Artificial islands incite images of grand, lavish floats that support modern human activity. In archaeological history, however, artificial islands have existed for as long as humans have known how to build things—including the hundreds of artificial islands lying beneath Scottish waters.
These “crannogs” are typically made of wood and stone. As crannogs were mostly underwater, methodological barriers prevented researchers from discovering and exploring them extensively. But one team of archaeologists based in the U.K. developed a novel strategy based on stereophotogrammetry, which records the landscape above and below water as a continuous image. This workflow enabled the team to investigate crannogs with much greater precision, according to a recent Advances in Archaeological Practice paper on the findings.
Notably, some crannogs were confirmed to be over 5,000 years old, which puts them at par with sites such as Stonehenge. The findings also push crannogs much further back in history than previously thought. Researchers had long assumed the structures were mainly built, used, and reused between the Iron Age and the post-medieval period.
“While we still don’t know exactly why these islands were built, the resources and labor required to construct them suggest not only complex communities capable of such feats but also the great significance of these sites,” said Stephanie Blankshein, the study’s lead author and an archaeologist at the University of Southampton, in a statement.
The findings stem from a larger project, Islands of Stone, which has studied crannogs spread throughout the Outer Hebrides, a particular hotspot for the artificial islets with 170 known sites out of 550 recorded in Scotland, since around 2021.
But the team had studied similar structures since the mid-2010s, during which they directly experienced the challenges of studying submerged archaeological sites.
“Straddling the boundary between land and water, [crannogs] preserve complex stratigraphies that offer key insights into past occupations and changing hydrological landscapes,” the researchers explained in the paper. But terrestrial survey methods don’t work in water, and marine geophysical equipment isn’t suitable for landscapes in shallow waters of less than 3.3 feet (1 meter), they added.
This constitutes a “white ribbon” of missing data, according to the paper. The team opted for a modified version of photogrammetry, which is already well-established as a method to create a 3D model of landscapes stitching together 2D photos taken from different angles. That on its own, however, is lacking when investigating objects underwater, explained Fraser Sturt, a marine archaeologist at Southampton.
“Fine sediments, choppy conditions, floating vegetation, and distorted or reflected light all hinder shallow water imaging,” added Sturt, a study co-author. “Photogrammetry is very effective in deep water but runs into problems at depths of less than a meter. This problem is a well-known frustration for archaeologists.”
The team’s solution was to use waterproof stereo cameras operated manually by divers. The researchers then compared the resulting dataset with aerial drone surveys, confirming that things added up nicely. From this “stereophotogrammetry,” the researchers were able to fill in that missing, white ribbon data for a Neolithic (between 10,000 BCE and 2,000 BCE) crannog in Loch Bhorgastail in Scotland.
Radiocarbon dating put Loch Bhorgastail crannog at more than 5,000 years old. The team’s analysis indicates it started as a circular wooden platform about 75.5 feet (23 meters) across, topped with brushwood. Then 2,000 years later, during the Middle Bronze Age, another layer of brushwood and stone were added to the island. Finally, the island preserved another phase of activity during the Iron Age, roughly 1,000 years after.
Hundreds of pieces of Neolithic pottery were scattered throughout the premises, which hinted to the researchers that the site was first established by humans from this period. According to Blankshein, some pottery with food residue suggests these islands could have been hotspots for communal activities like cooking or feasting.
But they’ll continue to work away for a clearer explanation. For the study’s final steps, the team assessed the scalability of its new methods, concluding that this should give crannog investigations a big boost.
Source: Gizmodo