCs MSc PCIfA

Leanne Swinbank BA ACIfA

10 January

2019 Draft 1.1

Internal corrections for

Project Manager review


Edward Burton BA PGCE PCIfA


Edward Burton BA PGCE PCIfA


11 January

2019

Draft 1.2

Corrections from Project Manager

Edward Burton BA PCGE PCIfA


Dr. Chrys Harris MCIfA

14 January

2019


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Abstract Magnitude Surveys was commissioned to assess the subsurface archaeological potential of a c. 6.1ha area of land at the northern end of Bourne, Lincolnshire. A fluxgate magnetometer survey was successfully completed, with the geophysical results reflecting archaeological activity, ridge and furrow ploughing, and natural variations in the soil and superficial geology. A potential archaeological complex of temporary or seasonal lowland stock enclosures with a droveway has been identified in the northern half of site, leading northwest to the higher slopes. Given the date of archaeological activity recorded in the vicinity of the site, these anomalies may indicate archaeological activity of a late prehistoric or Roman date. To the south of this enclosure complex is an apparent rectilinear configuration of diffuse, mottled anomalies on a similar alignment to the enclosures to the north. These anomalies may represent an additional enclosure, but this interpretation is less confident than of the enclosures to the north as these southern anomalies are more diffuse in nature and may be disturbed by the ridge and furrow ploughing.


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Contents Abstract ................................................................................................................................................... 3

List of Figures .......................................................................................................................................... 5

1. Introduction .................................................................................................................................... 6

2. Quality Assurance ........................................................................................................................... 6

3. Objectives........................................................................................................................................ 6

4. Geographic Background .................................................................................................................. 7

5. Archaeological Background ............................................................................................................. 7

6. Methodology ................................................................................................................................... 8

Data Collection ........................................................................................................................ 8

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

Data Visualisation and Interpretation ..................................................................................... 9

7. Results ........................................................................................................................................... 10

Qualification .......................................................................................................................... 10

Discussion .............................................................................................................................. 10

Interpretation ........................................................................................................................ 11

General Statements ...................................................................................................... 11

Magnetic Results - Specific Anomalies .......................................................................... 11

8. Conclusions ................................................................................................................................... 13

9. Archiving ....................................................................................................................................... 14

10. Copyright ....................................................................................................................................... 14

11. References .................................................................................................................................... 14


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List of Figures Figure 1: Site Location 1:25,000 @ A4 Figure 2: Location of Survey Area 1:5,000 @ A3 Figure 3: Magnetic Total Field (Lower Sensor) 1:1,500 @ A3 Figure 4: Magnetic Total Field (Upper Sensor) 1:1,500 @ A3 Figure 5: Magnetic Gradient 1:1,500 @ A3 Figure 6: Magnetic Interpretation 1:1,500 @ A3 Figure 7: Magnetic Interpretation Over Satellite Imagery 1:1,500 @ A3 Figure 8: Magnetic Interpretation Over Historic Maps 1:2,500 @ A3 Figure 9: Magnetic XY Trace Plot 1:1,500 @ A3


Magnitude Surveys Ltd 6 | P a g e

1. Introduction Magnitude Surveys Ltd (MS) was commissioned by CgMs Heritage (part of RPS Group PLC) on behalf of Longhurst Group & Alysia Care to undertake a geophysical survey on a c.6.1ha area of land at Beaufort Drive, Bourne, Lincolnshire (TF 09072127).

The geophysical survey comprised hand-carried, GNSS-positioned fluxgate magnetometer 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 written scheme of investigation (Price, 2018) prepared by Magnitude Surveys (2018).

The survey commenced on 16/12/2018 and was completed the same day.

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).

Director Dr. Chrys Harris is a Member of CIfA, has a PhD in archaeological geophysics from the University of Bradford and is the Vice-Chair of ISAP. Director Finnegan Pope-Carter is a Fellow of the London Geological Society, the chartered UK body for geophysicists and geologists, as well as a member of GeoSIG, the CIfA Geophysics Special Interest Group. Reporting Analyst Dr. Kayt Armstrong has a PhD in archaeological geophysics from Bournemouth University, is the Vice Conference Secretary and Editor of ISAP News for ISAP, and is the UK Management Committee representative for the COST Action SAGA.

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 geophysical survey aimed to assess the subsurface archaeological potential of the survey area.



4. Geographic Background The site is located at the northern edge of Bourne, approximately 1km north-west of the town centre (Figure 1). Survey was undertaken over a single arable field c.500m west of North Road (Figure 2). The site was bounded to the south and east by residential buildings; to the west and north by arable fields and single-track road, to the north-west corner by Conjury Nook.

Survey considerations:

Survey Area

Ground Conditions Further Notes

1 Mostly flat with a slight rise in the centre of the field, young cereal crop (wheat or barley)

The site was bounded to the east and south by wire fences and hedgerows, to the north and west by hedgerows. The ground surface was waterlogged in the south of the survey area, particularly in the south-west and south-east corners.

The underlying geology comprises interbedded sandstone and siltstone of the Kellaways Sand Member. A superficial deposit of glaciofluvial sand and gravel is recorded in the centre and south of the site (British Geological Survey, 2019).

The soils consist of loamy soils with naturally high groundwater (Soilscapes, 2019).

5. Archaeological Background The following archaeological background provides a summary of the known archaeology of the survey area and its surrounding environs, based upon information provided by CgMs Heritage’s archaeological desk-based assessment (Price, 2018).

Field walking surveys have been carried out within the survey area, identifying a spread of Middle to Late Bronze Age flint artefacts (HER 35053) across the centre and south-western areas.

Roman activity has been recorded c.200m east of the survey area, identified as pits and ditches containing animal bone and pottery (HER 33223), these artefacts are located in proximity of a possible Iron Age/Roman building found within a field bank (HER 34848). Additionally, surface finds of Roman roof tile and tesserae c.250m south of the site could indicate the presence of a villa (33229).

There is no recorded evidence of Medieval occupation in the survey area, while the presence of ridge and furrow in the vicinity of the site suggests that the site itself may have remained as agricultural land during the Medieval period.

Immediately north-west of the site lies the 19th century farmstead of Conjury Nook (HER 31534), which has been recorded as part of the Historic England Farmsteads Project in 2015.



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-carried GNSS-positioned system.

MS’ hand-carried 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.

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.

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 upper and/or 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 (Figure 9). 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 (2018) was consulted as well, to compare the results with recent land usages.



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 (Figure 7) and historic maps (Figure 8).

The fluxgate magnetometer survey has responded well to the survey area’s environment, with modern disturbance generally limited to the peripheries of the survey area. The results reflect a varied range of magnetic anomalies, including geological variations, ploughing and archaeological anomalies. Anomalies characteristic of natural variations in the soil and superficial geology have been detected in the south of the site, with mottled patches of minor variation likely reflecting recorded superficial deposits of glaciofluvial sand and gravel (see Section 4.3).

Two alignments of ridge and furrow cultivation have been identified within the site. This type of activity has already been recorded in the surrounding area (see section 5.5), but had not previously been identified within the site. In the northern half of the site, the ridge and furrow has been detected on a north-south alignment and is regularly spaced at c. 9m apart. The ridge and furrow alignment changes to an east-west orientation of more enhanced, wider anomalies in the southern half of the site, which are more closely spaced at c. 5m apart. The interface between these two alignments follows the line of a slight rise aligned east-west across the survey area, which is visible in LiDAR imagery and was noted at the time of survey.

A complex of probable and possible archaeological anomalies has been identified in the north of the site. This comprises a broad, diffuse, slightly sinuous anomaly aligned NW-SE, with a series of more discrete, continuous rectangular anomalies indicative of ditched enclosures adjoining it to the south. The enclosures themselves contain a small number of strong, discrete anomalies that may result from pit-like features, and some discontinuous linear responses have also been identified within the enclosures, which may represent subdivisions. The broad sinuous anomaly may result from sub-surface accumulations of material associated with an unbounded trackway or hollow-way feature, which would have linked the enclosures together. The NW-SE alignment of this



leads away to higher ground at Bourne Wood, c.1km west of the site, while the site itself is situated on lower-lying soils with naturally-high groundwater. Overall, the magnetic responses are relatively weak, suggesting that they were not used intensely or for long periods of time. Together, these features may have formed a system of temporary or seasonal stock enclosures situated on low grazing pastures, with an unbounded droveway leading north-west to higher, drier ground and possibly to contemporary settlements.

To the south of the possible droveway-enclosure complex, situated on a deposit of sand and gravel, a polygonal spread of varying magnetic responses has been identified on a similar alignment to the identified enclosures. This may represent an additional rectilinear enclosure, though this anomaly appears to have been differentially affected by subsequent ploughing activity in the south of the site, compared with the more consistent anomalies north of the east-west ridge-and-furrow ploughing system.

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. 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.

7.3.1.3. Ferrous (Discrete/Spread) – Discrete ferrous-like, dipolar anomalies are likely to be the result of modern metallic disturbance on or near the ground surface. A ferrous spread refers to a concentrated deposition of these discrete, dipolar anomalies. Broad dipolar ferrous responses from modern metallic features, such as fences, gates, neighbouring buildings and services, may mask any weaker underlying archaeological anomalies should they be present.

Magnetic Results - Specific Anomalies 7.3.2.1. Probable Archaeology – A series of weak, positive magnetic linear anomalies,

typical on ditch-like features, have been detected towards the centre of the survey area. These anomalies form three main rectilinear configurations [1a, 1b, 1c], which differ in size, including c. 40mx27m [1a], 20mx10m [1b], and 26mx22m [1c]. These anomalies share their northern extents with a broad, sinuous anomaly, [1d]. Anomaly [1d] ranges in strength but is generally weak, and its response peters out in the east of the survey area. Enclosures [1a, 1b, 1c] are relatively weak but consistent linear responses, suggesting the presence of ditch features with a low degree of magnetic enhancement, perhaps as a result of less intensive, seasonal, or temporary use. Anomalies of an



archaeological origin have been detected to the north of [1d], as well as within [1a] and [1c]; however, these are less continuous, which suggests that these result from different or less substantial sub-surface features.

7.3.2.2. Possible Archaeology [1e] – Approximately 15m south of [1c] is a series of variable spread-like responses [1e] which take a polygonal arrangement in the same orientation as [1c]. Close examination of multiple total field and gradient plots, particularly in narrow total field plotting ranges (Figure 4), has aided with the interpretation of these, showing that they do form a rectilinear arrangement. Many of the individual responses are oriented east-west, parallel with ridge-and-furrow trends in this part of the survey area. [1e] may therefore represent an enclosure similar to [1a], [1b] and [1c], which has been differentially disturbed by later ploughing.

7.3.2.3. Possible Archaeology [1f] – Adjacently west of [1a] is a curvilinear anomaly, [1f], which extends from the southwest corner of [1a] and curves to the north to meet [1d]. This may represent an additional enclosure, though is less enhanced and less continuous than the others, so may be less substantial in nature.

7.3.2.4. Possible Archaeology [1g] – A strong discrete anomaly measuring c. 3m x 5m [1g] has been identified c. 40m north of [1c]. Its relative proximity to the archaeological activity on site suggests that it may also be archaeological in origin, and it can be inferred from its high magnetic strength that it represents a more concentrated sub-surface deposit of magnetically enhanced material than other features on site. Examination of XY trace plots for this response shows that it is not characteristic of a burnt or fired feature, so this anomaly more likely represents an archaeological pit-like feature.



8. Conclusions A fluxgate magnetometer survey has been successfully completed across land at Beaufort Drive, Bourne, Lincolnshire. The survey has responded very well to the conditions of the site, with minimal interference from modern activity limited to the edges of the survey area. The survey has identified anomalies relating to probable archaeological activity, natural variations, and ridge and furrow ploughing systems.

Minor variations in the soils and natural geology have been classified as ‘spread’ responses and are consistent with a recorded superficial deposit of glaciofluvial sand and gravel.

Ridge and furrow cultivation has been detected across the survey area, visible along two different orientations of different spacings and strengths. Similar features are recorded in the vicinity of the site. This suggests that the site was likely under agricultural use during the Medieval period.

A complex of archaeological activity has been identified in the northern half of the site, which consists of a series of rectilinear anomalies indicative of ditched enclosures, joined along their northern extents by a broader, sinuous anomaly extending NW-SE across the site. There is some evidence of potential subdivisions and possible pit anomalies within these enclosures, and possible linear features have also been identified to the north-east, including a large discrete anomaly indicative of a pit-like feature. These weakly-enhanced anomalies have been interpreted as a late prehistoric or Roman system of seasonal or temporary enclosures for grazing livestock on lowland areas, joined together by an unbounded droveway leading to higher, drier ground north-west of the site. To the south of this complex, an additional group of varied, discontinuous responses has been identified which takes a rectilinear form on the same alignment. This may represent a similar enclosure feature which has been differentially disturbed by subsequent ploughing activity.

A previous archaeological field walking survey within the site identified concentrations of flint artefacts dated to the Middle and Late Bronze Age, which may suggest a possible, if very tentative, date for the possible and probable archaeological anomalies identified. The proximity of a number of Iron-Age and Roman finds and features to the south and east of the site suggests this complex may have formed part of an Iron Age and/or Roman agricultural landscape, though the possible droveway feature appears to lead only to the northwest, away from the Iron-Age and Roman activity.



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, 2019. Geology of Britain. [Bourne, Lincolnshire]. [http://mapapps.bgs.ac.uk/geologyofbritain/home.html/]. [Accessed 07/01/2019].

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, 2019. 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.

Price, S., 2018. Archaeological Desk-Based Assessment. Land at Beaufort Drive, Bourne, Lincolnshire. CgMs Ltd (Part of RPS Group PLC).

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, 2019. [Bourne, Lincolnshire]. Cranfield University, National Soil Resources Institute [http://landis.org.uk]. [Accessed 07/01/2019].

geophysical survey report land at beaufort drive, bourne

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