GEOPHYSICAL SURVEY, WEST RUDHAM LONG BARROW

Transcription

1 GEOPHYSICAL SURVEY, WEST RUDHAM LONG BARROW GPR, Magnetic and Resistivity Survey Resistivity survey underway of northern end of West Rudham Long Barrow. Photograph: Roy Loveday Site Code: WR16 NGR: TF Scheduled Monument: NHLE Norfolk HER: NHER 3611; ENF Giles Carey with David Ashby 2016 With financial support from the WR16: Geophysical Survey at West Rudham 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 Appendix 1: Technical data...11 Figures...12 List of Figures Figure 1: General Location Plan...13 Figure 2: Survey Areas...14 Figure 3: Resistivity Data Greyscale (raw)...15 Figure 4: Resistivity Data Greyscale (processed)...16 Figure 5: Resistivity Data colour plot...17 Figure 6: Resistivity Data Greyscale (low pass filtered)...18 Figure 7: Resistivity Interpretation...19 Figure 8: Magnetic Data Greyscale (Raw)...20 Figure 9:Magnetic Data Greyscale (processed)...21 Figure 10: Magnetic Survey Interpretation...22 Figure 11 GPR Time slice data Illustrated at depths of:...27 (A) M...27 (B) M...27 (C) M...27 (D) M...27 (E) M...27 Figure 12: Interpretative plan based on GPR survey results...28 Figure 13: Topographic survey of West Rudham Long Barrow (undertaken by Dr. Oliver Harris) WR16: Geophysical Survey at West Rudham 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 West Rudham (with work at Harpley 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 have been promising, although the area available for investigation was limited. In particular, the GPR survey demonstrated significant potential across the long barrow at West Rudham, and elucidated clearly the ditch plan and profile. This was largely confirmed in resistivity survey, although little differentiation in barrow makeup was noted. Unfortunately, the presence of a high number of localised dipolar magnetic responses at both sites precluded the collection of usable data. from the gradiometer survey. 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. WR16: Geophysical Survey at West Rudham Long Barrow 3

4 Introduction This report presents a programme of geophysical survey undertaken in April 2016 at the site of West Rudham Long Barrow (centred on SP ). 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, 2016). The site of West Rudham Long Barrow is a Scheduled Monument, and therefore Section 42 consent was applied for to undertake the survey. This was granted in March 2015 by Historic England (East of England Office). Project Background Site Location The long barrow mound lies c.2km SE of the village centre of Harpley, 190m north east of the parish boundary between West Rudham and Harpley. The long barrow at West Rudham stands on level heathland at the western end of the Good Sands region. The Harpley long barrow (the survey of which is subject of another report) lies just c190m to the south and a number of round barrows are situated around and between these mounds; a large bell barrow is located c600m to the south east. The barrow has been described as the best preserved example in Norfolk (Norfolk HER NHER 3611). The mound itself is surrounded by a complete circuit of concrete posts, which are reinforced with rebars. These posts demarcated the extent of the monument, as the surrounding area was previously under arable cultivation in the 1990s (Norfolk HER NHER 3611). The site is tightly fenced at its southern end with a post and chainlink fence. Several mature trees are present on the mound itself, and at the time of survey was covered by a fair blanket of bracken. The long barrow itself is well preserved, measuring some 60m in length x 16-21m in width. It stands to a maximum height of 1.5m and is orientated NNE-SSW. Topographic survey clearly indicated the survival of the eastern ditch of the barrow (figure 13); the western flanking ditch is more discontinuous and areas of possible disturbance were noted on this side of the barrow mound. It is known that the site was disturbed by warrening activity earlier in the 20th century (Sainty, 1938). The survey area was necessarily tightly focused around the southern end of the mound with as extensive coverage an possible across the mound and within its immediate vicinity. The magnetic survey provided coverage across the whole of the monument and an area to the east; GPR and resistivity survey were focused on the well preserved northern end of the barrow, with an extension of the GPR survey to the east to investigate the ditch profile. 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 West Rudham Long Barrow is recorded as having become largely infilled but surviving as a buried feature (NHLE ). WR16: Geophysical Survey at West Rudham 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 the chainlink fence at the southern end of the barrow mound and small-scale dumping on 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 West Rudham 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 West Rudham, it was hoped that the survival of the ditches could be accessed. Excavations in the 1930s (see below) had established, to some extent, the plan form and profile of the ditches from the site, but it is acknowledged that they have become significantly infilled and only survive as subtler earthworks ("slight linear hollows") today (NHLE ). Archaeological background The long barrow at West Rudham stands on level heathland at the western end of the Good Sands region. The Harpley long barrow (NHLE ) lies just c190m to the south and a number of round barrows are situated around and between these mounds; a large bell barrow is located c600m to the south east. The long barrow itself is well preserved, measuring some 60m in length x 16-21m in width. It attains its maximum height(1.5m) some 15m from the SSW end; from that point it slopes gently down to a height of 1m at its NNE end. Excavations undertaken on behalf of the Norfolk Research Committee in 1937 (Sainty et al., 1938) and 1938 (Hogg, 1940) exposed the major part of the southern end of the mound and investigated the rest by spaced trenching. This demonstrated that the mound was constructed of turves and completely enclosed by a moderately sized ditch (average. 3.5m wide x 1.2m deep) of rectangular plan with curving corners. There were no signs of typical upland flanking quarry features. The contents of the ditch had been used to furnish a gravel capping for the turf mound. Near the south end of the barrow this ditch lay under the mound rather than around its edge. Just within it was a gravel platform (0.3m high) that had been reddened by fire. A smaller WR16: Geophysical Survey at West Rudham Long Barrow 5

6 ditch ran from the south east and south west corners of the main ditch at this point to form an apsidal enclosure 11m long. It defined the southern end of the mound that appears to have been both an extension from, and a covering for, the platform. Neither structures nor burials were located during the excavation, the latter being explicable in terms the acidity of the sandgravel subsoil. The 1937 excavation located a spread of flint debitage and burnt flint within a distinct depression outside the eastern arm of the ditch near the centre point of the mound. This material lay in a matrix of black soil some 0.23m thick at the centre and extending some 9m (N-S) x 5.5m (E-W). Where it extended into the long barrow ditch (section V: Sainty et al., 1938) it overlay the primary silts. A sherd of Beaker came from just above this layer and a barbed and tanged arrowhead above the primary silts of the shallow extension ditch at the southern end of the mound. 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. WR16: Geophysical Survey at West Rudham Long Barrow 6

7 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. 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. The ring of reinforced concrete posts around the monument unfortunately limited the usefulness of the magnetic survey on the site. However, the GPR and resistivity survey have been successful in identify the extent of the mound and ditch deposits at the site. Interpretations offered are tentative only, based upon the limited area available for survey. Resistivity survey The resistivity survey confirmed the form of the earthwork barrow mound, recorded as a spread of high resistance material (A). This indicates that the material used for the bank is exceptionally dry, in comparison to the surrounding ground. The barrow ditch is clearly visible ringing the barrow mound, as an area of contrasting low resistance (B). It is clear as a 'spread' on the eastern side of the barrow mound, with it clearly extending beyond the area surveyed. Its form appears more disturbed on the western flank of the barrow, where there are a number of high resistance anomalies which are of indeterminate origin (C). Of particular note is a small 'gap' in the lower resistance ditch at the NE of the barrow mound (D). This is of particular interest given the identification of "partial causeways" in excavations in 1930, at the southern end of the mound (NHLE ). 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). The few discrete anomalies which are apparent all appear to be identifiable as the result of root activity (hardly suprising given the depth of the barrow mound material at this point). Magnetic survey The results of the magnetic survey have been severely limited by the presence of significant ferromagnetic responses, clearly corresponding with the presence of iron-reinforced concrete posts which ring the monument; these are evident as large dipolar responses (A). A second diploar response recorded from on top of the mound itself was identifiable with a discrete area of dumping on top of the monument (B). These dipolar responses have unfortunately, masked any potential archaeological responses within the survey. 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 WR16: Geophysical Survey at West Rudham Long Barrow 7

8 very high magnetic enhancement on the fluxgate gradiometers. These anomalously high readings also made processing of the data difficult. A number of weak magnetic responses have been identified, particularly at the southern end of the barrow mound (C). Unfortunately, for reasons outlined above, these features offer very little contrast with the material of the mound itself so it is difficult to interpret them further. 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 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, encircled by the ditch evident as a low amplitude response. At a depth of 1.50m bpgl the ditch appears to be c.3m wide; at a depth of 2.50m bpgl the ditch appears to be c.2.5m wide. 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. Some localised dumping across the southern end of the monument also caused some localised ferrous spikes. 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. This is particularly regrettable given that smaller-scale magnetic enhancement evident at the southern end of the barrow mound that had the potential to be of archaeological origin. The GPR and resistivity surveys at West Rudham have provided comparative material for the barrow plan form and some detail of the northern arc of the barrow ditch. Both surveys have shown that the ditch is more clearly defined on the eastern side, with the western extent more heavily disturbed, potentially by quarrying as evident on the topographic survey. Both surveys have hinted at the irregularity of the ditch cut, or at least the spread of material away from the ditch edge, away from the barrow. Some of this activity may relate to previous agricultural activity in close proximity to the barrow mound. A postulated causeway was most clearly identified in the resistivity survey carried out at the site. This can also be suggested in the GPR survey, most clearly evident in timeslices between m (evident as an interruption in the low amplitude response of the barrow ditch - circled below): m m m Overall, therefore, it is clear that the results from West Rudham contribute to the wider agenda of this project in establishing some concrete data on ditch depth, profile and plan form. It is WR16: Geophysical Survey at West Rudham Long Barrow 8

9 anticipated that further comparative work will be undertaken on the barrow ditches at both Pitsford (Carey, 2015) and Harpley (Carey, 2016), particularly the creation of pseudo sections, that will provide most effective direct comparison of the results. WR16: Geophysical Survey at West Rudham Long Barrow 9

10 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, Harpley Long Barrow: GPR, Magnetic and Resistivity Survey. Unpublished report. 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 West Rudham Long Barrow, Norfolk. Unpublished correspondence with Historic England Sainty, J. E Long Barrow at West Rudham, Norfolk The Antiquaries Journal 18 pp Sainty J. E., Watson, A. Q. and Clarke, R. R The First Norfolk Long Barrow. Norfolk Archaeology 26.III pp 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 WR16: Geophysical Survey at West Rudham Long Barrow 10

11 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). WR16: Geophysical Survey at West Rudham Long Barrow 11

12 Figures WR16: Geophysical Survey at West Rudham Long Barrow 12

13 FIGURE 1: GENERAL LOCATION PLAN WR16: Geophysical Survey at West Rudham Long Barrow 13

14 FIGURE 2: SURVEY AREAS WR16: Geophysical Survey at West Rudham Long Barrow 14

15 FIGURE 3: RESISTIVITY DATA GREYSCALE (RAW) WR16: Geophysical Survey at West Rudham Long Barrow 15

16 FIGURE 4: RESISTIVITY DATA GREYSCALE (PROCESSED) WR16: Geophysical Survey at West Rudham Long Barrow 16

17 FIGURE 5: RESISTIVITY DATA COLOUR PLOT WR16: Geophysical Survey at West Rudham Long Barrow 17

18 FIGURE 6: RESISTIVITY DATA GREYSCALE (LOW PASS FILTERED) WR16: Geophysical Survey at West Rudham Long Barrow 18

19 FIGURE 7: RESISTIVITY INTERPRETATION WR16: Geophysical Survey at West Rudham Long Barrow 19

20 FIGURE 8: MAGNETIC DATA GREYSCALE (RAW) WR16: Geophysical Survey at West Rudham Long Barrow 20

21 FIGURE 9:MAGNETIC DATA GREYSCALE (PROCESSED) WR16: Geophysical Survey at West Rudham Long Barrow 21

22 FIGURE 10: MAGNETIC SURVEY INTERPRETATION WR16: Geophysical Survey at West Rudham Long Barrow 22

23 11A WR16: Geophysical Survey at West Rudham Long Barrow 23

24 11B WR16: Geophysical Survey at West Rudham Long Barrow 24

25 11C WR16: Geophysical Survey at West Rudham Long Barrow 25

26 11D WR16: Geophysical Survey at West Rudham Long Barrow 26

27 11E FIGURE 11 GPR TIME SLICE DATA ILLUSTRATED AT DEPTHS OF: (A) M (B) M (C) M (D) M (E) M WR16: Geophysical Survey at West Rudham Long Barrow 27

28 FIGURE 12: INTERPRETATIVE PLAN BASED ON GPR SURVEY RESULTS WR16: Geophysical Survey at West Rudham Long Barrow 28

29 Area of eastern ditch Areas of disturbance FIGURE 13: TOPOGRAPHIC SURVEY OF WEST RUDHAM LONG BARROW (UNDERTAKEN BY DR. OLIVER HARRIS). WR16: Geophysical Survey at West Rudham Long Barrow 29