1. Background

The excavation of the Late Roman and Anglian settlement at West Heslerton, North Yorkshire, England, has been the setting for a number of experimental developments in archaeological techniques.

Figure 1. Aerial Photograph of excavation area in progress, Summer 1995.

The excavation, funded by English Heritage as part of the national rescue archaeology programme, is among the largest ever undertaken in Northern Europe, covering an area of nearly 20 hectares. The site is situated on the north facing slopes of the Yorkshire Wolds on the southern side of the Vale of Pickering, to the east of the present village of West Heslerton. Approximately 70% of the occupied area is located on a series of low chalk knolls which give way to sand or sand and gravel deposits in the northern part of the site. The main objective of the excavation has been the recovery of the full plan of an Early Anglo-Saxon or Anglian settlement, the excavation of an associated cemetery having been completed in 1986 (Haughton & Powlesland forthcoming). It was not realised at the outset of the project in 1986-7 that the settlement site would be so large or that it contained important Late Roman and Middle Saxon complexes, which more than double the temporal span of the settlement.

Occupation began in circa AD360 with the site being deserted by the middle of the ninth century. Although stone buildings of the Late Roman period have been identified in the southern part of the site, most of the structural evidence examined comprises either post-hole or post-in-trench structures and Grubenhauser or cavity floor buildings, which characterise the Early Anglo-Saxon period in England. Much of the settlement was not enclosed, but the southern half included a complex network of ditched and fenced enclosures spanning the life of the settlement.

The chalk and chalk-gravel bedrock upon which much of the site is situated is an ideal substrate for survey using the fluxgate gradiometer, and the site was surveyed using a Geoscan Research FM18/FM36 fluxgate gradiometer by staff of English Heritage (Ancient Monuments Laboratory), prior to the start of excavation. Unfortunately, owing to technical difficulties, the survey over the northern third of the site produced no results. However, the remaining two-thirds are covered by a magnificent series of surveys. These were extensively used in planning the excavation of these areas (Figure 2).

Although these surveys produced a clear picture of the major features, some parts of the site were obscured by deposits of colluvium, and even after removal of the modern ploughsoil many features remained difficult to see on the ground. It was therefore decided to experiment by undertaking a higher resolution fluxgate gradiometer survey after removal of the ploughsoil, with a view to producing a more detailed survey which could form the basis of a pre-excavation plan. The first trials, also using a Geoscan Research FM36 fluxgate gradiometer, undertaken over four small areas in 1991, produced excellent results and the method has been applied as a standard technique to a 1.44 hectare area prior to the beginning of excavation in 1995.

Figure 2. English Heritage (AML) fluxgate gradiometer surveys 1990-1991.

2. Methodology

The development of archaeological techniques at West Heslerton over the duration of the excavations (1977-1995) has been mirrored by the increasing availability of more and more powerful computers, coupled with lower cost and greater memory capacity. This has allowed the project to redefine the way in which large-scale excavations can be managed on a day to day basis. The primary change has been the integration of all the various data sets generated by an excavation into a single Geographic Information System (GIS) and Geographic Database Management System (GDMS); in this case the in-house developed G-Sys software. The software provides facilities for the superimposition of multiple surveys, at any resolution or placement. These include satellite imagery, high altitude imagery (for instance multi-spectral data) and ground based geophysical survey. G-Sys is a 3-D package and includes facilities for representing the geophysical surveys as 3-D surfaces, or draped onto Digital Elevation Models (DEMs). In addition, the software can display Ordnance Survey maps (Raster or Vector), the digitised plans of excavated features, and the 3-dimensional plots of survey points. These can be directly related to multiple databases (for instance context and object records), which can be actively filtered and interrogated on screen.

The ability to carry out all these functions is important, as it is through these developments that the excavation strategy used at West Heslerton could be modified during the excavation phase on a day by day basis. All context and object points were 3-dimensionally recorded using a Nikon DTM A20 electronic distance meter (EDM). A system was put into place to ensure that the data collected each day was downloaded to the site computer each evening, allowing a constant check to be kept on the progress of the excavation. The development of recording using hand-held computers in the field meant that context and object records could be merged with the 3-D data collected by the EDM each day, providing an invaluable tool for error checking while still in the field, thus eliminating many problems usually not encountered until the post-excavation phase. The combination of daily updating the computer record, together with the results of the spatially referenced high resolution geophysical survey, was a key factor in the ability of the director to modify the emphasis of the excavation strategy on a daily basis, and allowed the maximization of resources from day one, rather than finding out in the last week that the most complex areas remained to be excavated.

Recommended standards for undertaking commercial fluxgate gradiometer surveys in Britain state that the sampling interval should be no greater than, 1m x .5m, although a narrower vertical spacing of .25m is preferable (English Heritage 1995). For the high resolution surveys at West Heslerton, a greater resolution has been applied to the east/west axis. When applied to large areas the results have been quite spectacular. Data are dumped from the FM36 to a laptop computer and processed in the field using G-Sys Geophysics software and then displayed using the G-Sys GIS Geographic Data Management Software. Very little secondary processing of the data is required; only the removal or reduction of spikes, generated by ferrous objects near the surface, de-striping to equalise offsets, resulting from surveying bi-directionally, and balancing to bring all the grids into the same data range.

All of the surveys at West Heslerton have used a sensitivity setting of 0.1 nanoTesla, and thus very minor magnetic fluctuations can be detected. It should be stated that it is unlikely that an increase in east-west resolution would have resulted in much greater clarity of data had the ploughsoil not been removed prior to the survey being carried out, thus the technique described here will only have relevance to sites where the area has already been stripped. This technique provides greater clarity for two main reasons. Firstly, the higher resolution of the east-west axis means that the anomalies detected, particularly linear anomalies, have a much clearer definition of their edges, as well as enabling the detection of features much smaller than a metre in diameter, which might otherwise have been missed using the more usual one metre east-west resolution. Secondly, the removal of the ploughsoil prior to surveying has the dual benefits of removing modern iron objects such as broken ploughshares and horse shoes, which create spikes in the survey data, as well as removing the effects of modern ploughing from the data (seen in the English Heritage survey data as faint east-west oriented linear anomalies) which tend to obscure the underlying archaeological anomalies.

The limited (0.36 hectare) high resolution surveys carried out in 1991 had demonstrated the potential usefulness of this technique, particularly in areas where feature definition was poor on the surface. It was thus decided to adopt the technique as a standard before the commencement of excavation at West Heslerton in 1995. After the removal of the topsoil by machine (varying from a few centimetres to half a metre in depth), the survey was carried out. The area surveyed consisted of a number of different underlying local geologies and subsoils; low chalk knolls, colluvial hillwash and sands & gravels. The survey was conducted using a 25cm resolution in both axes, thus quadrupling the data gathered (14400 as opposed to 3600 data points for a 30 metre square grid or 16 data points per square metre). Although also quadrupling the time taken to cover the area using the standard 25cm by 1 metre resolution, the resulting data were of an exceptionally high quality, and proved to be an invaluable tool in the day to day planning of the excavation strategy, for a number of reasons. Firstly, highly stratified sequences could be targeted. Secondly, feature types could be recognised and thirdly, broad initial phasing of features could be carried out. These three criteria are discussed in detail below.

3. The recognition of stratigraphic sequences in fluxgate gradiometer data

The high resolution survey of the initial sample area 11CE in 1991 (Figure 3 and Figure 4) was important, since in this area, even after careful cleaning, it was very difficult to see many of the features indicated in the original survey, most of which were cut into a sandy hill-wash which was very similar to the fills of the features themselves. The survey was carried out using a resolution of 25cm in the north-south axis and by 50cm in the east-west axis, effectively doubling the normal east-west resolution. One of the most important aspects of the first survey was the fact that stratigraphic relationships demonstrated through excavation could, with hindsight, be seen in the results of the geophysical survey. This discovery had ramifications which were fully explored during the 1995 season. The ability to identify testable stratigraphic relationships not visible to the naked eye was vital to the successful completion of one of the prime aims of the excavation, namely the recovery of securely stratified sequences spanning the late Roman and early Saxon period, and the middle to late Saxon period. In addition, with the knowledge gained from the 1991 surveys, it was possible in some cases to predict the sequences prior to excavation, an ability which proved to be exceptionally useful during the arduous 1995 season.

Equally important was the fact that some of the larger features had low magnetic susceptibility and were thus less visible in the survey data than many, in reality, much slighter features. This was particularly noticeable in area 11CE, where a middle Saxon slot cut through an earlier Roman enclosure. The slot, which showed as a strong positive magnetic anomaly, proved to be a relatively narrow feature, whereas the magnetically weak positive anomaly which it was seen to cut through, proved to be a deep, metre-wide Roman enclosure ditch. This difference in the magnetic susceptibility of the fills was also noted in the 1995 survey results, allowing the phasing of features to be predicted with a high level of confidence. See Figures 5 and 7 and compare with Figures 5 and 8 to highlight this.

Figure 3. Detail of area 11CE. (English Heritage AML survey 1990)

Figure 4. High resolution survey of area 11CE. (Heslerton Parish Project 1991)

Figure 5. Normal resolution survey of Site 12. (English Heritage AML 1990)

Figure 6. High resolution survey after removal of ploughsoil. (Heslerton Parish Project 1995)

Figure 7. Detail view of south-east area 12AD (standard resolution survey) before the removal of the ploughsoil (English Heritage 1990).

Figure 8. Detail view of the same area (high resolution survey after the removal of the ploughsoil (Heslerton Parish Project 1995).

4. The identification of feature types

The combination of using high resolution and surveying after the removal of the ploughsoil allowed, for the first time, features of less than 40cm in diameter to be seen in the results of a fluxgate gradiometer survey. This was particularly clear on the survey data from area 12 AE (see Figure 9 and Figure 10). The survey results clearly indicate the position of two post-hole structures (later confirmed by excavation); the first, to the south, being an earlier Anglian post-hole building and the second, to the north, proving to be a later Middle Saxon post-in-trench building. The eastern sides of both buildings have been severely truncated by ploughing. Many of the post-holes visible in the survey remained invisible on the ground even after initial cleaning, not becoming visible to the naked eye until after heavy rain and subsequent differential drying. Some of the post-holes seen in the survey data proved to be less than 20cm in depth, and would certainly not have been detected on a normal resolution survey of the same area. However, as can clearly be seen by comparing the survey data and the digitised plan of the excavated features (Figure 11), even the high resolution survey did not reveal the complexity of the archaeology in this area. Very shallow post-holes were not detected, and the paired post building technique of the earlier post-hole building was not evident.

Figure 9. High resolution survey of Area 12AE. (Heslerton Parish Project 1995). The superimposed square shows the location of the detail illustrated in Figure 8 and Figure 9 below.

Figure 10. Detail of Area 12AE showing individual post-holes of two Anglo-Saxon buildings, the southern an earlier post-hole building, and the northern a Middle Saxon post-in-trench building.

Figure 11. Digitised plan of excavated post-hole buildings seen in the survey data above.

The recognition of potential Grubenhauser was of prime importance to the excavation strategy, not least because of the time it takes to 3-dimensionally record each find from the fills of these features. For example, the Grubenhaus shown in Figure 12 and Figure 13 contained over 5000 finds, most of which were animal bones. Thus it was vital that each of these features could be identified at an early stage, in order to allocate the resources necessary to excavate the Grubenhauser in a way which would obtain the maximum level of information from these poorly understood structures. In this way, 11 potential Grubenhauser were flagged for attention at an early stage in the excavation, out of a total of 14 Grubenhauser excavated in the 1995 season ( the other 3 were not covered by the original survey).

Figure 12. Detail of 1990 survey showing a potential large feature. (English Heritage AML)

Figure 13. Detail of 1995 survey showing typical Grubenhaus magnetic anomaly. (Heslerton Parish Project)

The survey data also showed a number of related ditched enclosures, spreading north-south and east-west across the site. The excavation of similar enclosures in the 1992 season, combined with the analysis of the geophysical results, allowed us to make reasonable assumptions about the time allocations necessary to extract the full stratigraphic sequences from these enclosures which, as predicted, proved to be multiple phase re-cuts of long standing enclosure systems.

5. Identification of an earlier landscape

The survey data in 1995 also demonstrated the existence of an earlier phase in the life of the settlement (Figure 14). Due to the scope and remit of the 1995 excavation, which stipulated the full recovery of the Anglo-Saxon and late Roman phases, it was not possible to devote more than a cursory investigation to this earlier phase. However, the survey data allowed this phase to be identified and plotted, and as such can act as an aid to planning should any future excavation be envisaged. It was particularly noticeable in this part of the survey area that, as had first been noted in 1991, stratigraphic relationships were evident in the survey data (Figure 15). The earlier east-west aligned ditch was already in existence when a Grubenhaus was placed just inside this enclosure to the north. A later curvilinear slot was then cut through both the ditch and the Grubenhaus.

Figure 14. Detail of area 12AD demonstrating the presence of an earlier phase in the life of the settlement.

Figure 15. Detail of survey data demonstrating a number of stratigraphic relationships

Figure 16. As Figure 14 with superimposed digitised plan of excavated features

6. Justification

Many excavators might argue that as the project strategy was to carry out a full open-plan excavation anyway, what justification could we have for also carrying out this full high resolution gradiometer survey? Firstly, the project had at its disposal the fluxgate gradiometer and all the software necessary for processing the survey data, so the only costs incurred were the 14 person days it took to carry out the survey and process the results. Secondly, we feel that the quality of the result speaks for itself. The techniques developed at West Heslerton could be utilised at other sites where full open-plan excavation was not feasible, but where it was possible to open up an area larger than the proposed excavation area alone. In this way, a full ground plan could be recovered without the expensive option of total excavation. The dual benefits of this would be; firstly, that should further excavation prove possible in the future, the survey data would make the development of an excavation strategy much easier, and secondly, the interpretation of the excavated features could be placed into a wider context, much more satisfying than trying to tie together the evidence from a few randomly situated trial trenches.

The net result of the 1995 high resolution survey was in effect a spatially referenced pre-excavation plan. As was the case in previous seasons, many of the anomalies visible in the survey data were not initially visible on the ground, due to the nature of the site, with many features cut into colluvium and subsequently infilled with colluvium, rendering them almost invisible to the naked eye. The high resolution survey thus allowed excavation strategies for each area to be developed prior to the commencement of excavation. This was particularly important in areas where the survey clearly showed the potential for the recovery of stratigraphic sequences, but which remained invisible on the ground. Resources could thus accordingly be allocated to the areas requiring the most attention, maximising the recovery of securely stratified data. With hindsight, we can now say that without this ability to plan the excavation strategy using the knowledge gained from the high resolution survey, it would not have been possible to excavate a quarter of the total features from the West Heslerton Anglian settlement in a single excavation season, and as such is both a justification of the techniques developed and a tribute to all the volunteers who gave their time and labour willingly during the long, hot summer of 1995.

7. Acknowledgements

This work could not have been undertaken without the support and funding provided by English Heritage. The English Heritage Ancient Monuments Laboratory undertook the initial gradiometer surveys and provided both the detailed results and primary data. Thanks are also due to The Landscape Research Centre which provided both the gradiometer and survey equipment and data processing facilities used for the high resolution surveys. The high resolution gradiometer surveys were undertaken by James Lyall assisted by Heather Clemence following the initial trials undertaken in 1991 by Tim Carew and members of the excavation volunteer team. We are indebted to the large team of excavation supervisors and volunteers who undertook the massive excavation over a period of nearly nine years and are too numerous to mention here. The digitising of the excavation plans was undertaken by James Lyall using G-Sys software.

8. Bibliography

English Heritage 1995 Geophysical Survey in Archaeoogical Field Evaluation, Research and Professional Services Guideline 1. English Heritage, London

Haughton, C & Powlesland, D (forthcoming) West Heslerton: the Anglian Cemetery. English Heritage/The Landscape Research Centre


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