ABOVE: A test stake consisting of a perspex stake, with standard steel test rods attached at intervals along its length. Embedded to a known depth in the seabed the stake monitors changes in sediment height. After recovery the amount of steel lost to corrosion can me measured, giving a measure of how agressive the local environment is.
ABOVE: A diver recording data off a test stake in situ.
We have used the techniques developed at Cellardyke to investigate a number of sites around the Scottish coast:
HMS Dartmouth is a protected wreck site in the Sound of Mull. It is very difficult to appreciate as a diver, however, as it is in a thick kelp forest which obscures the remains. After consultation with marine biologists, to ensure that no lasting damage would be done to the biological communities on the site, the licensee cleared paths to permit divers greater access.
In doing this, however, there was a slight chance that the clearings could influence the preservation of the site, possible stimulating errosion by removing the protective kelp. To investigate this possibility we deployed a number of test stakes, into cleared areas and areas still protected by kelp.
The stakes showed that there was no increase in either errosion or corrosion in the cleared areas, vindicating the limited clearance technique for increasing access to this site.
The John Preston is a small wooden coasting vessel that sank again in the Sound of Mull. Ths site has been the subject of study for some years by the Nautical Archaeological Society (Scotland). The site was exposed in the 1980's when her cargo of slates was salvaged.
To be preserved it is important that a protective layer of sediment builds up over the site. Data from the stakes, however, indicate that the site is not receiving a great influx of sediment, and burial is unlikely.
The Kinlochbervie designated site consists of a collection of pottery, some large iron objects, and very little else. The site is enigmatic, and the results we obtained from our stakes agreed with this! (See Monitoring the Kinlochbervie site for further details). At Kinlochbervie a range of unusual features were noted. One of these was the absence of sedentary life - usually prevelant on hard substrates of the type found at Kinlochbervie.
It appeared that an unusual preservation environment and an unusual biology were occurring on the same site. The question is, are these things related, and can we predict one from the other?
We have found test stakes to be very useful, providing unambiguous corrosion kinetics and information about sedimentation patterns. It would, however, be useful to have additional supporting evidence. Further, stakes and other test systems are only relevant at the place where the test was carried out. A complex environment changes by the metre, and being able to follow this by observing changes in species distribution would be a very powerful tool.
The biology of an area provides instant feedback on conditions in that area, conditions that may also influence the preservation or archaeological remains. Distribution patterns may allow us to plan comprehensive test strategies, while minimising the number of tests needed.