SURVEY RESULTS

2002 / 59 Athelney II, Somerset

1.	Survey Area

1.1	Gradiometer survey covering approximately 2ha investigated most of the hilltop on which the 'Fort' is located (Areas A, B and C). Further survey totalling 1ha was conducted to the west of the former abbey (Area D).  

1.2	GPR survey examined a part of the abbey immediately to the east of the monument.  Figure 1 shows the location of the these survey areas and those of the previous work, at a scale of 1:2500.

1.3	The survey grid was set out by GSB Prospection and tied in to the base map by Dr Henry Chapman using a Trimble GPS system.

2.	Theory and Methodology of Ground Penetrating Radar (GPR)

2.1	The primary advantage of GPR is its ability to provide a three dimensional view of a buried site. A short pulse of energy is emitted and echoes return from interfaces with differing dielectric constants. These reflections may respond to the changes at the interface between strata or materials. The travel times are recorded and converted into depth measurements, giving a depth section.

2.2	The equipment consists of a battery pack to generate the signal, a transmitter and receiver antenna system, which is dragged by an operator along a traverse, and connecting cables. The unit is controlled by a portable computer that also records all the reflections. This allows the data to be viewed in real time while on site and subsequent processing techniques to be applied to improve the quality of the image.

2.3	The GPR technique relies on dielectric contrast between differing materials. Under suitable conditions the following features may be identified: walls, floors and rubble spreads; refilled pits and ditches; voids e.g. tunnels and chambers; buried paths and roadways; stone coffins and soil / bedrock interfaces.

3.	Display

3.1	Figures 2 and 3 show summary results and an interpretation of the gradiometer survey data superimposed on the Ordnance Survey map at a scale of 1:2000.  Figure 4 is a summary interpretation of the GPR time slice maps.  

3.2	Figures 5 to 14 are data plots and interpretation diagrams of the gradiometer survey results produced at a scale of 1:625.  These display formats are discussed in the Technical Information section at the end of the text

3.3	The data from the GPR survey are displayed in Figure 15, as time slice maps. This form of display combines the data from all the traverses and provides plan views of the results at different times or depths.

3.4	Letters indicate gradiometer survey anomalies while numbers refer to GPR anomalies highlighted on the relevant interpretation diagram.

4.	General Considerations - Complicating Factors

4.1	The conditions for survey were good, both fields were under short pasture at the time of the survey.

4.2	Whilst the soils per se are suitable for magnetic survey, the prolonged waterlogging in some areas will have diminished the level of magnetic response.  However, anomalies associated with settlement and industrial activity, as at the 'Fort' site at Athelney, should be detectable.

4.3	Isolated and small-scale anomalies have been recorded throughout each of the survey areas. It is likely that modern debris in the topsoil accounts for these anomalies. However, given the context, some of these responses may be due to iron objects of archaeological interest, slag and isolated features containing strongly magnetically enhanced material.

4.4	A 225MHz antenna was used for the GPR investigations.  This will record data up to a depth of around 2m.

4.5	While depths have been indicated on the diagrams, these have to be viewed with caution. The conversion from time to depth depends on the velocity of the electromagnetic signal through the ground. Given the nature of the site, this is likely to vary markedly over a small distance, as a result, any depth conversion is only an approximation. An average velocity of 0.06ns/m has been used for the time to depth conversions.

4.6	Where there is a strong electromagnetic contrast, the GPR signal can be inter-reflected or reverberated and this produces a delay in the reflection of the signal. This is termed 'ringing'. This happens, to some extent, with all reflections and results in a greater apparent depth than actually exists. As a result, it is often not possible to detect the base of features; only the tops of buried features/deposits are detected with certainty.

5.	Results of Gradiometer Survey

The 'Fort' (Areas A-C)

5.1	A strong magnetic response (A), recorded in the western part of the survey area, may reflect part of an archaeological ditch. However, it aligns roughly with an existing boundary extending north from the A361 road and could indicate a former field boundary. The fact that it does not appear on any of the available maps could could support the former (archaeological) interpretation. Regardless of origin, the magnetic strength of anomaly (A) suggests that the fill of the feature is strongly enhanced and may represent material associated with industrial activity
 
5.2	A group of pit type anomalies lie immediately to the east of anomaly (A). They appear to run parallel with the ditch anomaly and may be associated with it. They could represent settlement features or further remains of the boundary.

5.3	Anomaly (B) comprises a linear alignment of ferrous responses aligned north-south in the western half of the survey. Although the strength and form is different, it represents a continuation of a curvilinear ditch type anomaly detected by the earlier survey. The majority of archaeological type responses lie to the east of this feature and it was initially thought to have an archaeological origin, indicating a boundary to the site. However early Ordnance Survey maps show a boundary feature in this position in the 19th Century and it seems more likely that the recorded anomalies reflect the remains of this more recent feature.

5.4	A broad band of increased magnetic response (C) lies immediately east of (B). It follows the boundary feature northwards then turns northeastwards and extends to the edge of the grid. The shape of (C) is suggestive of a boundary or enclosure rather than a centre of settlement activity. A few truncated linear anomalies (D) & (E) can be discerned which could reflect discrete ditch segments, but in general the anomalies within band (C) lack any coherent patterns. The island has been the focus of agricultural settlement over hundreds of years and it is possible that ploughing has truncated or destroyed archaeological remains.

5.5	Excavation trenches were positioned over one each of the anomalies in groups (D) and (E). Both were found to be substantial ditch features containing slag and other strongly enhanced material thought to relate to industrial activity. As the majority of the soils of the area are magnetically weak, the archaeological features may only be visible by the presence of slag and burnt material in the fill.

5.6	To the east of (D) an intermittent linear anomaly (F) appears to continue the line of the possible enclosure ditch and there are hints (G) that it might turn a right angle. Alternatively, a pair of short linear responses (H) and a cluster of anomalies (I) may indicate the continuation of the putative enclosure, though the limits of the survey and the isolated nature of the responses make this interpretation tentative.

5.7	At the other end of the putative enclosure, an apparent gap (J) in the band of increased response could indicate an entranceway, though the ill defined nature of the responses casts some doubt on this interpretation.

5.8	Strong linear anomalies, that appear to represent parts of a ditch system or enclosures, have been recorded in the centre of the survey (K). Amongst these are some very strong responses suggesting industrial deposits. Trenches positioned over two of these anomalies confirmed the presence of features containing slag and other burnt debris indicative of nearby industrial activity.

5.9	Numerous linear and curving trends have been highlighted. Although many of these are very weak, the wider context might support an archaeological interpretation. They could indicate features with fills that have low levels of magnetic enhancement and/or that have suffered from plough damage. However, a modern origin, such as recent agricultural activity is equally plausible.

5.10	Extensive ferrous disturbance in the northern part of the survey area is due to the presence of an adjacent electricity junction box, while the broad ferrous anomaly on the southern edge of the grid is the product of recently constructed flood defences.

5.11	A number of small scale ferrous responses have been recorded by the survey. These are typical of small pieces of ferrous debris scattered in the topsoil and are usually attributed a modern origin. In this case, however, it seems likely that at least some of these responses will have been produced by fragments of archaeological slag. Looking at the combined results of both surveys, there appears to be greater number of these ferrous responses around the periphery of the survey area than in the centre. This distribution could be a result of topography - the centre of the site is the highest point and material may have been washed downslope to the east west and south. Alternatively it could be a consequence of fieldwalking, which was carried out as part of the first investigation of the site in 1983.

The Abbey (Area D)

5.12	Area D is located to the west of the abbey complex and the eastern end of the survey area coincides with part of the abbey remains.

5.13	The level of response recorded in this area is in marked contrast with that in the area of the 'Fort'. There is little evidence of industrial activity in the data, a single strong response (L) being the exception. This may represent modern debris , however, given the context, an archaeological explanation is possible.  

5.14	A curious square response (M), measuring about 5m x 5m, may represent a large well-defined feature, such as a sunken floor. However, a modern origin, such as recent agricultural disturbance, cannot be discounted.

5.15	Other archaeological type responses consist of magnetically weak trends and pit responses for which the interpretation remains tentative. Slightly stronger anomalies within a region of increased magnetic response recorded in the eastern part of the survey area may be due to the remains of abbey buildings.

6.	Results of Ground Penetrating Radar Survey

GPR survey was carried out over the eastern part of the Abbey complex, immediately to the east of the monument. The survey area corresponds to part of the presbytery, ambulatory and a possible southern transept or side chapel. It was hoped that survey might identify a possible crypt.

6.1	The results show a very good correlation with the results of the resistance survey from the first Time Team programme. A number of wall lines can clearly be seen on the time slice maps beginning at around 0.6m and ending around 0.9m. The most well defined responses, indicating clear wall lines (1), appear in the 0.6 to 0.7m and 0.7 to 0.8m time slice maps.

6.2	Of particular interest is the southwestern corner of the survey area where the resistance survey recorded a broad area of high readings indicative of a spread rubble. The GPR results suggest a possible curving (2) or apsidal wall, a feature that is not unusual in this part of a complex monastic building. This interpretation is cautious, however, as the spread of rubble and looting of stonework could result in the curving anomaly.

6.3	Below the 0.9m level there appear to be few reflections, suggesting that the structure is about 1m in depth. The results appear to indicate that a crypt is not present in the this part of the abbey complex.

7.	Conclusions

7.1	Gradiometer survey of the 'Fort' site detected responses and debris that are likely to relate to industrial activity. Much of the material appears to be confined within substantial ditch features that are otherwise poorly defined. It is likely that plough damage has been severe; consequently no clear plan of the 'Fort' has been obtained.

7.2	Several anomalies of archaeological interest have been detected to the west of the abbey complex. The results give the impression of low activity in the area, though it is not possible to gauge the effect of plough damage.

7.3	Ground Penetrating Radar (GPR) survey over part of the abbey complex was successful.  Elements of the abbey are clearly visible in the GPR data although they are not as striking as the resistance results obtained in 1993.  However, excavation indicated poor survival of the walls.

Project Co-ordinator:	Dr C Gaffney & J Gater
Project Assistants:	Dr S Ovenden, C Stephens & E Wood
Report Author:		D Shiel

Date of Survey:		18th June - 
Date of Report:		9th January 2003

References:

GSB 1993	Report on the Geophysical Survey at Athelney, Somerset Report No. 93/55.  GSB Prospection 1993.  Unpublished report.

SSEW 1983	Soils of England and Wales.  Sheet 5, South West England.  Soil Survey of England 		and Wales.

SITE SUMMARY SHEET

2002 / 59 Athelney II, Somerset

NGR: ST 346 293 (Abbey) and ST 341 292 ('Fort')

Location, topography and geology 

Athelney Hill, or Island lies immediately to the south of the A361 between the villages of East Lyng and Burrowbridge, Somerset.  The island is a prominent rise overlooking the Somerset Levels and, while the survey areas occupy level ground, steep downward slopes are present on all sides.  The reddish fine loams and silts overly clay soils that characterise the Whimple 3(572f) association.  The geology is Permo-Triassic and Carboniferous reddish mudstone (SSEW 1983).

Archaeology

Athelney is known as a fortress and a monastic site with strong links to Alfred the Great.  The site of the abbey is marked by a monument placed there in the 19th century and the island is a Scheduled Ancient Monument (SAM 367).  A resistance survey carried out in 1987 by the Ancient Monuments Laboratory, English Heritage failed to locate a clear building plan of the monastic site.  GSB Prospection carried out a spectacularly successful geophysical survey as part of a Time Team investigation on behalf of Channel 4 (GSB 1993).  A detailed plan of an extensive abbey complex was revealed.  In addition, evidence of both occupation and possible industrial activity were detected in the area of the 'Fort' site.

Aims of Survey

The site was re-visited during the 100th Time Team programme celebrations.  It was hoped that new geophysical techniques would shed light on the layout of the 'Fort' site and add more detail to knowledge available on the abbey complex.

Summary of Results *

Gradiometer survey of the 'Fort' site recorded a number of anomalies that appear to be associated with industrial activity.  Some linear anomalies were found to relate to possible enclosure ditches but no clear plan of the 'Fort' was obtained.  It is likely that ploughing over many years has disturbed and/or destroyed archaeological deposits.  Several anomalies of archaeological interest have been detected to the west of the abbey complex, within a magnetically quiet level of background response.

Ground Penetrating Radar (GPR) survey over part of the abbey complex was successful.  Elements of the abbey are clearly visible in the GPR data although they are not as striking as the resistance results obtained in 1993.  However, excavation indicated poor survival of the walls.

* It is essential that this summary is read in conjunction with the detailed results of the survey.

List of Figures

Figure 1		Location of Survey Areas						1:2500
Figure 2		Summary Greyscale Image					1:2000
Figure 3		Summary Interpretation Diagram					1:2000
Figure 4		Comparrison of Resistance and GPR Data				1:2000
Figure 5		Area A: XY Trace Plot						1:625
Figure 6		Area A: Dot Density Plot						1:625
Figure 7		Area A: Interpretation Diagram					1:625
Figure 8		Area B: XY Trace Plot						1:625
Figure 9		Area B: Dot Density Plot						1:625
Figure 10	Area B: Interpretation Diagram					1:625
Figure 11	Area C: XY Trace Plot, Dot Density Plot & Interpretation Diagram	1:625
Figure 12	Area D: XY Trace Plot						1:625
Figure 13	Area D: Dot Density Plot						1:625
Figure 14	Area D: Interpretation Diagram					1:625
Figure 15	GPR - Time Slice Maps						1:500