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It now widely accepted that excavation of all archaeological features should have a rigorously focused approach. Increasingly those responsible for the management of excavation projects employ all available geoprospection techniques as an integral part of their project planning. Such techniques can be used to examine large areas and identify those which will repay more detailed investigation and can assist in the planning of curatorial strategies or curation.
There is a need, ever more apparent, since the implementation of PPGI6, for integrated geophysical and close contour topographical surveys, to establish and characterise the archaeological record. In conjunction with aerial photographic reconnaissance, with which it has many parallels, geophysical survey can provide additional and complementary evidence of subsurface archaeological features and surface compiled data. This can be achieved rapidly and economically, allowing the recording and assessment of the cultural resource to assist in the formulation of a management strategy. Both Exploring our Past (English Heritage 1991) and the Draft Research Agenda (Williams 1997) have affirmed the value of geophysical prospection as a means of enhancing knowledge of the archaeological record non-destructively and cost-effectively. The scheme outlined here concerns the archaeology of the area immediately around Birdoswald Roman fort on Hadrian's Wall and the potential offered by geophysical and topographic survey to contribute to our understanding of the fort and its environs.
The recent Hadrian's Wall management plan (1996) quite clearly delineates not only the extent of the area encompassed as a Scheduled Ancient Monument but also recommends an Agreed Setting (or buffer) zone around the monument. At present these scheduled areas are protected by the 1979 Ancient Monument and Archaeological Areas Act. However, the setting zone is not. In neither area is there any restriction on existing agricultural operations because Class Consents have already been granted. Although the unscheduled archaeological sites around the Wall benefit from the same protection as sites elsewhere, such protection does not include legislation specifically enacted to protect ancient monuments. In any event, the extent of the archaeology surrounding Hadrian's Wall is not known in detail.
The previous excavation and history of the Fort has been well documented by a number of authors, most recently by Wilmott (1997). That the extent of the resource is not yet fully realised is evidenced by the fairly recent assertion that the continuing strength of the vicus at Nether Denton (which survived the abandonment of the fort) has been adduced to explain the apparent absence of a vicus at the neighbouring Wall Fort of Birdoswald (Shotter 1986: 50).
Clearly, the evidence shows this is not the case and the true extent of the settlement to the east of the fort is an unknown entity. An unpublished resistivity survey showed a large number of linear anomalies which embody the entire expanse of Chapel Field (Walker 1986) and suggests that further research might yield informative results.
The settlement evidence to the west of the fort is equally evocative. A much more ephemeral type of building is seen conforming to the expected linear pattern following the road leading from the fort. The evidence for archaeological features extending beyond the limit of the preliminary survey is tangible. Again this is reinforced by the admirable, but austere survey produced by Walker. The importance of the cemetery site has been thus described.
The Society for the Promotion of Roman Studies (1985), regarded the location and excavation of cemeteries as the most urgent research priority for the military sites of Roman Britain, as cemeteries in the northern military zone are very little understood (Wilmott 1993).
The extent of the archaeological remains north of the Wall and fort is unknown except for the resistivity survey of Tank Field (Walker 1986). The route and position of the road system leading from the outlying fort at Bewcastle to Birdoswald is largely conjectural near to Birdoswald (RCHME 1987: 3). The series of linear earthworks, just north of the fort, also merit further investigation.
During preliminary discussions concerning the project described here the initial response has been encouraging, especially from County Archaeologists and statutory bodies in the north of England. All have expressed interest in an intensive multidisciplinary approach to non-invasive survey. The major criticism of previous surveys has been that only one method has been employed and that the interpretative element has not been fully exploited. It is clear that relatively low cost intensive survey can provide substantial benefits in developing a resource management plan. Cumbria County Council has used the initial survey results precisely in this way. A comprehensive sites and monuments record is an essential archaeological resource management prerequisite. This is particularly important in view of the proposed development of the Roman Wall footpath.
The aim of this present project is to complete the preliminary geophysical and close contour survey of Birdoswald Roman fort and settlement begun by J.A. Biggins and D.J.A. Taylor of the University of Durham. This will include another principal area of interest, the site of the Roman cemetery, known to lie west of the fort and vicus. The preliminary survey of the fort and vicus indicated a much more extensive and complex development than previously realised (Biggins and Taylor, forthcoming).
It is proposed that the area covered by the preliminary survey, completed in 1997, which included both magnetometry, resistivity and close contour surveys within the fort should be extended to cover a zone around the fort, beyond that covered by the earlier work.
The survey will incorporate magnetometry, supplemented by close contour survey and aerial photographic rectification (if appropriate). It should be stressed that close contour survey is not a substitute for an earthwork survey and both methods will be incorporated into the record. Close contour survey provides an accurate representation of both the surface terrain model and an accurate record of elevation.
The survey will be conducted in four distinct phases. After each stage, there will be a period of re-evaluation and consultation with the Project Officer. The proposed stages are:
The magnetometry survey will routinely employ 1m parallel traverses with 0.5m sample intervals, using an FM36 fluxgate gradiometer. This technique which, whilst slower than zig-zag traverses, is the method of choice if archaeologically 'quiet' areas are envisaged. Although it is estimated that a parallel survey takes 25-30% longer to conduct than a zig-zag survey, if the grid is pre-surveyed only one person is required to perform the survey and move the guidelines. A zigzag survey requires at least two people and there is consequently very little difference in cost. This system has been used with some success on this and other sites, although it relies upon an unobstructed site and the presence of trees, fences can require the presence of more than one person. It also alleviates the problem of striping, which upon occasion can never be completely removed by processing. This will prove particularly important in the area of the cemetery as such sites have proven to be particularly intractable to geophysical survey. If suspected areas of activity are encountered, it is proposed to re-survey the selected areas using 0.5m traverse intervals and possibly 0.25m sample intervals.
The close contour survey, using a TC403 total station, will employ at least 1000 points per hectare. Initial data collection will utilise the same grid system used for geophysical survey. This ensures uniform coverage on the ground and thus alleviate the problem of 'blind spots'. Data processing and the production of terrain modelling will be conducted using Landscape processing software. The value of this approach, besides physically identifying features on the ground and producing an earthwork plan, also provides valuable confirmatory evidence of sub-surface features. The large-scale survey produced by RCHME (1987) when overlaid over the geophysical data confirmed that topographical features were well represented in the geophysical record. This is an invaluable addition to interpretation.
Resistivity survey was conducted within the intramural area of the fort. In many ways this proved disappointing, although certain features represented by magnetometry were confirmed by this method (e.g. kilns and drainage systems). It might be that local conditions (particularly high rainfall) and the inherently waterlogged condition of the ground may have been partially responsible for these results.
A resistivity survey has already been completed by Walker (1986), encompassing the fields towards the east and west of the fort. This showed only general linear features, rather than precise building outlines. It may be that the re-interrogation of the original results, using improved software may yield an improved representation of these features. A reassessment of the original data (provenance unknown) may prove constructive.
The suggestion by staff of the Ancient Monuments Laboratory that further resistivity survey at Birdoswald could be conducted using a variety of probe configurations and- electrode spacing has considerable merit. The use of a resistivity multiplex survey at Wroxeter shows substantial promise. Whether this method would be successful at Birdoswald is uncertain, although a considerable dry spell may facilitate such a survey towards the end of the year. Such a survey would represent a further stage of work and is not planned as part of the proposal set out here.
The data will be deposited with archival authorities in a form suitable for replication and transmission by electronic means and the report will be deposited with the Ancient Monuments Laboratory, the regional inspector, RCHME and the Cumbria Sites and Monuments Record. Publication in an archaeological journal will be sought after discussion with the project officer, Mr. T. Wilmott.
Area A comprises the field (Chapel Field) to the east of the fort. This is delineated by the extant Wall to the north and the scarp falling into the valley of the River Irthing to the south. Towards the extreme east of the survey area the site contiguous with milecastle 49 provides a natural boundary. This phase will augment and incorporate those magnetometry grids previously completed, together with the close contour and earthwork survey.
Area B incorporates the field to the west of the fort, encompasses the area to the south of the Wall and is delineated by the scarp of the Irthing to the south. The western limit is defined by the field boundary fence. This phase will incorporate those areas south of the fort to the limit of the scarp. Although this area has been partially excavated, valuable information about the nature and extent of the vallum and egress routes from the fort may be elicited. If the undergrowth in the heavily wooded area to the northwest of the fort can be cleared, geophysical survey may be conducted in that area. This may prove difficult, but is not impossible, although wire fencing does surround the area. This area (as well as area A) is known to contain evidence of prehistoric features, including an Iron Age promontory fort and a possible Neolithic burial site (Wilmott a and b forthcoming).
Area C, the area of the cemetery, which lies some 400m south-west of the fort may prove to be the most difficult to determine. The problems of conducting such a survey have been highlighted (David 1995: 11) and the difficulties have not been underestimated. The general survey method (1m traverse, 0.5m sample interval) may be augmented by closer interval magnetometry survey. It is perhaps in this area (rather better drained) that resistivity survey may most usefully be employed. The prospect of conducting both resistivity and magnetic susceptibility has been discussed with Dr. A. David, but at this stage, the agreed method of choice remains magnetometry survey.
Area D, the survey area north of the Wall, incorporates a 40m transect directly north of areas A and B. This will include the route of the Maiden Way and a number of earthworks of unknown provenance.
The completion of this project should ensure that a number of questions are answered. These include the extent and physical dimensions of the vicus and the route of the Maiden Way, which leads to the outpost fort at Bewcastle. The earthworks to the north of the Wall and Birdoswald fort, so far of unknown provenance, will be mapped and their geophysical properties determined. It is expected that the extent and site of the cemetery, so far only generally located, can be delineated with some confidence.
The information derived from this survey will give the management team responsible for the development of Birdoswald and its environs as a heritage resource, a much clearer idea of its potential. The survey will provide a communication medium which will aid public understanding of archaeological research methods. In more general terms, a substantial composite survey of a major Wall fort will perhaps stimulate and encourage more research of this type.
Biggins, J.A. and Taylor, DJ.A. forthcoming Survey of the Roman Fort and Settlement at Birdoswald.
English Heritage 1991 Exploring our Past: Strategies for the Archaeology of England. Historic Buildings and Monuments Commission for England.
English Heritage 1996 Hadrian's Wall World Heritage Site: Management Plan July 1996 Historic Buildings and Monuments Commission for England.
RCHME 1987 Birdoswald (Banna) Roman Fort and Environs. Unpublished survey.
Shotter, D. 1986 The Roman Frontier in Britain, Preston
Walker, R. 1986 Birdoswald Geophysical Survey. Unpublished report commissioned by English Heritage
Williams, T. 1997 Archaeology and English Heritage: Agenda for the Future, (Draft Research Agenda), English Heritage Archaeology Division
Wilmott, T. 1993 The Roman Cremation Cemetery in New Field, Birdoswald Transactions of the Cumberland and Westmoreland Antiquarian and Archaeological Society (series 2) 93: 79-85
Wilmott, T. 1997 Birdoswald English Heritage Archaeological Report 14, Historic Buildings and Monuments Commission for England
Wilmott, T. forthcoming a A possible Neolithic burial from Birdoswald, Cumbria Proceedings of the Prehistoric Society
Wilmott, T. forthcoming b Excavations on the Birdoswald Spur 1997 Transactions of the Cumberland and Westmoreland Antiquarian and Archaeological Society (1998)
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