A new radiocarbon date for a shell-filled pit at The Cairns Iron Age site in South Ronaldsay shows that it was in use in the fifth or sixth century AD.
The pit appears to have been used to cook shellfish and after consumption, their shells, all 18,637 of them, were put back in it.
The site director at The Cairns is University of the Highlands and Islands Archaeology Institute lecturer Martin Carruthers.
He said: “This is an astonishing number of shells for a short-lived, single-event context. This suggests it may have been part of a special food event, a feast involving the whole community of the site or even beyond.”
The majority of the shells, which were analysed by UHI Archaeology Institute Masters student Holly Young, belonged to limpets (84 per cent), with common periwinkles making up the rest.
The radiocarbon date shows the pit was in use at the same time as the nearby souterrain.
“One of our project research aims has been to investigate the role of souterrains and this extraordinary contemporary feasting is adding to our picture that souterrains may have been very special places involving social and ritual practices, in addition to whatever other roles they may have had in food production or storage.
“Indeed, during the construction of The Cairns souterrain another cache of shells was placed over the slab roof of the structure along with a special deposit of rotary querns!”