29 November 2012

Received some interesting feedback on the summary report, concerning vitrified forts. So I thought I would outline why we think that vitrification took place at the end of the fort’s occupation and not at the time of its construction.

Vitrified forts are found mainly in Scotland and the Massif Central of France. Archaeological excavations have demonstrated that vitrified ramparts are timber-laced stone faced ramparts that have been set alight and burnt, producing such a heat that the rubble core, not the outer stone faces of the rampart, has melted or vitrified (obviously purely timber ramparts would simply be reduced to ashes and purely stone ramparts would not burn).

As Julius Caesar remarked when he encountered such timber-laced stone ramparts during his conquest of Gaul, such defences are very hard to destroy because the timber lacing makes the walls impervious to battering (unlike a stone wall) while the stone face of the ramparts makes them very difficult to set alight.

However, clearly it was possible to set them alight.Two experiments have been undertaken (one by Prof Vere Gordon Childe in the 1930s and another by Prof Ian Ralston in the 1980s – both Professors of Archaeology at the University of Edinburgh), which both demonstrated that it is possible to set alight timber-laced stone ramparts and so vitrify the rubble core. However, both these experiments showed that a lot of timber fuel and a lot of time was needed simply to produce a little vitrified stone. The key to vitrification occurring is after the stonework of the rampart has collapsed after the initial burning, sealing the inner timber beams, and thus reducing them to charcoal; with charcoal much higher temperatures can be attained and the temperature of the inner core of the collapsed rampart rises to above 1000 degrees celsius, sufficient to vitrify the rubble core closest to the charcoal beams.

So, given that timber-laced stone ramparts are very strong (and, given the amounts of stone, timber and nails as well as labour needed to construct them, very expensive to build), it is unlikely that they were so constructed just to be set alight to form one large mass of fused stone. Indeed excavations have shown that in several such sites only part of the ramparts have been vitrified and that during the process of burning the wall has collapsed outwards and inwards, leaving only the vitrified rubble core, often to not a great height or even in a continuous line around the fort.

The process is clearly destructive, not a form of construction.

However, given the amount of time and fuel needed to properly vitrify the rubble core of the ramparts, it is highly unlikely that this was accidental. Furthermore, given that the vitrification is often most evident nearer the inside edge of the rampart rather than the outer edge, it is unlikely that such destruction was effected during an attack on a fort.

Instead is it much more credible that such destruction was wrought after a fort had been captured. Such destruction did not simply destroy the ramparts and so prevent an enemy using the fort after the victors had departed, but was a very visible, indeed spectacular, form of destruction apparent to everyone in the surrounding countryside; as much a political statement as a pragmatic action.

The excavation at Trusty’s Hill corroborates this. We found evidence of timber lacing in the form of large upright post-holes in the core of the rampart. These were ringed with vitrified stone, which was concentrated in these areas, but not a continuous mass, within the rubble core of the rampart. Most of the vitrified stone was near the interior side of the rampart not the exterior side. Indeed the entire outer stone face of the rampart had been ripped away before the core of the rampart was set alight – the large stones on the exterior side were entirely unaffected by heat. Likewise for the inner stone face. The thick charcoal rich layer presumably derived from the burning process was only apparent on the interior side of the rampart. We suspect that the stone faces were deliberately removed to better allow draughts of air to aid the burning. The top part of the rampart had collapsed outwards and inwards and was composed of predominantly heat reddened but not vitrified stone, though there were small nuggets of vitrified stone amongst this. This collapse had sealed the vitrified rubble core, what had originally been the base of the core of the rampart. Furthermore, given that vitrified stone was apparent at both the east and west sides of the summit of the hill, and had collapsed into the rock-cut basin on the south side, and was also apparent on the north side, it appears that the entirety of the summit rampart at Trusty’s Hill was set alight and subjected to prolonged burning. There was no evidence of occupation of the summit of Trusty’s Hill after this destruction of the ramparts.

We can surmise that the destruction of Trusty’s Hill, at some point probably in the first half of the seventh century AD, was for the people in the surrounding countryside, a spectacular and ominous sight on the horizon for several weeks.

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