THE SCOTTISH TRUST FOR UNDERWATER ARCHAEOLOGY

Loch Tay Sidescan Sonar Survey Fieldwork Report Summer 2008 T N Dixon and B L Andrian

Tay Goblin 20 foot Hardy motor sailer ideal for sidescan sonar survey

The Scottish Crannog Centre Kenmore, Loch Tay , PH15 2HY Tel: 01887 830583 email: [email protected] www.crannog.co.uk Loch Tay Sidescan Survey 2008 Introduction Sidescan sonar has been used on many occasions as a tool in underwater archaeology, in most cases as a remote sensing aid for locating wrecks in the sea but sometimes in a bid to view the underwater landscape in freshwater. Its most high profile use in in the recent past was an effort to locate the Loch Ness monster when a fleet of small boats with sonar detectors scanned the length of the loch. The monster was not located but many other features, such as a Wellington bomber from WWII, were discovered. The STUA has been surveying and excavating in Loch Tay since 1979 when the first survey of the loch looking for crannogs was carried out. This led to the ongoing long-term excavation of Oakbank Crannog, off the village of on the north shore of the loch, which started in 1980. A broader survey of the north shore from village to the west, took place as part of the Historic Landscape Project under the auspices of the National Trust for Scotland with funding from Historic Scotland and the HLF. This covered the shoreline, the shallow water and the slightly deeper water down to about 8 metres. A stated long-term aim of the underwater survey was to examine the whole of the loch using sidescan sonar. Over the years, as well as the crannogs in the loch, a number of small wrecks have been located ranging from the remains of a steamship that was blown up in shallow water in the 1950s to an 11 metre long logboat dating to the Middle Bronze Age around 1500 BC. Another feature discovered recently is an area of submerged woodlands with tree stumps and branches dating from 8000 years ago in the Mesolithic Period, throughout the Neolithic Period and from 1500 years ago in the Dark Ages. Further work on these remains brought to light evidence of beaver influence in the loch which has now been dated to the latest Neolithic Period. In the summer of 2008, we carried out the first sidescan trials at the east end of Loch Tay in an effort to assess the usefulness of relatively inexpensive sidescan units that have become available. While it is ultimately planned to survey the whole of the loch, in 2008 the main effort was to survey shallow areas down to about 15 – 20 metres. At these relatively shallow depths, effective archaeological work can be carried out using Fig 1 Tay Goblin, survey boat basic diving equipment without involving the problems of long decompression stops. Another benefit of the shallower areas of the loch is that the silt is not so deep as to be a major problem and features and objects, such as small shipwrecks, should still be visible and not completely buried. Another benefit for divers checking targets is that light levels are reasonable in shallow water while it is pitch black deeper down. The equipment The STUA tested two systems: an Imaginex Sportscan unit, supplied by Sula Diving of Orkney and a Starfish unit supplied by Tritech of Aberdeen. The units consisted of a towfish and associated electronics. Both are attached to a standard laptop computer running proprietary software that can be freely downloaded from the internet in both cases. The units were towed behind a 20 foot Hardy motor sailer (Fig 1, 2) which provided a very steady platform with plenty of undercover space for the laptop and operators even in bad weather. In use, the Sportscan fish was heavy and could be difficult to handle. However, it was steady in the water. The gps readout did not work on some occasions and it is not clear why although it is 2 suspected that there was a resource conflict in the computer. While it was possible to take spot gps readings with a handheld unit this was much less satisfactory than having the co-ordinates permanently displayed on the screen with the sidescan images. The software that comes with the sidescan unit was adequate and stable but somewhat clumsy to use. A simple on/off recording situation would be best but the existing software required the appropriate folder to be opened before a run could be recorded. Overall the images from the unit were good but the output was restricted in what it could do, which is not surprising given the price. The Starfish sidescan unit was very small and light and came in one relatively small container. The fish was very easy to handle. It was linked to the laptop through a small box that needed its own power supply. Fig 2 Sidescan equipment There were problems in getting the gps to take readings and it is not clear why this was so. In some cases, while the unit seemed to be recording there was nothing readable on the sidescan trace although sometimes the gps was still taking accurate readings. At other times the sidescan trace was recorded but not the gps information. The reasons for this are not clear but it may be that the computer that was being used was not modern enough. Another possibility was the problem of ensuring a good power supply to both computer and sidescan unit. The Starfish software was easier to use with a simple on/off method of recording runs. Discussions are taking place with the company that supplied the unit, Tritech of Aberdeen, to sort out these teething problems. Both units gave good clear pictures that were readily understandable. Features on the bottom of the loch stood out vividly from the usual flat sediments and even anchor chains to moored boats could be seen extending up from the lochbed moorings. Small wrecks were readily identifiable, as were the wooden piles of a pier and the circular stone structure of one of the Loch Tay crannogs. Many unidentifiable features were also noted and when the project is fully underway, it is planned that these will be examined by a team of divers working along with the sidescan team. Results A number of sonar runs were made in the east end of Loch Tay mostly in Kenmore Bay but also as far west as Fearnan on the north shore. A run was also made along the south shore from opposite Fearnan to Kenmore Bay. Some runs were made at Fearnan, to see what might be gained from observations at Oakbank Crannog where substantial excavation over the years has created a modified site with large stone piles around the periphery on the east, south and west sides. East of Priory Island, Kenmore The area to the east of Priory Island has been an important place for moorings in the past as it is directly associated with the Campbell lands and the gardens for inhabitants of Priory Island, Balloch Castle and, more recently, . Priory Island was inhabited for a long time and latterly it was the siting of a repair facility for steamships, with the remains of a sturdy slipway and the boiler of a ship still lying on the island. The wreck of a small ship has been known of, and dived on, for at least twenty years and the remains of a freight steamship lie near the remains of an old pier. The ship was blown-up in the 1950s for the shaft and bearings.

3 Fig 2 Sportscan image of early boat Fig 4 Starfish image of early boat

Runs from the east side of Priory Island east towards the bridge at Kenmore passed over the site of the small wreck and good images (Figs ?, ?) were acquired by both sidescan units over a period of a few days. Even relatively small details of stern features could be made out clearly as well as the overall shape. The remains of the blown up steamship were also clear and appeared as a jumble of material (Fig 5). It will be interesting to take a printout of it down and see how it fits in with the actual remains. A number of other features were also noted on the lochbed, such as sunk mooring lines where even the rope could be discerned (Fig 6). Other features were noted that will require diving to identify them. When both sidescan units were operating correctly the latitude and longitude of the targets was recorded on the screen so they can be relocated.

Fig 6 Sunk mooring in Kenmore Bay Fig 5 Blown-up steamship

4 Dalerb To the west of Priory Island there is a level area now used as a community picnic site. The west end of the site has the remains of a large slipway that used to be used for repairing the steamships in the loch (Fig 7). A sidescan run some 20 metres off the pier produced an image that clearly showed the piles of the slipway underwater (Fig 8). Dumps of material to west and east of the slipway are the material dredged up and dumped in the 19th century when the facility was built. The

Fig 7 Dalerb steamship repair pier Fig 8 Sidescan image of Dalerb pier dredged channel that the slipway sits in can also be clearly seen. To the west of the slipway, about 30 metres out into the loch, there are the remains of Mary’s Distaff Crannog. The site was first surveyed in 1979, (Dixon 1981) and appears as a large, roughly circular mound of stones (Fig 9). The shape of the crannog is clear in the sidescan trace and other features around it can be made out (Fig 10). Two particularly large objects lying to the south-west of the crannog (top right in the picture) are probably large stones but diving is needed to check out those and other features to both east and west. While the crannog shows up clearly as a mound the large black shadow running down in the trace is the part of the site that was projecting above the water on the day of the survey. The image would have been more complete if the site had been completely submerged and further survey when the loch level is higher would be useful.

Fig 9 Mary’s Distaff Crannog Fig 10 Sidescan image of Mary’ Distaff Crannog

The image shows quite clearly that crannogs can be recognised by sidescan survey and will make locating sites much quicker and easier prior to diving to observe them in more detail.

5 Fearnan Bay A number of runs were made to the east of Fearnan Bay at the site of Oakbank Crannog. The crannog itself and features associated with it, such as stone mounds created from the excavations, are a familiar area and it was hoped that specific elements would be recognised. However, there was a strong wind on the day which caused short, choppy waves. These made the sidescan image more difficult to interpret and created a herring-bone pattern across the image. There is software that can allow for these anomalies but it was not available at the time so further work is needed in the area. The crannog off the hotel at Fearnan could also be seen clearly on the sidescan image. The whole bay is of considerable interest and will be surveyed more completely in the future.

South shore of Loch Tay The south shore of Loch Tay from opposite Fearnan east to Kenmore was also surveyed as an exercise. A number of features are found along this shore including Croftmartaig Crannog off the village of Acharn. The site did not show up well on the sidescan and it is thought that this is because of the very shallow water between the crannog and the shore distorting the picture. More work will be carried out here in future. Other features were noted along the shore and they will be examined by underwater archaeologists in the future.

Conclusion The pilot sonar work with portable, relatively inexpensive equipment clearly demonstrated the potential for locating crannogs, wrecks, structures and a wealth of other features in detail. It also demonstrated that systematic coverage can be obtained in relatively short periods of time, weather permitting, and that repeated runs over the same areas from different positions help to clarify obscure features. Interpretation of the scans clearly requires experience and an understanding of many factors including the nature of the lochbed sediments, contours, and the affect of weather and signals on performance and recording of the information. Proper identification of unknown targets will still rely on visual inspection by divers and/or remotely operated cameras. Just as the Ben Lawers Historic Landscape Project discovered many cup-marked rocks and other features along the foreshore, sonar mapping – particularly out to the 30m contour – would be particularly worthwhile and is planned for 2009. A number of issues still need to be sorted out, in particular the problems with the gps readouts, but the overall efficiency and benefits of the method are very clear and can be a great addition to the tools of the underwater archaeologist.

References: Dixon, T. N. 1982. A survey of crannogs in Loch Tay, Proc. Soc. Antiq. Scot.. Vol 112. pp17-38.

Acknowledgements: Thanks to Tamara Brilz of Tritech, Aberdeen and Bobby Forbes of Sula Diving, Orkney for supplying the sidescan units and offering endless advice. Thanks also to Barrie Andrian, Scott Cruickshank, David Carlin and Najat el Hafi, the support crew who helped with the equipment and operating the boat. Funding support was given by Historic Scotland.

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