Archaeologists reveal secrets of prehistoric human-made island

Archaeologists from the University of Southampton have excavated and recorded a large timber platform hidden beneath what today appears to be a stone-built island, located in a Scottish loch.

They used a technique called stereophotogrammetry to record the human-made island above and below the waterline as a single continuous structure, providing a perspective that wouldn't have been possible using land or underwater survey alone.

The researchers, working with the University of Reading, examined  the ‘crannog’ in Loch Bhorgastail on the Isle of Lewis to reveal a structure built more than 5,000 years ago. Their fieldwork uncovered a layered wood and brushwood construction under the stone capping of the island, along with hundreds of pieces of Neolithic pottery submerged in the surrounding water.

University of Southampton archaeologist Dr Stephanie Blankshein explains: “Crannogs are small artificial islands that are typically thousands of years old. Hundreds exist in the lochs of Scotland and many remain unexplored or undiscovered.

“While crannogs were long thought to have been built, used and re-used, mainly between the Iron Age and the post-medieval period, we now know that some were first constructed much earlier during the Neolithic between 3800 and 3300 BC.”

Older than Stonehenge

Over several years of fieldwork visits , using traditional excavation techniques, coring, sophisticated surveying, and radiocarbon dating, the archaeologists have revealed the different stages of development of the Loch Bhorgastail crannog.

It was first established over five-thousand years ago, making it older than well-known monuments such as  Stonehenge. It started as a circular wooden platform, around 23 metres across, topped with brushwood. Some two-thousand years on, in the Middle Bronze Age, another layer of brushwood and stone were added, before another phase of activity took place around one thousand years after that during the Iron Age. A stone causeway, now underwater, leads from the loch shore to the island.

Over the years, archaeologists have discovered hundreds of pieces of Neolithic pottery, such as from different types of jars and bowls, scattered in the surrounding water. This suggests the site was first established by people of this period, before the Bronze Age.

“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. Large quantities of pottery, often still containing food residue, and worked stone found on and around the islands, suggest their use for communal activities such as cooking or feasting.” says Dr Blankshein.

Seeing clearly in shallow water

During fieldwork in 2021, archaeologists designed and applied a new technique for using stereophotogrammetry in shallow water. They used this image-capturing process  to examine the loch bed surrounding the crannog. Their technique  is described in a newly published article in the journal Advances in Archaeological Practice .

Photogrammetry is a well-established way of creating 3D computer imagery using multiple 2D photos. Photos of a subject are taken from multiple different angles and then ‘stitched together’ by specialist software to create a high resolution digital model .

Successfully conducting photogrammetry in shallow water is, however, fraught with obstacles, as Principle Investigator and Director of the Southampton Marine and Maritime Institute , Professor Fraser Sturt explains: “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.”

To help solve this, the researchers used two small waterproof cameras – with low-light performance and a wide-field of view. Locked at a set distance apart on a frame, this ‘stereo’ method provides 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, matching that achieved by an aerial drone.

“By combining stereophotogrammetry, drone technology and some innovative post-processing of the data, we have managed to set out an accessible approach that is portable and cost effective,” says Dr Blankshein, lead author on the paper.

This innovative surveying work represents the first publication from the University of Southampton’s Coastal & Inland Waters Heritage Science Facility . This work has provided extra clues to the mysterious past of the crannog structures, and the team hopes the method can now help further investigations of other, similar sites.