Birka is famous for being one of the key Viking Age trading sites, and in 1993, UNESCO awarded it World Heritage Site status.
At the time, UNESCO stated that Birka bears "exceptionally well-preserved testimony to the wide-ranging trade network established by the Vikings during the two centuries of their phenomenal economic and political expansion."
Research at Birka on Björkö Island in Lake Mälaren has a long tradition, but until recently, it has mostly focused on the trading town's area within the city wall.
What remains hidden outside the city's walls?
As Forskning & Framsteg writes, the common perception has been that only graves were to be found outside the walls.
However, experts have recently been looking at other areas with the help of modern technology. Archaeologists Sven Isaksson and Sven Kalmring have explored Björkö's northern cape with geo-radar technology and drones, as well as excavations.
"A site like this has never been found before, it is the first of its kind, but the finds convincingly show that it was a shipyard," Isaksson, a professor of Archaeological Science at Stockholm University, stated.
Remains that have not been excavated until now have previously been described as boathouses and sites where boats were pulled up to shore. But the researchers made an exciting discovery – a Viking shipyard site.
Among other things, the archaeologists carried out excavations along the former shoreline, discovering a stone-lined hollow 22 meters long and 10-12 meters wide.
"But we did not find any traces of any wall or roof construction, so it was not a boathouse. However, there was a stone gasket at the bottom. Most likely, it has been a boat slipway through which boats have been pulled up to be serviced.
"The finds of artifacts from the area show with great clarity that this is where people have served their ships," Isaksson added.
A row of rivets in the longitudinal direction of the slip, traces of a wooden construction, and remnants of woodworking tools point to the site being a shipyard.
Furthermore, organo-chemical analysis of soil samples shows traces of spruce or pine resin, which also indicate woodworking. There are also indications that tar has been used.
A digital 3D model of stone packing in one of the investigated trenches. On top of the stone packing was a layer of sand, and originally there was also a wooden boat slip at the bottom. Traces of this can be seen in the form of post-holes and a stone frame at the bottom. Photo: Fredrik Lundstrom
The first Viking age shipyard to be excavated
In Denmark and on Gotland in Sweden, traces of boat repairs have been found at piers or harbors, but the boat slipway makes the facility at Birka the first Viking-era shipyard to be excavated.
Boats may also have been built on the site, which was used from the end of the 9th century and more than 100 years onwards.
But the facility may have been in use earlier than that.
The exact type of the shipyard is not yet clear, and further research will look into whether it was a free port outside the city or an official port connected to it.
There are signs of other buildings on the site, which may have been the smithy, another workshop, or a timber warehouse.
The shipyard may have been even larger than what has now been excavated. In Korshamn, which is close by, there is a 20-meter-long foundation for a building, which is still unexcavated.
With further funding, there may also be excavations there, as well as continued exploration of Kugghamn.
Detailed map of Björkö's northern part, with the results of mapping of near-shore remains between Birka's Black Earth and Korshamn. The features recorded as possible jetty or boat landing remains have a light red fill and are marked 1-9. Photo: Stockholm University
The town ramparts around Birka functioned not only as a defense but also as a legal, economic and social boundary.
Previous investigations of harbor facilities in Birka have mostly been carried out inside the town rampart, in the area known as the Black Earth harbor area, and below the so-called Garrison.
The newly discovered shipyard at Kugghamn is located, along with a number of other maritime remains, outside Birka's town rampart, along the northern shore of Björkö.
"By investigating various maritime elements in connection with a possible house site in Kugghamn, we are now trying to get an overall view of a very exciting and previously archaeologically completely unexplored environment", Kalmring added, according to a press release from Stockholm University.
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