Bridges and Dendrochronology Dates

Excavating for a bridge at Landsjö, on an industrial scale (photograph by Simon Terbrant Säfström).

Because Landsjö Castle was built of incredibly heavy materials (the mortar alone would have required tons of sand and limestone) on an island, we always assumed that it was accessed from the shore via a long causeway or bridge. Although we know the castle was deconstructed over successive winters by dragging the large stone blocks across the ice sometime in the 1500s and 1600s, such a methodology would have been impractical for building the castle during short, winter days. 

Our initial idea was confirmed last year when we found used horseshoe nails intermixed with debris from the construction of the castle in Trench D. Although horses can swim and although they may have arrived on a barge, it seemed likely they came across a bridge. 

Two of the students, Henrik and Daniel,  enjoying the muddy work.

This year, we applied to dig a 1.5m x 150m trench in cooperation with the landowner Micke whose house we stay in during the dig. His foreman drove the digger out today, and we began the enormous trench. At our current numbers and pace, it would take us seven months to dig as much as he accomplished in a few hours. 

Posts retrieved from the trench. Note the tapered ends on the right two stakes. All three are probably large enough for dendro dates (photograph by Simon Terbrant Säfström).

The results were spectacular. By searching in the trench for the posts broken off as the trench was excavated, we were able to retrieve 15 or so, three of which can probably be dated using dendrochronology to the nearest-decade—if not the exact year—the tree was cut. Dendrochronology is ostensibly the science of quantifying tree rings based on large, known assemblages of preserved wood. In the waterlogged, oxygen-poor mud near the lake, organic materials do not degrade quickly, and there is a chance the posts may be even older than the medieval period.

The smaller pieces that cannot be dated using dendrochronology can still have their intrinsic ages determined using radio-carbon dating. Thus, the assemblage of posts we recovered, slices of which are now resting in the large freezer downstairs until they can be moved to a lab, is invaluable in determining the construction sequence of the castle. There was another spectacular find made along with the posts, but you’ll have to check back tomorrow for more on that.

Part of the full assemblage of posts recovered.