Dorset WreckMap 2001
An NAS Project for the Summer of 2001
It is recognised that Studland Bay in Dorset is an area of high archaeological potential. Old Harry Rocks, at the southern extent of the Bay are the remains of the land bridge that connected Studland Bay with the Needles on the Isle of Wight until the inundation caused by the end of the last ice age approximately 7000 years ago. Immediately north of the Bay is Poole Harbour which has a long history of maritime use. In the 1960s a log boat was discovered off Brownsea Island, Poole Harbour and radiocarbon dated to c300BC, indicating maritime activity in the area since pre-historic times. In the 1st century AD Poole was used as an invasion port by the Romans and by the 15th century the Harbour was awarded Staple Port status as one of the few ports in the country where wool could legally be exported. The medieval period saw trading links with the Baltic, Spain and Italy and by the 17th and 18th century, transatlantic trade links were commonplace.
The close proximity of Studland Bay to Poole Harbour with its long and varied history of maritime trade and use would therefore suggest a high archaeological potential for neighbouring Studland Bay, particularly with respect to maritime material. Indeed, the discovery in 1984 of the Studland Bay Wreck, one of only 52 wrecks designated nationally under the Protection of Wrecks Act 1973 would seem to confirm this. As a 16th century vessel of the Iberian-Atlantic tradition (Thomsen 2000: 69-85), she certainly attests to maritime activity in the area in the 16th century.
Studland Bay, Dorset
Buoy marking the designated wreck site, Studland Bay, Dorset
The National Trust have taken responsibility for the conservation and storage of this timber. They arranged for a dendrochronologist to examine it with a view to obtaining a date. Unfortunately, the timber was identified as fast growing oak of an unsuitable nature for dendro-dating. Mike Markey of the Poole Bay Archaeological Group and licencee of the designated Studland Bay Wreck was consulted regarding the possibility that the timber had eroded from the protected site. He felt that the timber was too large to have been from that particular wreck.
Timber found on Studland Beach, January 2001.
The project aimed to test a methodology for an effective multi-disciplinary approach to seabed evaluation, while encouraging public involvement within a modest project budget.
Specifically the project aimed to:
- Utilise Skills & Involve amateurs - Provide an opportunity for those involved in the NAS Training Programme to put their newly acquired skills into practice.
- Characterise the seabed (ground truthing)
- Record sites & add information to SMR/NMR - Provide information for the Dorset Maritime Sites and Monuments Record, the National Monuments Record (Maritime) and the Dorset Coast Forum Marine Conservation Group.
- Disseminate information - Publish the project results among the participants, supporting bodies and more generally in the public domain.
The project was divided into four broad phases:
- desk based assessment
- geophysical survey
- ground truthing of the geophysical targets and selected SMR records
- dissemination of the information acquired through the Sites and Monuments Record and publication.
Desk based assessment
A desk based assessment was carried out to assimilate existing archaeological and geophysical survey data in the area. This involved consultation of the Dorset Maritime Sites and Monument Record (DMSMR) which currently has 44 records for the Studland Bay area, comprising the designated wreck (see above), reported obstructions, anomalies from BP geophysical survey, Admiralty listings, multi-period shipwrecks and Valentine Tanks form the Second World War. This information, combined with that obtained from local dive literature enabled the prioritisation of areas to conduct the geophysical survey.
NAS members participating in Dorset WreckMap 2001.
NAS member Martin Openshaw during one of the ground truthing dives
The geophysical survey was undertaken between the 6th and 8th June 2001. The aim of the geophysical survey was to map an area of seabed and identify targets of archaeological potential. Two survey systems were used, a Swath bathymetry system provided by Submetrix Ltd (SEA) and a Tritech 375Khz side scan sonar provided by Atlantas Marine of Langport Somerset. The Swath system gathers depth data in a swathe across the seabed. The data can be viewed as a two-dimensional colour plot of bathymetry that clearly shows seabed features. The side scan sonar system produces an image, not based on depth, but based on the signal strength of the returned sound energy, providing a very clear indication of seabed relief, highlighting anomalies that could potentially be archaeologically important. The two systems were used in conjunction with one another as the Swath system produces quantitative high resolution data, whereas the side scan produces a wider qualitative image of the seabed. The simultaneous use of these two systems optimises the information gathered on the seabed topography. The geophysical data were then processed over a 5 day period to produce data that could be assimilated into a Geographical Information System (GIS) and provide targets for ground truthing.
The survey vessel Snapper was mobilised with the two acoustic systems. The data was collected and integrated with the Navigation system in the survey computer. The tow fish was trailed behind the survey vessel at a set distance to give an acoustic image of the seabed. Positioning for both systems was given by Differential GPS.
The ground truthing of geophysical targets was carried out by NAS volunteer sport divers under the direction and guidance of NAS staff during the Wreckmap 2001 field school 28th July - 3rd August 2001. Targets identified during the geophysics phase of the project were located using a Differential GPS (DGPS) receiver carried on a rigid inflatable. Divers were subsequently deployed to carry out a pre-disturbance survey of the anomaly using the Diving with a Purpose Dorset Dive Slate (see Appendix C) which encourages the collection of information concerning archaeology, environment and ecology.
Divers during the project
The information gained during the course of this project has been/will be disseminated in the following ways:
- Report distributed to participants and sponsors
- Report on the NAS website
- Report in the NAS quarterly Newsletter
- Completed Diving with a Purpose forms forwarded to Dorset Coast Forum & Dorset Wildlife Trust
- New data added to the DMSMR where appropriate
NAS Project Report distributed in November 2001.
Results
Geophysics
Time restraints led to the decision to have two discreet survey areas, one to be surveyed on each day. Area A1 was to the North East of the Harbour entrance. This area was chosen, as a zone of high sediment mobility. Area A1 has historically given up artefacts at different times, most recently ships timbers have been revealed and noted by Mike Markey, in an area close to the known Antler wreck site. The second area A2 was chosen as part of the Bay that had minimal coverage during previous surveys and logistically its location optimised the ground truthing phase of the project.
Survey lines in both areas were run with a North South orientation. Table 1 shows the survey scheme undertaken for the project. We wanted to achieve total ensonification of the sea bed with the swathe bathymetry system. This system is dependent on water depth and appropriate line spacings were chosen accordingly. The majority of Area 1 was surveyed with a line spacing of 40m because of its shallow nature. Area 2 was surveyed with a line spacing of 50m.
The data interpretation from the Swathe bathymetry system has produced a total of forty targets. The majority of these targets are in Area 2. The targets range from small ephemeral impression on the seabed, that are most likely to be bed forms, to targets that are more likely to be of a cultural nature. Twelve of these targets were dived during the ground truthing phase of the project. From the side scan image produced by the Swathe system we can identify a series of reef structures indicating emergence of the bed rock geology from the sediment. This was confirmed at the ground truthing stage, targets 20, 26, 36, 40 proved to be pronounced eroding faces of rocky reefs. The faces of the reef were perpendicular to the tidal flow which would suggest that they are in a continual stage of development. From our findings during the ground truthing phase, these structures form a natural "trap" for anything flowing over the seabed, including cultural material.
It is apparent that the positions resulting from our own geophysical survey differ from those already held on the SMR. This could of course be down to a number of factors including different positioning systems (e.g. GPS, DGPS DECCA), active sedimentary regimes leading to covering/uncovering of targets or simply many closely located targets.
It is intended that further interpretation of the data will take place. This will entail a detailed analysis of the bathymetry and an investigation into the sediment regime in the areas and their effects on the morphology of the bedforms.
Side scan geophysical anomaly
To encourage a multi-disciplinary approach to seabed evaluation and promote a holistic appreciation of the dive sites, a Marine Species Identification course was organised as part of the project. The course was tutored by Peter Tinsley of the Purbeck Marine Wildlife Reserve and attended by NAS volunteers and a visiting group of SeaSearch volunteers. This enabled project participants to identify environment and species as part of the seabed assessment during the ground truthing phase of the project. Feedback from project participants and professional archaeologists indicated that this was a worthwhile exervise and served to raise awareness of the natural environment.
During the WreckMap Dorset 2001 project, 12 geophysical targets were ground truthed. The results of these ground truthing dives are detailed in the dive log sheets which are being held in the project archive in the NAS office. The information from these sheets has been assimilated into the Ground truthing Sheets.
Diving with a Purpose Forms
The project aimed to familiarise participants with the NAS Diving with a Purpose form as a means of rapid pre-disturbance assessment of a potential site. The form used during the project was specifically designed for the Dorset region by the inclusion of five marine species of particular interest to the Dorset Marine Conservation group of the Dorset Coast Forum. During the ground truthing of targets, information concerning these species could therefore be collected. This information was of use to both the marine conservationists and the geophysicist (for interpretation purposes).
Project participants found the DWAP form useful and easy to use in the initial stages of the project. Experienced recorders however, found it restrictive and eventually reverted to their own recording methods. NAS recognises the limitations of the DWAP form but sees it as a useful introductory tool to promote constructive recording techniques.
NAS Diving with a Purpose form
During a ground truthing dive on 31/07/01 an intact glass bottle was found between rocks in a small reef. After consultation with the National Trust as landowners it was decided to raise the bottle for the following reasons:
- The intact bottle, though apparently not very old, was unusual in appearance and felt to be at risk from the elements if left in-situ.
- As an educational exercise in the administrative, logistical and practical implications of carrying out such an activity.
The National Trust were consulted with respect to the proposed retrieval of the bottle. They approved and agreed to undertake responsibility for the conservation of the bottle once it had been recovered. The necessary materials for safe retrieval were obtained and two project participants raised the bottle on the 2nd August 2001, the final diving day of the project. Having been successfully recovered, the bottle was photographed and put in passive conservation.
The bottle is now in the keeping of the National Trust Archaeologists based in Corfe, Dorset. The bottle was found to have the letters DAWS on it. The bottle was reported to the Receiver of Wreck on Droit No: 209/01.
Bottle recovered during the project
- Successful ground truthing of geophysical targets
- Better understanding of the dynamic nature (unquantified) of the seabed in Studland Bay
- Provided information on marine species for Marine Conservation Group
- Public involvement & development of stewardship
- Promote use of the Diving with a Purpose form
- Increased public awareness (evening talks by Gordon Le Pard & Ed Cumming)
- Educational value
- Public involvement in archaeological ground truthing process
- Understanding of process of artefact retrieval
It is felt that amateurs have a clear role in the ground truthing of the underwater cultural heritage of the UK. The project clearly showed that volunteers, with some guidance and direction from archaeologists, can make a valuable contribution to the archaeological record. While this is not a new concept, NAS is trying to provide a national framework (Diving with a Purpose) within which numerous groups can participate. Such groups will have access to an organisation that can respond to requests for support and provide guidance. We would anticipate that information retrieved from these groups would, in the first instance, go to the local SMR and other appropriate bodies. Sport divers have a valuable role to play in the long term monitoring of sites as well as ground truthing and discovery. They can therefore play a positive role in the subsequent management of the coastal resource.
The Future : Geophysics
Conduct dedicated archaeological geophysical survey
This option has significant resource and funding implications. Although sponsors can often be found to support small scale surveys as demonstrated by this project, it is less likely that this will be possible for a survey of an area the size of Studland Bay.
Continue to raise sponsorship for further geophysical surveys
The two days of geophysical survey at Studland cost the NAS approximately £500. This figure is significantly subsidised by time and equipment provided free of charge by project sponsors. This approach could be continued in future years.
The first option gives complete control over the progress of the project whereas the second option allows the project to continue relatively cheaply but at the discretion of your sponsors.
The Future : Ground truthing
Use existing SMR as a basis for future ground truthing, or use targets generated by dedicated geophysics
Who does the ground truthing?
- NAS continue to ground truth targets using teams of volunteers, or
- NAS publish positions of geophysical targets (e.g. via NAS website) and invite sport divers to ground truth targets independently
The first option offers more control and ensures that any information finds its way back into the archaeological record. The second option provides an opportunity for a greater number of participants, but does rely on the integrity of the contributing sport divers. Independent groups may not provide information in a usable form and with this approach there is also the possibility that an important site could be found by a group and not reported.
The Future : Monitoring
Establish monitoring stations on selected sites e.g. Antler wreck, Valentine tank, reef, or other appropriate site
Such activity would provide quantifiable information concerning the dynamic nature of the seabed and environment in Dorset, an essential factor for decisions in future coastal management.
Who does the monitoring?
NAS would recommend that NAS supported volunteer teams carry out this work under the Adopt a Wreck/Site scheme, thus promoting public participation and developing a sense of local ownership of the underwater cultural heritage.
Dorset Coast Forum might wish to consider other 'at risk' areas where this methodology could be applied.
Conclusion
NAS is committed to continuing with its involvement in Dorset and would like to enter into discussion with all interested parties concerning the issues raised above. It is important that all views are taken into consideration in the formulation of a future plan.
Dorset WreckMap 2002 ?
The NAS would like to thank:
Captain Ron Howes and his boat Snapper who diligently steered survey lines for many hours beyond the call of duty. Steve Pierce from Poole Harbour Commissioners supplied tidal data and valuable information on the survey area. Charlie Foll from Atlantas Marine supplied the side scan sonar and finally Paul Byham from Submetrix Ltd (SEA). Everyone provided their time and equipment free of charge. We also have to thank Simon Corfield from DERA for his support. NAS would also like to thank David Kemp and staff at the National Trust Studland Bay Visitors Centre.
Boat launch at Studland during the project
Videoing the environment and the methodology
Writing up results at the end of each day
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