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.

Abandon Ship!

Although we've been very lucky with the weather so far—rain tends to be less frequent in Sweden during late June and all of July—today we had a really wet day. With thunderstorms forecast about half an hour after the video below was shot (and indeed they did come) we decided to make a hasty retreat back across the lake. Later, after lunch, with the sun shining we set out to return, but only made it to the boats before the sky turned dark and the thunder started again. Better luck tomorrow...

Photogrammetry

Simon getting an aerial view from a tree.

Following on last year, I've been using photogrammetric models to obtain orthogonal aerial views (taken at a 180º angle looking down onto the trenches). Photogrammetric software estimates the three-dimensional coordinates of an object (in this case an excavation unit), by making measurements between objects present in multiple photographs. From a cloud of photographs taken at different angles and heights (the model below uses 47 different views), I am able to obtain a perfect aerial shot—though there are other applications as well.

Photogrammetric model of Trench F, Stensö. Although only the aerial view is visible here, the model is fully 3D and can be viewed from any angle. 

Archaeologists traditionally had to resort to all kinds of methods for aerial photography—from balloons to ladders to heavy gantries for lifting up a camera. This method, which uses the Russian software Agisoft, is quite simple and only requires a camera on site. Both high and low angle views are desirable, though not strictly necessary—at Stensö we can stand on the tower to get high angle shots, and at Landsjö we can use the trees.  These shots do not have to be directly overhead. Back at the dig house, the photos need to be carefully cropped to remove areas outside the area of interest (trees, for instance) and any elements that are not in focus.

Trench F, Stensö, drawn.

From there, I am able to use the models later on for drawing or for direct documentation in the report. The drawing above was made using a series of photogrammetric models documenting the excavation of the first 20cm of Trench F at Stensö. Although drawings like this can also be done by making what is basically a downward facing photographic panorama, this actually takes less time. I also think it represents shapes better than the panoramic method. Photogrammetry is a great digital method for archaeology. 

Life on the Dig

Lunch on the site at Stensö on a sunny day.

The crew eating dinner at our Stensö accommodations at Smedstorp farmstead.

Playing a game at Landsjö manor, where we stay for the second two weeks.

The wood-fired hot tub at Smedstorp.

My co-director Martin surveying the ruined perimeter wall at Landsjö.

Landsjö

Landsjön at sunset.

On Friday we finished at Stensö and made the move to Landsjö, a little to the west. Although the other trenches were closed on Wednesday, Trench F, inside the tower at Stensö, was not finished until Thursday evening. With such a large crew this year (15 as of this morning), we're moving faster than last year. We'll see what we find here...

In The End

A 2015 ten-pence piece.

In best archaeological practice, we excavate to the point where archaeology becomes quaternary geology. Depending upon how limited our time and how ambitious our goals have been, this may or may not occur, but it is always a shame to close a trench before reaching what is termed “the Natural” level. 

Today in Trench F, we reached a deep fissure in the bedrock against the wall of the tower, which despite containing fragments of brick, was judged to be below the deepest level in which we were likely to still recover more finds given the large amounts of unmixed, blueish-gray glacial clay constituting the layer. Therefore, we closed the trench by backfilling.

Before backfilling though, we always “salt” the trenches with modern coins. For two weeks before the excavation, I carefully went through my change in Cambridge and pulled out all the low-denomination, 2015 coins for this purpose. Swedish coins were added as well. The purpose—half in jest—is that if any future archaeologist excavates in the tower again but has not read the report for whatever reason or has chosen to re-excavate because they believed we missed something (both of which occur from time to time), they will find evidence of the deepest level we reached. 

However, this is fantastically unlikely to occur. On a philosophical level, the coins will rest just above the bedrock under the tower at Stensö until the end of time. They are, in the larger scope, some of the only things I ever possessed that I know the location of, precisely and definitively, in time after my death. And after I cease to know that, well, they’ll be just like everything else: an artifact that in the absence of a historical record—and even then—will never, under all powers of scientific scrutiny, reveal my identity. 

Coins and backfill aside, that’s the archaeological conundrum. As W.G. Sebald wrote in The Rings of Saturn, paraphrasing the East Anglian antiquarian Thomas Browne,  “We study the order of things… but we cannot grasp their innermost essence. And because it is so, it befits our philosophy to be writ small.”

Medieval Man's Best Friend

A paw print found on one of the bricks at Stensö.

Sometime in the 1300s, a brickmaker had just finished laying out the still-moist forms of future bricks, ready to be fired, when a dog ran through her work. Some 700 years later, we uncovered evidence of that canine romp, because apparently it wasn't enough of an issue to warrant a do-over. Undoubtedly, other bricks were marked similarly—the dog didn't just step on one brick. I can just imagine how guilty it must have looked when it got yelled at.

It's actually somewhat common to find animal and human prints on archaeological sites and materials, but this is the first time I've seen it with a dog. Strangely, the piece of brick is harder than most and seems to have survived because of how the paw print distributed the heat during the firing process. Neat!