The underwater “anomaly” has come to light in a project looking at prehistoric sea level change in Orkney. The project, The Rising Tide: Submerged Landscape of Orkney, is a collaboration between the universities of St Andrews, Wales, Dundee, Bangor and Aberdeen.
But although it is tempting to speculate that the ring-shaped feature, which lies just off the loch’s shore, is the remains of a henge — a circular or oval-shaped flat area enclosed and delimited by a boundary earthwork (usually a ditch with an external bank) — or perhaps a prehistoric quarry, at this stage the project leaders are urging caution.
Orkney-based archaeologist, Caroline Wickham-Jones, a lecturer at the University of Aberdeen, explained: “The preliminary results from the high-resolution geophysical sensing are suggesting that there is an unusual ‘object’ in the shallow water just off the shore, but more work is needed before we can identify it or even confirm whether it is a natural, perhaps geological, feature, or something man-made.”
Dr Richard Bates from the School of Geosciences, St Andrews University, added: “The character and size of this feature — approximately 90m in diameter — are about the size of the main Ring of Brodgar. If it turns out to be artificial, the massive anomaly has to predate the influx of the sea into the Stenness Loch basin.”
When prehistoric Orcadians started to build the stone circles in Stenness, the landscape would have been much different to what it is today and the sea would have been about a metre below current levels. Prior to the sea coming in, the loch area was stands of open freshwater, with reed beds — probably much like the landscape around the Loons, in Birsay, today.
Previous studies have shown that the sea around Orkney reached its present level about 2000BC, but even then, because there is a rock “lip” at the Brig o’ Waithe which held the sea back, the impact of the rising water in the Loch of Stenness was a bit slower.
“We think there was a gradual incursion of the sea over time, preceded by a number of storm events that saw seawater crash over the rock lip and begin to form what was to become the Loch of Stenness,” said Ms Wickham-Jones.
One of the reasons the team began surveying the Stenness Loch was because of an aerial photograph from Andrew Appleby. The photograph, showing the Unstan Chambered Cairn, appeared to show earthworks running off into the loch.
Ms Wickham-Jones said: “With Unstan as a starting point, we worked our way around the loch and now, through sediment analysis as well, we’re getting precise information on the changing landscape as well as researching when the Loch of Harray changed from being a marshy landscape to water.
“In addition, we’re looking at things like possible deposits from the Storegga tsunami in the loch sediment.”
Possible evidence of the Storegga tsunami is particularly interesting, because evidence of the prehistoric event has been confirmed in Shetland and other areas of northern Scotland, but has not found in Orkney to date.
The Storegga slides comprise massive landslides that occurred underwater, about 100km north-west of the Norwegian coast, around 6100BC. The landslides, thought to have been triggered by an earthquake, saw an area of underwater sediment the size of Scotland slip from the continental shelf and into the deeper waters of the North Sea.
This resulted in huge quantities of sea water rippling outwards, in fast-moving waves, from the site of the slide toward the coastline of Scotland. In Shetland, evidence has shown that the resultant wall of water could have been as high as 25 metres.
As well as the Stenness Loch, the project has also focused on Hoy, Hoxa and the Bay of Firth.
In the latter, the surveys have revealed how the landscape was transformed from the start of the Mesolithic period (c.7000BC), when the “bay” was dry land, to the late Neolithic/Bronze Age (c.2000BC), when sea water had filled in the lower-lying areas leaving Damsay as a tidal island.
Dr Martin Bates, of the University of Wales, commented: “Survey has identified a possible lake site in the Finstown basin before the sea flooded the area, and this may have been a focus of activity during the Mesolithic, when hunting and fishing groups exploited the rich resources in, and around, the lake. In future, diving at the margins of this lake might reveal evidence for such activity”.
Ms Wickham-Jones added: “The Bay of Firth results are particularly interesting as they show that, even as late as the early Bronze Age, there were tracts of land which are now under water.
“We’ve got prehistoric activity all around the bay, at Stonehall, Wideford Hill and Cuween Hill for example, but it is curious to me that the tombs at Cuween and Wideford are both later Neolithic structures.
“Archaeologists study what’s there, but sometimes it’s more interesting to ask what’s not there. The early Neolithic tombs around the bay for example: where are they? Many other early Neolithic tombs in Orkney — such as Unstan — are found near present sea level, on low-lying land. Were the earliest tombs around the Bay of Firth built on land that has since been covered by sea?”
The surveys have detected two intriguing anomalies in the bay, one of which is visible in aerial photographs by the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) and also appears on the remote sensing results.
A seismic survey in the bay has helped to shed light on the possible structure of one of these, and now the team plans more diving work to confirm the results.
In the South Isles, money from the Scapa Flow Landscape Partnership has enabled work in Mill Bay and Hoxa to interpret the early development of Scapa Flow.
During the Mesolithic, lower sea levels meant that the Flow comprised a large land-locked bay, open to the sea at the south.
This would have been a good area for the hunter-gatherer population of Orkney, and the research is starting to identify areas where sites may have existed.
By the time of the first farmers, the Flow had opened up to the west and much of the coast had been submerged, but areas suitable for farming still existed, for example in Longhope Sound, Widewall Bay and Water Sound.
“We might think we know an awful lot about Orkney’s archaeology, but there’s a whole hidden side to it, and that is the remains that lie underwater.
“Until we get to grips with this, how much do we really know?”
The project work is funded by many different bodies, most recently the Crown Estate, Historic Scotland, the Royal Archaeological Institute and the Society of Antiquaries of London.