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Geophysical Survey Report of Former Lowfield School Acomb, York For York Archaeological Trust On Behalf of City of York Council Magnitude Surveys Ref: MSSE148 June 2017

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Geophysical Survey Report

of

Former Lowfield School

Acomb, York

For

York Archaeological Trust

On Behalf of

City of York Council

Magnitude Surveys Ref: MSSE148

June 2017

Former Lowfield School, Acomb, York MSSE148 - Geophysical Survey Report

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Unit 17, Commerce Court

Challenge Way

Bradford

BD4 8NW

01274 926020

[email protected]

Report Written by:

Leanne Swinbank BA

Figures Produced by:

Leanne Swinbank BA

Report Checked by:

Hannah Brown BA MA MSc PhD

Report Issued:

29 June 2017

Abstract Magnitude Surveys was commissioned to assess the subsurface archaeological potential of a c. 3.4ha area over the site of the former Lowfield School in Acomb, York. A fluxgate gradiometer survey was successfully completed and no anomalies of probable or possible archaeological origin have been detected. The geophysical results primarily reflect modern activity, relating to the playing fields, and agricultural anomalies. Strong anomalies following a typical drainage pattern have been detected, as have former field boundaries, and ferrous responses relating to boundaries and sports equipment.

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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 .................................................................................................................. 7

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

7. Results ....................................................................................................................................... 8

Qualification ...................................................................................................................... 8

Discussion .......................................................................................................................... 8

Interpretation .................................................................................................................... 9

General Statements .................................................................................................... 9

Magnetic Results - Specific Anomalies ........................................................................ 9

8. Conclusions ............................................................................................................................. 10

9. Archiving ................................................................................................................................. 11

10. Copyright ................................................................................................................................. 11

11. References ............................................................................................................................... 12

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List of Figures Figure 1: Site Location 1:25,000 @ A4 Figure 2: Location of Survey Area 1:5000 @ A3 Figure 3: Magnetic Greyscale 1:1000 @ A3 Figure 4: Magnetic Interpretation 1:1000 @ A3 Figure 5: Magnetic Interpretation Over Satellite Imagery 1:1000 @ A3 Figure 6: Magnetic Interpretation Over Historic Mapping 1:2000 @ A3 Figure 7: XY Trace Plot 1:1000 @ A3

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1. Introduction Magnitude Surveys Ltd (MS) was commissioned by York Archaeological Trust on behalf of City of York Council to undertake a geophysical survey on a c. 3.4ha area of land at the former Lowfield School, Acomb, York (SE 57480 50953). A c. 0.6ha area could not be surveyed due to overgrown vegetation.

The geophysical survey comprised hand-pulled, cart-mounted 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 commenced on 23 June 2017 and took one day to complete.

2. Quality Assurance Project management, survey work, data processing and report production have been carried out by qualified and professional geophysicists to standards exceeding the current best practice (CIfA, 2014; David et al., 2008, Schmidt et al., 2015).

Magnitude Surveys is a corporate member of ISAP (International Society of Archaeological Prospection).

Director Graeme Attwood is a Member of the Chartered Institute for Archaeologists (CIfA), the chartered UK body for archaeologists, as well as the Secretary of GeoSIG, the CIfA Geophysics Special Interest Group. 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. Director Chrys Harris has a PhD in archaeological geophysics from the University of Bradford.

All MS managers have postgraduate qualifications in archaeological geophysics. All MS field staff have relevant archaeology or geophysics degrees and supervisors have at least three years’ field experience.

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

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4. Geographic Background The site is located in Acomb, approximately 2.67km west of York, bounded by houses and gardens off Dijon Avenue to the north, Green Lane to the east, Tudor Road to the south, and Gale Lane to the west (Figure 1). Survey was undertaken over the area of the former school’s playing fields, which comprised flat topography and well-maintained grass. A c. 0.6 ha section east of the playing field was covered with overgrown vegetation and could not be surveyed (Figure 2).

Survey considerations:

Survey Area

Ground Conditions Further Notes

1 Flat, playing fields. Goal posts were still in situ on the southern half of the playing fields. A small circle of concrete was present roughly central to the site. High metal fences bounded the north, south and west, a large shipping container and the former school track bounded the east.

2 Flat, overgrown. Unsurveyable.

The underlying geology comprises Sherwood sandstone group - sandstone. Superficial deposits are Alne glaciolacustrine formation – clay, silt (British Geological Survey, 2017).

The soils in this area are unclassified (Soilscapes, 2017).

5. Archaeological Background The following archaeological background summarises the results of a Heritage Gateway (2017) search, focused on the village of Acomb and the landscape in the close vicinity of the site. The site was subject to a watching brief in 2007, which did not record any significant archaeological activity (ADS AIP No. E.36.4191). However, archaeological activity has been recorded within the vicinity of site, documented through archaeological investigations in Acomb.

Previous geophysical survey and excavation to the east recorded a possible Iron Age/Romano British field boundary off West Bank, Acomb (ADS AIP No. C.36.0018). Further possible Roman activity has been identified to the north-west of site, off Front Street, including a Roman site (ADS AIP No. C.92.4000) and three small pits of possible Roman origin (ADS AIP No. C.92.4354). An assessment undertaken on land towards The Green recorded multi-period Roman and Medieval activity (ADS AIP No. B.36.2009). An earthwork survey undertaken at Hob Moor Junior School to the south-east of site recorded ridge and furrow and a raised platform of unknown dates (ADS AIP No. E.92.U032).

A number of other archaeological investigations undertaken in the surrounding area have not recorded any material or features of archaeological significance (e.g ADS AIP No. E.92.F018; E.92.F035; E.92.4192).

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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.

6.1.3.1. MS’ cart system was comprised of Bartington Instruments Grad 13 Digital Three-Axis Gradiometers. Positional referencing was through a Hemisphere S321 GNSS Smart Antenna RTK GPS outputting in NMEA mode to ensure high positional accuracy of collected measurements. The Hemisphere S321 GNSS Smart Antenna 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. Rows of temporary sight markers were established in each survey area to guide the surveyor and ensure full coverage with the cart. Data were collected by traversing the survey area along the longest possible lines, ensuring efficient data 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.

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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. Multiple greyscales images at different plotting ranges have been used for data interpretation. Greyscale images should be viewed alongside the XY trace plot (Figure 7). 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 mapping, satellite imagery, historic mapping and soil and geology mapping. Google Earth (2017) was also consulted, 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 5), historic mapping (Figure 6) and XY trace plots (Figure 7).

The fluxgate gradiometer has responded well to the survey area’s environment; despite the overall “noisy” background magnetic levels of the site, a range of responses have been detected. Broad-scale and discrete ferrous responses have been detected across the site. A buried iron/steel pipe has been detected in the northeast corner. Strong linear responses are present positioned in a herringbone pattern, this pattern is typical of drainage. Much weaker, less defined, parallel linear anomalies classified as “Agricultural” may also represent drainage features, however a former ploughing regime is also possible. A number of anomalies form broad linear spreads which follow precisely the recorded alignment of former field boundaries.

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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. Drainage Features – A distinct herringbone patterning of anomalies in the

northeast of the survey area is characteristic of land drains; the strong magnitude of these anomalies is unusual however. The area directly around these drains is extremely magnetically enhanced which may be adding to the magnitude of the anomalies themselves. This magnetic enhancement may be due to the backfill used when the drains were first added; it is likely this was infused with rubble, metallic or burnt material.

A series of weaker parallel linear anomalies to the west of the dominant drainage features are also likely to be land drains. This weaker response, with a slight dipolar element, is more typical of drainage features than those of a much stronger magnitude.

7.3.2.2. Agricultural – Two irregular broad bands have been detected in the survey area running through the playing fields on a northwest-southeast alignment. The location and orientation of these features correlate with former field boundaries denoted on historic mapping (Figure 6). The eastern most of these bands has been cut through by a modern ferrous response and the strong drainage features, but the band appears to resume to the north and south of these features.

7.3.2.3. Ferrous – Broad-scale ferrous responses have been detected around the perimeter of the survey area. These represent the high metal fencing which surrounds the former school grounds to the north, south and west, as well as the presence of a shipping container and a subterranean service to the east. The anomalies labelled [1a] are the responses from in situ goalposts on the

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playing fields; [1b] is a small concrete circle (perhaps used in athletic throwing events), also present at the time of survey. [1c] was not extant at the time of survey, but can be seen in recent satellite imagery (Google Earth, 2017) with a different ground surface, which is likely to have also been used in a sports capacity. Each of these playing field related features have left a strong ferrous response.

Discrete small-scale dipolar responses are common across the site. These will be caused by modern ferrous debris on or near the ground surface. Concentrated patches of this modern ferrous material have been classified as “Ferrous (Spread)”.

Undetermined – Strongly positive anomalies in the northeast corner of the site have been classified as “Undetermined”. These possess a similar shape and alignment to the strong drainage features; however, they are less well defined and the magnetic response is different, not having the same negative magnetic shadow as those with a confident drainage classification. With these anomalies being at the edge of the survey area the context is lacking to provide a confident interpretation, though a modern origin is likely.

Weak linear trends have been identified which do not follow the alignment of drainage, former field boundaries, or any modern features visible on recent satellite imagery (Google Earth, 2017). It is likely that these linear trends are natural or modern in origin, however an archaeological origin cannot be entirely ruled out.

8. Conclusions A fluxgate gradiometer survey has been successfully completed across the surveyable extent of the site. The survey has detected a range of different types of responses, including agricultural and modern activity, as well as anomalies of undetermined origin. The detection of features with weak and strong magnitude demonstrate the technique has been effective at this site. An area of c. 0.6ha was unsurveyable due to overgrown vegetation.

Agricultural activity is evident in the magnetic results by the presence of two former field boundaries. Land drains are also a dominant feature of the site, the most prominent in the northeast corner, and the weaker to the west of these.

Modern activity is primarily demonstrated by ferrous responses, these run along the perimeter of the site due to fencing and a service line, and internally they represent elements of the playing fields, such as goalposts, and a concrete circle used in athletics throwing events.

Anomalies have been classified as “Undetermined” where a specific origin of the response is ambiguous through the geophysical results. These are considered more likely to represent natural changes or modern processes; however, an archaeological origin cannot be ruled out.

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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 all reports to the ADS Grey Literature Library subject to any time embargo dictated by the client.

Whenever possible, MS has a policy of making data available to view in easy to use forms on its website. This can benefit the client by making all of their reports available in a single repository, while also being a useful resource for research. Should a client wish to impose a time embargo on the availability of data, this can be achieved in discussion with MS.

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.

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11. References British Geological Survey, 2017. Geology of Britain. [Acomb, North Yorkshire]. [http://mapapps.bgs.ac.uk/geologyofbritain/home.html/]. [Accessed 27/06/2017].

Charted 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, 2017. Google Earth Pro V 7.1.7.2606. 53°57'04.5"N 1°07'35.6"W. Eye alt 678m. ©2017 Google © DigitalGlobe.

Heritage Gateway, 2017. Heritage Gateway. [http://www.heritagegateway.org.uk/gateway/]. [Accessed 20/06/2017].

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.

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, 2017. [Acomb, North Yorkshire]. Cranfield University, National Soil Resources Institute [http://landis.org.uk]. [Accessed 27/06/2017].