GEOPHYSICAL SURVEY, HARPLEY LONG BARROW

Transcription

1 GEOPHYSICAL SURVEY, HARPLEY LONG BARROW GPR, Magnetic and Resistivity Survey The slight earthwork remains of the Long Barrow at Harpley, with minor road cutting through northern flank, viewed from the NNE. Photograph: Giles Carey Site Code: HRP16 NGR: TF Scheduled Monument: NHLE ; Norfolk HER: NHER 3637; ENF Giles Carey with David Ashby 2016 With financial support from the HRP 16: Geophysical Survey at Harpley Long Barrow 1

2 Contents Contents...2 List of Figures...2 Summary...3 Summary...3 Acknowledgements...3 Introduction...4 Project Background...4 Site Location...4 Aims and objectives...5 Archaeological background...5 Geophysical survey...6 Standards...6 Field methods...6 Resistivity survey...6 Magnetic survey...6 GPR survey...6 Data processing...6 Results...7 Conclusions...8 Bibliography...9 Appendix 1: Technical data...10 Figures...11 List of Figures Figure 1: General Location Plan...12 Figure 3: Resistivity Data Greyscale (raw)...14 Figure 4: Resistivity Data Greyscale (processed)...15 Figure 5: Resistivity Data colour plot...16 Figure 6: Resistivity Data Greyscale (low pass filtered)...17 Figure 7: Resistivity Interpretation...18 Figure 8: Magnetic Data Greyscale (Raw)...19 Figure 9:Magnetic Data Greyscale (processed)...20 Figure 10: Magnetic Survey Interpretation...21 Figure 11 GPR Time slice data Illustrated at depths of:...26 (A) M...26 (B) M...26 (C) M...26 (D) M...26 (E) M...26 Figure 12: Interpretative plan based on GPR survey results...27 Figure 13: Topographic survey of Harpley Long Barrow (undertaken by Dr. Oliver Harris) HRP 16: Geophysical Survey at Harpley Long Barrow 2

3 Summary A small-scale programme of geophysical survey was undertaken in April 2016 at two neighbouring barrows, at West Rudham (centred on TF ) and Harpley (centred on TF ). This report is focused on survey at Harpley (with work at West Rudham being reported on separately; Carey, 2016). This geophysical survey comprised ground penetrating radar (GPR) survey, magnetic (fluxgate gradiometry) and resistivity survey. This work was undertaken as part of a wider project to characterise the geophysical 'signature' of well attested long barrow sites in the Midlands and East Anglia. The results were promising and confirmed the original layout of the barrow, now truncated on its northern edge by construction of the Harpley to Weasenham St. Peter road. It confirmed the survival of below-ground ditch deposits, now largely infilled. Little variation in geophysical response was identified on the barrow mound itself. Acknowledgements The survey was undertaken by a small project team: Dr. Roy Loveday and Dr. Oliver Harris (University of Leicester), Giles Carey (independent archaeologist) and David Ashby (University of Winchester). Oliver Harris undertook the dgps survey of both monuments. The assistance of Dr. Mark Gillings (University of Leicester) with the processing of data is also gratefully acknowledged. The geophysical survey would not have been possible without equipment loaned by the Universities of Leicester and Winchester and access to processing software. Access to both sites was facilitated through the good offices of David Robertson, Historic Environment Officer (Countryside), Norfolk County Council - who is greatly thanked for his assistance. The site is scheduled and therefore Section 42 consent was required; this was applied for and duly granted by Historic England. Nick Carter, Assistant Inspector of Ancient Monuments (Bedfordshire, Norfolk and Suffolk) is thanked for his assistance with this matter. The fieldwork undertaken during this phase of the project was supported with a grant from the Prehistoric Society; their financial assistance is gratefully acknowledged. All conclusions/interpretations are the author s responsibility alone. HRP 16: Geophysical Survey at Harpley Long Barrow 3

4 Introduction This report presents a programme of geophysical survey undertaken in April 2016 at the site of Harpley Long Barrow (centred on TF ). The survey comprised ground penetrating radar (GPR) survey, magnetic (fluxgate gradiometry) and resistivity survey. The work was undertaken as part of a wider-ranging project: Characterising Survivors, Explaining Absence, a Survey of Midland and East Anglian Long Barrows (Loveday, 2015). Harpley Long Barrow is a Scheduled Monument, and therefore Section 42 consent was applied for to undertake the survey. This was granted in March 2016 by Historic England (East of England Office). Project Background Site Location The long barrow mound lies c.650m ESE of Cross's Grave to the SE of the village centre of Harpley, positioned along the Harpley to Weseanham St Peter minor road. The long barrow at Harpley stands on a strip of set-aside, former heathland that stood at the western end of the Good Sands region in north-west Norfolk. The West Rudham long barrow (the survey of which is subject of another report) lies just c190m to the north. A number of small round barrows are dispersed around these two long barrows, including one placed adjacent to the southern end of the Harpley monument. This long barrow is visible as an oval mound 1.2m high and 31m long by 23m wide. The extant earthwork is part of a larger mound that has been destroyed by ploughing and the construction of the Harpley to Weasenham St Peter road - the extant earthwork measures just 0.15m in height at its N end, reduced from a height of c1.2m at the south. This reduction may have resulted from the passage of traffic along the track between Harpley and Weasenham St Peters; the current narrow, tarmac road presumably restricting the breadth of such movement. It is considered possible that the northern extent of the barrow lies beneath this road. Prior to the ploughing of the heath (post 1938) the mound was recorded as 46m (N-S) x 27.5m (E-W) with a visible ditch c4.5m wide (Norfolk HER NHER 3637). It has the appearance of a short oval long barrow enclosed by its ditch, but may extend further beneath the road. The mound itself was previously surrounded by a complete circuit of concrete posts, reinforced with rebars, of which a few remain. The survey area for all techniques was the same, covering the full area of the earthwork monument and continuing as close to the road as possible. It was possible to carry out GPR survey across the road surface to try and characterise the extent of below-ground survival in this area. As with previous fieldwork carried out for this project, the survey was particularly targeted on establishing the geophysical 'signature' of the ditch, with the aim of providing a dataset for cross-comparison over a wide geographical area (see Carey, 2015). The ditch at Harpley Long Barrow is recorded as having become largely infilled but surviving as a buried feature (NHLE ). HRP 16: Geophysical Survey at Harpley Long Barrow 4

5 The site sits upon sand and gravel on a parent chalk geology (BGS, 2015). The underlying soils of the area are recorded as acid and sandy, on the edge of lowland heath (Soilscapes, 2015). Aims and objectives The purpose of any geophysical survey is to as far as reasonably possible, determine the nature of the detectable archaeological resource within a specified area using appropriate methods and practices (English Heritage, 2008: 3). The key aim of the survey was to assess the potential of the site to respond to a range of geophysical survey techniques. It was quickly identified that the presence of the reinforced concrete posts around the site, coupled with disturbance associated with road construction on the northern flank of the monument, would severely affect the results of any magnetic survey undertaken on the site. It was envisaged, therefore, that GPR and resistivity survey would provide critical survey data for this site. The project at Harpley was undertaken as part of a wider project to explore the geophysical signature of barrow sites in the Midlands and East Anglia. As stated by Loveday (2016), the key aim of this project is to acquire data on barrow ditches in particular to: 1) Clarify the external characteristics of a Neolithic monument type that is rare in the two regions, and 2) furnish a secure gauge for the interpretation of plough-levelled long enclosure sites as either former barrows or embanked enclosures. Given that the focus of the project is on ditch depth and profile, it was felt that GPR was a crucial technique to apply. In particular, at Harpley, a key research question was the survival of the ditch below ground. GPS topographic survey carried out in parallel with the geophysical survey clearly indicated the survival of the quarry ditch as a slight earthwork (figure 13; contra Castleden, 2014: 144). The extent to which the ditch extended under the road, and hence, the plan form of the monument remained a key focus of the GPR survey in particular. Archaeological background The Harpley long barrow stands on former heathland near the western edge of the Goodsands upland region of Norfolk. The earthwork was under plough in the mid 20 th century but is now protected in a strip of road side pasture. It comprises a sub-oval mound c31m x 23m x 1.2m in height, aligned north - south. The north end is much reduced but is still marked for c12m as a slight rise of c 0.15m. The fact that the road deviates slightly around this northern end suggests the mound may extend further, the road representing a narrowed and metalled formalisation of a wider track responsible for the erosion. In 1938, before the heath was ploughed, the mound was recorded as being surrounded by a ditch-hollow c4.5m wide (Hogg, 1941: 315-6). Dimensions at that time were 46 x 27.5m. The Harpley long barrow has never been the subject of investigative fieldwork and thus only has only received incidental comment in the literature (Lawson, 1981:21; Clarke, 1961). A small round mound is recorded almost touching the enclosing ditch at the SSE point of its circuit. Similar small round mounds are scattered between the Harpley long barrow and the HRP 16: Geophysical Survey at Harpley Long Barrow 5

6 larger West Rudham long barrow 192m to the north, and a very large bell barrow (Cross s Grave) lies 650m to the ESE. Geophysical survey Standards The surveys and subsequent reporting were carried out in accordance with English Heritage s guide to Geophysical Survey in Archaeological Field Evaluation (2008), the IfA s Standard and Guidance for Archaeological Geophysical Survey (Draft) (IfA, 2010) and the ADS Geophysical Data in Archaeology: A Guide to Good Practice (Schmidt, 2001). Field methods An overall survey grid was established using tapes, with reference to known points on Ordnance Survey mapping. This grid was subsequently surveyed using dgps. Data collection was carried out using a standard methodology, with all grids walked in the same direction (N-S), in a zig-zag traverse pattern. A standard grid size of 20m 2 was used for all surveys. Resistivity survey An RM15 resistivity metre, with PA5 multiplexer, in parallel twin probe array mode was used to conduct the survey. The transect interval was 0.5m and sample interval was 1.0m. Magnetic survey A Bartington Grad dual fluxgate gradiometer was used to conduct the magnetic survey; the sample interval used was 0.25m with a traverse interval of 0.5m. This amounted to 3200 readings per 20m 2 grid. All data was logged in continuous mode. GPR survey The GPR survey was carried out using a pulse EKKO PRO 250 MHz transducer connected to a Digital Video Logger (DVL) to control and record the data from the unit. The traverse interval was 1.0m, with readings being taken to a depth of 3 m. The readings were automatically logged at m intervals, giving a resolution of 400 readings per 10 m linear traverse. Data processing Geoplot software (version 3) was used to download and process the magnetic and resistivity survey data. Greyscale plots of both raw and processed data were produced in Geoplot. Processed data and interpretation diagrams, which have been georectified in AutoCAD Map 3D and ArcView GIS, are presented in this report, with raw data included in the project archive. The raw data has been subject to minimal editing to remove operator error, with data subsequently processed to remove geological and background biases and interpolated to aid interpretation (see appendix 1). GPR data was processed using GFP_edit 4 for processing, and remapped to form timeslices. This included reversing alternate lines, so the data shows as zig-zag. The data was further processed in EKKO-Mapper 4 (see appendix 1). A series of timeslices were produced, every 0.10m. A selection of these timeslices were georeferenced in ArcView and are reproduced in this report to give an overview of results. A full set of timeslices and pseudosections are included in the project archive. HRP 16: Geophysical Survey at Harpley Long Barrow 6

7 The full raw dataset is held in the digital archive for this project. Results The results of the magnetic survey have, as anticipated, been adversely affected by modern disturbance and ferromagnetic and other obstacles in the survey area. It is clear that the reinforced concrete posts which survive on site once formed part of a complete circuit around the monument. What remains in the ground clearly contains a significant quantity of nearsurface magnetic material with a distinctive dipolar response, which has severely limited the usefulness of the magnetic survey on the site. However, the GPR and resistivity survey have been successful in identifying the extent of the mound and ditch deposits at the site. It is clear from all surveys that the general ditch plan surrounding the mound is oval in form rather than sub circular as indicated on modern Ordnance Survey mapping. The GPR survey (see below), particularly indicated that the ditch on the northern flank of the monument has been severely truncated and this may explain previous interpretations. It is clear, taking the results of the surveys together, that barrow mound material does not extend under the road as has been previously postulated. The road appears, in both the resistivity and GPR surveys to 'clip' the northern ditches of the monument but suggests that the vast majority of the monument survives to the south of the road corridor. Resistivity survey The resistivity survey provided the clearest plan of the monument. The mound itself is evident as a high resistance anomaly, surrounded by a halo of mid range resistance representing the barrow ditch, c.4m wide. Little detail is apparent on the mound itself, even when running a low pass filter on the data in an attempt to provide some differentiation to this area of the survey (figure 6). A possible quarry has been identified to the east of the quarry mound, although this is offered as a tentative interpretation, given that the feature is not immediately apparent in either GPR survey or topographic survey (see figure 13) carried out on this site. Magnetic survey As stated above, the results of the magnetic survey have been severely limited by the presence of significant ferromagnetic responses. The way that these dipolar responses surround the monument suggests that significant reinforcing material remains in the ground although the posts have largely been removed. These dipolar responses have unfortunately, masked any potential archaeological responses within the survey. This has been demonstrated by superimposing the plan of the barrow mound as recorded in resistivity survey on the magnetic survey results (figure 10). These type of dipolar responses cause "very large spurious magnetic anomalies...that are very difficult to suppress" (Gaffney and Gater, 2003: 82). Regrettably, the effect of this disturbance was widespread across the survey area, reflecting the 'lag' effects of very high magnetic enhancement on the fluxgate gradiometers. These anomalously high readings also made processing of the data difficult. Further ferromagnetic disturbance was noted at the northern edge of the survey area, associated with the road and its construction. HRP 16: Geophysical Survey at Harpley Long Barrow 7

8 GPR survey A selection of timeslices have been reproduced in this report (figure 11), with a full set available in the project digital archive. Key features have been identified from overall comparison of this data (figure 12). No internal features have been identified on the barrow mound by the GPR survey. The mound itself is identifiable as consisting of high amplitude material, consistent with being of turf construction. This barrow mound was encircled by the ditch evident as a low amplitude response. The ditch is easiest to recognise at a shallower depth, becoming more difficult to identify below c.1.50m bpgl. At a depth of 0.50m bpgl the ditch appears to be c.4m wide, narrowing down to c.2m at a depth of 0.80m bpgl- 1.40m bpgl. The ditch and mound remain difficult to identify in GPR survey at the northern end of the mound, particularly at depths greater than c0.70m bpgl. This appears to indicate that there has been some truncation in this area, possibly associated with road construction. This would also appear to be the case in the resistivity survey, where road construction appears to have disturbed ditch material. Conclusions As was clear on site, the data from the magnetic survey has been severely compromised by the presence of large dipolar anomalies caused by the iron reinforcing of the concrete posts ringing the site. As discussed above, these anomalies had a large impact on the quality of magnetic data collected for the site, causing a 'lag' in the magnetic enhancement of the data that was very difficult to process. Through comparison with the other survey data from the site it was evident that no magnetic signature of the barrow was recoverable from the data. The GPR and resistivity surveys at Harpley have provided comparative material for the barrow plan form and have indicated the survival of mound material in a largely intact form south of the road. Both surveys have suggested that the construction of the road has directly implicated on ditch deposits at the northern flank of the monument, although survival appears to be good elsewhere. Unfortunately, both datasets provide little evidence of internal features, with little differentiation evident in the geophysical response of the barrow mound. Overall, therefore, it is clear that the results from Harpley contribute to the wider agenda of this project in establishing some concrete data on ditch depth, profile and plan form. It is anticipated that further comparative work will be undertaken on the barrow ditches at both Pitsford (Carey, 2015) and West Rudham (Carey, 2016), particularly the creation of pseudo sections, that will provide most effective direct comparison of the results. HRP 16: Geophysical Survey at Harpley Long Barrow 8

9 Bibliography BGS, British Geological Survey Geology of Britain < (accessed 27/05/2016). Carey, G Geophysical survey, Longmans Hill Long Barrow, Pitsford, Northamptonshire: GPR, Magnetic and Resistivity Survey. Unpublished report. Carey, G Geophysical survey, West Rudham Long Barrow: GPR, Magnetic and Resistivity Survey. Unpublished report. Castleden, R Neolithic Britain: New Stone Age Sites of England, Scotland and Wales London: Routledge English Heritage, 2008 Geophysical Survey in Archaeological Field Evaluation < (accessed 28/04/2015). Gaffney, C. and Gater, J Revealing the Buried Past: geophysics for archaeologists Stroud: Tempus Geoscan Research 2005 Geoplot Instruction Manual < (accessed 26/06/2015). Hogg, A. H. A A Long Barrow at West Rudham, Norfolk Norfolk Archaeology 27.II pp IfA, 2010 Draft Standard and Guidance for Archaeological Geophysical Survey < (accessed 28/04/2015). Lawson, A. J. Martin, E. A., Priddy, D. with Taylor, A The Barrows of East Anglia, East Anglian Archaeology 12 Loveday, R Application for a Section 42 licence to carry out geophysical survey at Harpley Long Barrow, Norfolk. Unpublished correspondence with Historic England Sensors and Software, EKKO_Mapper User's Guide Soilscapes, Soilscapes Viewer. National Soil Resources Institute (University of Cranfield) < (accessed 29/05/2016). Schmidt, A Geophysical Data in Archaeology: A Guide to Good Practice York: Archaeology Data Service HRP 16: Geophysical Survey at Harpley Long Barrow 9

10 Appendix 1: Technical data 1. Resistivity survey data Raw Clip (limits maximum and minimum values for display and subsequent processing): -1/+1 SD. Processed Low pass filter (to remove minor geological variance in the dataset): Mean, Gaussian 21x21 radius Interpolation (smoothes greyscale appearance by adding extra data points into the dataset, calculated with reference to surrounding collected data) on both the Y and X-axis. 2. Magnetic survey data Raw Clip (limits maximum and minimum values for display and subsequent processing): -3/+3 nt. Zero Mean Traverse Despike (removes large anomalies above a certain threshold): x-radius 2; y-radius 2; threshold: 1 Processed Interpolation (smoothes greyscale appearance by adding extra data points into the dataset, calculated with reference to surrounding collected data) on both the Y and X-axis. 3. GPR data Velocity calibrated using a hyperbolic curve, set to m/ns. Amplitude Equalization Gain set to manual and Max Gain increased to For mor detail on Processing PGR data please see: Sendor & Softwear Inc. (2009). EKKO_Mapper User s Guide. For more technical information on data processing for resistivity and magnetic data, see (Geoscan Research, 2005: Chapter 6); for more detail on processing GPR data see (Sensors and Software, 2009). HRP 16: Geophysical Survey at Harpley Long Barrow 10

11 Figures HRP 16: Geophysical Survey at Harpley Long Barrow 11

12 FIGURE 1: GENERAL LOCATION PLAN HRP 16: Geophysical Survey at Harpley Long Barrow 12

13 FIGURE 2: SURVEY AREAS HRP 16: Geophysical Survey at Harpley Long Barrow 13

14 FIGURE 3: RESISTIVITY DATA GREYSCALE (RAW) HRP 16: Geophysical Survey at Harpley Long Barrow 14

15 FIGURE 4: RESISTIVITY DATA GREYSCALE (PROCESSED) HRP 16: Geophysical Survey at Harpley Long Barrow 15

16 FIGURE 5: RESISTIVITY DATA COLOUR PLOT HRP 16: Geophysical Survey at Harpley Long Barrow 16

17 FIGURE 6: RESISTIVITY DATA GREYSCALE (LOW PASS FILTERED) HRP 16: Geophysical Survey at Harpley Long Barrow 17

18 FIGURE 7: RESISTIVITY INTERPRETATION HRP 16: Geophysical Survey at Harpley Long Barrow 18

19 FIGURE 8: MAGNETIC DATA GREYSCALE (RAW) HRP 16: Geophysical Survey at Harpley Long Barrow 19

20 FIGURE 9:MAGNETIC DATA GREYSCALE (PROCESSED) HRP 16: Geophysical Survey at Harpley Long Barrow 20

21 FIGURE 10: MAGNETIC SURVEY INTERPRETATION HRP 16: Geophysical Survey at Harpley Long Barrow 21

22 11A HRP 16: Geophysical Survey at Harpley Long Barrow 22

23 11B HRP 16: Geophysical Survey at Harpley Long Barrow 23

24 11C HRP 16: Geophysical Survey at Harpley Long Barrow 24

25 11D HRP 16: Geophysical Survey at Harpley Long Barrow 25

26 11E FIGURE 11 GPR TIME SLICE DATA ILLUSTRATED AT DEPTHS OF: (A) M (B) M (C) M (D) M (E) M HRP 16: Geophysical Survey at Harpley Long Barrow 26

27 FIGURE 12: INTERPRETATIVE PLAN BASED ON GPR SURVEY RESULTS HRP 16: Geophysical Survey at Harpley Long Barrow 27

28 Barrow ditch FIGURE 13: TOPOGRAPHIC SURVEY OF HARPLEY LONG BARROW (UNDERTAKEN BY DR. OLIVER HARRIS). HRP 16: Geophysical Survey at Harpley Long Barrow 28