Pen Selwood Geophysical Survey

Transcription

1 Pen Selwood Geophysical Survey Prepared for: The South Somerset Archaeological Research Group and the Pen Selwood History Society Prepared by: Geoff Parcell 9 February 2018 Report Number: /GS/1015! 1 of 11!

2 SUMMARY Between December 2015 and January Ha were surveyed using a Bartington Grad gradiometer. The work was carried out when people were available between other activities and when the field was available after grass cutting. The survey established the magnetic method does work over a Cretaceous Greensand geology. A number of anomalies have been identified which may warrant further investigation. BACKGROUND The village of Pen Selwood is situated on a prominent Upper Greensand ridge rising to 240 metres marking a major topographic divide between the Chalk ridgeway to the East and the Somerset levels to the West. It occupies the southern end of a prominent wooded area which formed the core of the ancient forest of Selwood. The ridge marks the watershed of the rivers Stour to the East, Cale to the West and Brue to the North. Pen Selwood is in an area steeped in history including an Iron Age hillfort, Kenwalch s castle, Bronze Age settlements, Egbert s Stone marking the junction of three counties, Somerset, Wiltshire and Dorset, three motte and bailey castles each within 2 miles of Pen church, and 20,000 Pen Pits. The pits have been identified as pre-norman quern quarries, probably of more than one period, from the Iron Age onwards though their purpose has been the cause of much debate. The Iron Age hillfort, three motte and bailey castles and two areas of Pen Pits are scheduled monuments. Given the strategic nature of the ridge, was asked by Pen Selwood History Society to help evaluate a large field at the southern end of the ridge survey to determine subsurface features (centred on NGR ST ). (Figure 1) Figure 1. Location of survey! 2 of 11!

3 Aerial photos on Google Earth show a significant number of features which may relate to pits, though they have never been mapped and they are outside the area of scheduled monument. (Figure 2) Figure 2. Aerial photo from Google Google 2018 An initial 23 grids (0.9 Ha) were surveyed by gradiometer in December 2015 in an adjoining field (NGR ST ) to test if the geology (Cretaceous Boyne Hollow Chert member sandstone, part of the Greensand formation ) was suitable for giving distinctions in magnetic response. Having established that good results were achieved the survey was moved to the main field and between September 2016 and January contiguous whole and partial squares were surveyed covering the whole of the field (Figure 3). The survey area covered c ha.! 3 of 11!

4 Figure 3. Location of grids! 4 of 11!

5 EQUIPMENT USED Fluxgate gradiometer - Bartington Grad The Bartington Grad is a dual system gradiometer, a form of magnetometer. It comprises two sensor rods carried on a rigid frame, each sensor including two fluxgates aligned at 90 degrees to each other, one set 1m above the other. It measures variations in the magnetic field between the two fluxgates, recorded in nanotesla (nt) at each sampling point within a grid. The manufacturer claims a depth range of approximately three metres. The instrument is most effective when carried at a consistent height, not exceeding 0.3m above the ground. Magnetometers are especially effective for discovering thoroughly decayed organic materials, such as those which accumulate in ditches and pits, and matter exposed to intensive firing, including industrial areas, hearths and larger ceramics. All of these are likely to give a positive magnetic response, sometimes with a negative halo, giving a dipolar effect. Non-igneous stone features, such as walls and banks, are usually perceived as negative anomalies against a background enhanced by decayed organics. Software: Geoscan Geoplot 3.00v Geoplot 3.00v allows the presentation of data in four graphical forms: dot-density, grey scale, pattern and X-Y (or trace) plots. The latter are particularly effective when used in conjunction with other graphical modes to emphasise ferrous magnetic anomalies or other distortions which show as accentuated peaks or troughs. The program supports statistical analysis and filtering of the data. METHOD The survey area was measured in 20m grids orientated northwest to align with the eastern field boundary and tied into the OS grid post survey. Readings were logged at 0.25m intervals along northwest to southeast traverses set 1m apart, in a zig zag pattern. Approximately 25 grids were walked per day surveying. To the west of the survey area some portions of the field are on a steep slope. Preliminary processing revealed very little impact from modern ferrous magnetic features, which are characterised by sharp dipolar fluctuations ranging from approximately 30nT to over 3000nT. Two processing sequences were carried out to mitigate the impact of modern ironwork. 1. Readings exceeding 30nT either side of 0 were replaced by null (dummy) entries. 2. Any anomalous isolated readings were similarly replaced. 3. Typical regular error due to the zig zag operation of the gradiometer was removed. 4. The mean reading for every traverse was reset to The asymmetric data collection pattern was mitigated by the positive interpolation of data points along the Y axis using the calculation of sin(x)/x.! 5 of 11!

6 RESULTS The survey results reveal a network of magnetic anomalies on differing alignments across most of the field. There are two distinct areas. In the northern half there are strong non linear anomalies including some circular anomalies up to 10 m in diameter. To the south there are fewer anomalies although with some distinct linear and curvilinear features. To the west separate bands up to 20 metres in width are relatively quiet zones which may be related to geology. (Figure 4b) In this area the field slopes steeply to the west so the surface could be revealing different geological formations. The quiet zone is offset about half way along by a clear linear positive anomaly. Figure 4a. Interpretation Figure 4b. Interpretation highlighting quiet zones Positive anomalies Negative anomalies Historic field boundary Circular anomalies Ferrous magnetic anomalies Trig point! 6 of 11!

7 Figure 4c. Interpretation without data Positive anomalies Negative anomalies Historic field boundary Circular anomalies Ferrous magnetic anomalies Trig point! 7 of 11!

8 DISCUSSION Reviewing the Tithe map (Fig 5) on Somerset HER shows the field was previously divided into two with a clear dogleg which explains some of the anomalies and the distinct character of the areas to the north and south. Along this line of field division the anomalies are interpreted as ferrous magnetic anomalies associated with fencing. Aerial photographs also highlight the different surface appearance of the two portions (see Figure 2). Field names from the Tithe map of 1842 names the two separate fields as Home Pits and New Close. Home Pits New Close 100 m Figure 5. Tithe Map South West Heritage Trust HER A triangulation point is located in the south west corner of the field. Having recognised the historic field boundaries, a number of additional anomalies are identified, consistent with ditches and pits, and which are worthy of further evaluation. (Figure 4a and Figure 4c). These include: A. The two linear anomalies in excess of 50 metres in length. B. A number of orthogonal anomalies. C. Circular anomalies in the north east which may well be pits. D. A linear east-west aligned feature which is apparent on the magnetics as well as the aerial photo.! 8 of 11!

9 A. Two strong linear anomalies with a gentle curve within a range of 3 to 9 nt, in the southern quieter portion of the field consistent with ditches containing thermo remnant materials. The field drops away steeply to the south down to Long Lane which was an early long distance trackway from east to west. However the anomalies do not appear to follow the contours. An apparent entrance midway along the length of the combined anomalies appear to have slightly unturned terminals. Figure 6. Linear anomalies B. Orthogonal positive anomalies suggesting three sides of a rectilinear enclosure boundary within a range of 1 to 3 nt (the normal range for ditches). There is no correlation with any features visible on the surface. These are of archaeological interest. Figure 7. Rectilinear features C. Circular anomalies in the North East Circular anomalies up to approximately 10 metres in diameter are visible. These are within a range of 3 to 6nT consistent with pits though locally some of the anomalies are in excess of 10 nt. Only some of these circles have been highlighted on the interpretation. These can be matched with similar surface features identified on aerial photos and depressions on the surface. They are similar in scale to the pits in the scheduled monuments and and are in the portion of the field marked as Home Pits on the Tithe map. This identifies a further area of pits beyond those currently recognised as scheduled monuments. Figure 8. Circular anomalies! 9 of 11!

10 D. A linear east-west aligned positive magnetic anomaly which separates two offset quiet zones with fewer magnetic anomalies, within the range 2 to 6 nt consistent with ditches, with some ferrous magnetic anomalies at the western end. The linear anomaly is oriented at right angles to the natural slope of the land. Beyond the field boundary the ground dips steeply downwards towards Underhill, but there is no obvious route to an existing dwelling. Figure 9. East-west linear anomaly In the adjoining field where the testing was conducted in 2015, the only significant features are a pair of parallel curvilinear anomalies set 20 metres apart and running north to south (Figure 10). These anomalies do not align with field boundaries from historic mapping. See for example the smallholding boundaries on the Tithe map (Figure 5). It is also difficult to make any alignment of these with anomalies in the field to the south. The remaining few anomalies are associated with ferrous-magnetic interference. Figure 10.Survey in northern field CONCLUSION Between December 2015 and January Ha were surveyed using a Bartington Grad gradiometer. The work was carried out when people were available between other activities and when the field was available after grass cutting, hence the timescale. The survey established the magnetic method does work over a Cretaceous Greensand geology. A variety of anomalies have been identified which warrant further investigation to ground truth results and collect dating evidence. ACKNOWLEDGEMENTS Thanks to the landowner Charles Buckler for his cooperation in providing access to the land.! 10 of! 11

11 Figure 11.Survey overlaid over aerial photo Copyright Unless otherwise stated, the copyright of the geophysical survey report is owned by No part of this report may be reproduced or transmitted by any means, electronic, mechanical, (including photocopying), recording or by any information storage and retrieval system, without prior permission from the copyright owner. Limitation of liability To the full extent permissible by law shall have no liability for any damage or loss (including, without limitation, financial loss, loss of profits, loss of business, loss of goodwill, loss of reputation or any indirect or consequential loss), however it arises, resulting from the use of this report or any material appearing on it or from any action or decision taken as a result of using the report. Home Farm, Sutton Montis, Yeovil BA22 7HF Tel: ! 11 of! 11