Archaeologists have found the well-preserved remains of a forty-metre-long late-medieval drainage sluice near Monnickendam. “In the Netherlands, about five medieval locks have been excavated so far, but they are all much smaller,” says Jeroen Flamman, who is leading the excavation on behalf of archaeological company Vestigia. Also special: the lock was found exactly on the spot where a new pumping station is being built.
The find is unexpected, but not entirely. A lock was marked on a historical map from 1590. However, on the basis of the map it was not possible to determine exactly where the lock might still be in the three hundred meter long Nieuwendam. It was therefore decided that an archaeologist would be present during the construction of the pumping station in case something was found. Meanwhile, the archaeologists took the opportunity to record the historical structure of the dam. “Two weeks ago we were told that they had discovered huge wooden beams,” Flamman recalled. Since then, archaeologists have been excavating and mapping the lock at the Purmer Ee and four meters below the raging traffic on the N247.
It is already clear that the lock is built on a heavy wooden foundation. The lock itself is made entirely of oak. The walls are made of recycled ship wood, among other things. Notches on the east side, on what is now the Markermeer, indicate that the lock must have had at least one door that opened at low tide and closed at high tide, Flamman explains. “The door itself is no longer there, it will have been reused at some point, just like the ship planks that were used for the walls before.” The lock was 6.5 meters wide and an estimated 3.5 meters high. And he must have been overprotected, Flamman says. “Otherwise he would have collapsed from the lateral pressure. So it was not a lock; only small flat-bottomed boats could have sailed through it with lowered masts.”
Filled with soil and waste
The lock was probably built around the middle of the sixteenth century. “We haven’t done tree ring research yet,” says Flamman. “We have already been able to establish that there was a small dam before. According to the sources, it was built at the beginning of the fifteenth century. The lock must therefore be younger. The wood was sawn and that only happened in the sixteenth century.”
The lock was decommissioned in the course of the seventeenth century. “Then two wooden walls, which we excavated, were placed in the lock and then the whole thing was filled with soil and waste. The pottery in it gives the dating of those works.”
So far, few medieval locks have been archaeologically explored. In Rotterdam, some were excavated near the Hoogstraat in the 1940s. A lock was also found during the construction of the railway tunnel in 1990. Some of the Rotterdam locks were already partly made of stone.
Flooding during storm surges
“However, a wooden lock was cheaper,” says Diederik Aten, historian of the Hollands Noorder-Kwartier Water Board, which is having the pumping station built. Since the find was unexpected, he has not yet been able to do much historical research. We know that in 1403 Albrecht van Beieren, Count of Holland, granted permission to build a lock in the Nieuwendam, which closed the inlet between the Zuiderzee and the Purmermeer and to prevent flooding during storm surges in the villages around that lake. That could be this lock, which has been renovated and extended over the years.”
In the near future, the archaeologists will record everything three-dimensionally using 3D scans and photogrammetry. “I don’t think it will be possible to keep everything,” says Flamman. “But a scale model can also give a good impression later on. And on December 11 there is an open day for the public.”
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