Archaeologists have used innovative techniques to reveal the secrets of an artificial island older than Stonehenge in a Scottish loch, discovering a large timber platform hidden beneath it.
The island in Loch Bhorgastail on the Isle of Lewis appears to be made of stone – however, University of Southampton researchers have found that timber was used for the basis of the structure.
While it was known that there was some wood underneath the island, the extent of the timber structure was not discovered until excavation and analysis started.
During fieldwork at the crannog, archaeologists uncovered a layered wood and brushwood construction under the stone, and found hundreds of pieces of Neolithic pottery submerged in the surrounding water.
University of Southampton archaeologist Dr Stephanie Blankshein said: “When we actually started excavating is when we realised that it was actually this coherent, quite large timber structure that was under what you would see as the stone island today.
“So that was a big surprise, and that was in 2021 when we actually started digging down.
“Before that, we didn’t know. We just knew that there were pieces of wood sticking out and that there must have been some sort of structure there.”
Working with experts from the University of Reading they have now used a technique called stereophotogrammetry – a way of creating 3D computer imagery using multiple 2D photos – to show how the island looks above and below the waterline as a single continuous structure.
Dr Blankshein said: “Through excavating and through the photogrammetry work that we did, we were able to build a picture of this coherent timber structure, not just bits of wood supporting a stone mound with a kind of timber platform underneath, but actually, the timber itself was the basis of the structure.
“A lot of work went into it, we’ve had a lot of really good dates come out of it, and all the dates are aligning to about 3500 to 3300 BC across all the sites we’re seeing.
“So we know that this was an activity that wasn’t just taking place at this site, but other sites nearby and even on other islands throughout the Outer Hebrides.”
Crannogs are small artificial islands that are typically thousands of years old, and hundreds of them exist in Scotland’s lochs.
Dr Blankshein said: “While we still don’t know exactly why these islands were built, the resources and labour required to construct them suggests not only complex communities capable of such feats, but also the great significance of these sites.”
The archaeologists revealed the different stages of development of the crannog over the course of several recent years of fieldwork visits to the site using traditional excavation techniques, radiocarbon dating, surveying and coring.
They discovered that it was first established more than 5,000 years ago and started out as a circular wooden platform, around 23 metres across, topped with brushwood.
Around 2,000 years later in the Middle Bronze Age, another layer of brushwood and stone were added, with more activity around 1,000 years after that during the Iron Age.
A stone causeway, now underwater, leads from the loch shore to the island.
The site was first discovered in 2009.
The experts have now provided new perspectives on the crannog using photogrammetry in which photographs of the subject are taken from multiple different angles and then “stitched” together by specialist software to create a high resolution digital model.
Carrying out photogrammetry in shallow water such as that around the crannog poses difficulties so the archaeologists created a new technique for doing so during their fieldwork.
Principal investigator and director of the Southampton Marine and Maritime Institute, Professor Fraser Sturt, said: “Fine sediments, choppy conditions, floating vegetation and distorted or reflected light all hinder shallow water imaging.
“Photogrammetry is very effective in deep water but runs into problems at depths of less than a metre. This problem is a well- known frustration for archaeologists.”
Using a technique developed during fieldwork in 2021 the researchers used two small waterproof cameras – with low-light performance and a wide-field of view.
These were locked at a set distance apart on a frame, providing precise overlapping of imagery, to help compensate for any missing or disrupted data.
The cameras were manoeuvred through the water by a diver with positioning controlled to centimetre accuracy, which is said to match that achieved by an aerial drone.
Their technique is described in a newly published article in the journal Advances in Archaeological Practice.