Say What? Iron Age People Set Fire To Their Own Hillforts

Traprain Law Hillforts on Fire 600x390

Sounds crazy, but new research suggests that by setting fire to their walls, Iron Age communities were actually strengthening them!

For years, archaeologists have been stumped by a strange feature present in numerous Iron Age (800BC-AD42) hillforts across Western Europe. The walls of these forts bare evidence of extreme fire damage, with vast areas of stonework that had literally melted in the heat. In the process, the walls had been partially ‘vitrified’, meaning the melted stone and mortar had re-solidified into a glass like substance.

In the past, theories on why this burning occurred have ranged from it being accidental, to the result of ritual burning, though the main argument concluded that it was caused by hostile forces laying siege to these forts.

In the 1930s, and more recently in the 1980s, controlled experiments were carried out on replica walls to prove that burning would cause instability within the stonework and structural collapse. The theory had been given further weight due to evidence suggesting that the burning had occurred towards the end of occupation, which helped ensure that the argument for aggression remained more or less unchallenged, until now…

Trial by fire

Enter a trio of volcanologists, who have set out to prove that the fire damage could actually have been intentional. They think they’ve found significant flaws in the size and structure of walls used in the previous experiments, along with some of the methods employed in the wall construction.

By carrying out new controlled burning tests on samples of sandstone and mortar – designed to represent the blocks in a typical Iron Age fort wall – they found that whilst the blocks themselves were weakened, the mortar had been transformed into strong, dense glass substance.

Incredibly, this served to strengthen the overall structure of the wall, despite the damage caused to the stones.

An Iron Age Firestarter

But how did they manage to produce a fire strong enough to melt the stone? The presence of carbonised pieces of wood found within the vitrified substances of the hill-forts indicates that the walls had large pieces of timber inbuilt into the structure – a feature that Caesar had also described during his military campaigns in Gaul.

Whether they were placed there for the purpose of burning remains unknown, but during the controlled experiments in the 1930s, archaeologists found that their inclusion would have produced heat capable enough of melting the stone.

Defensive not offensive

Whilst further exploration and experimentation is needed on different types of material and structural composition, the volcanologists are confident that the overall structure of these forts were consequently strengthened, rather than weakened, by the process.

The resulting transformation of the mortar into a super strong glass substance is likely to have ensured that the occupants’ defences would have been more efficient at withstanding invaders, and implies that the action was deliberately employed as a defensive manoeuvre, rather than as an act of aggression by invading tribes.


As in all things with archaeology, the theory is an idea alone, and though the science can be proved, the motives will always be a speculation. In some cases, it’s entirely possible that the burning was due to hostility, and in those instances, just imagine that you were an invading tribe setting fire to your enemy’s fort, only to find your actions had served to strengthen it. Talk about frustrating…

H/T: Science Direct & National Geographic

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Written by Rosanna Ring

Rosanna is one of DigVentures' intrepid Community Archaeologists. She's busy turning Barrowed Time (our dig at Morecambe's Bronze Age burial mound) into a real seaside adventure, and getting the Pop-Up Museum on the Prom ready for you to visit!

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