Finland has a lot of cairns, usually sitting on hill tops near the sea. Unlike a mound, the cairn consists only of stones, and so it lets rain water percolate through. This messes up the contents of the cairn. Bones and burial goods are rarely preserved, and it seems that the ancient Finns didn’t stock their cairns with a lot of interesting stuff to begin with anyway. This makes individual cairns difficult to date, though seen as a class their chronology is fairly well understood.
Despite the fact that few Finnish cairns contain anything interesting or valuable to a layperson, a lot of them have central depressions indicating that people have delved into them some time after their construction. There is no evidence to suggest that the depressions are due to the collapse of any internal wooden chambers.
During the excursion last Saturday at the Bronze Age conference in Helsinki, Tapani Tuovinen let us in on an interesting methodological development. How do you know at what date a burial cairn was looted?
The bedrock in southern Finland weathers in a characteristic way visible in a microscope. During the last Ice Age, a lot of nice round pebbles were produced through abrasion, and they’re really good for cairn building. When you retrieve one them from the sea or the glacial till, they show no signs of microweathering at all to begin with. But if you look at pebbles on an undisturbed 3000-y-o cairn, you find heavy microweathering on the upper half of the stones. Their lower halves are far less weathered.
Tapani & Co looked at the stones scattered around the central depressions in looted cairns, and found that many of them were upside down: the undamaged bit was no longer pointing downward. This showed that they had been moved around relatively recently, most likely during the 19th century. The method doesn’t give absolute dates, but it’s still useful information.
Then the team looked at the bedrock beneath cairns they excavated, and found that it was much less weathered than the rock outside the bases of the cairns. This confirmed that they are pretty damn old: the cairns have been sitting there long enough for a slow weathering process to produce a visible difference.