Vitrified fort is the name given to certain crude stone enclosures whose walls have been subjected in a greater or less degree to the action of fire^[1]. They are generally situated on hills offering strong defensive positions. Their form seems to have been determined by the contour of the flat summits which they enclose. The walls vary in size, a few being upwards of 12 feet high, and are so broad that they present the appearance of embankments. Weak parts of the defence are strengthened by double or triple walls, and occasionally vast lines of ramparts, composed of large blocks of unhewn and unvitrified stones, envelop the vitrified centre at some distance from it.

No lime or cement has been found in any of these structures, all of them presenting the peculiarity of being more or less consolidated by the fusion of the rocks of which they are built. This fusion, which has been caused by the application of intense heat, is not equally complete in the various forts, or even in the walls of the same fort. In some cases the stones are only partially melted and calcined; in others their adjoining edges are fused so that they are firmly cemented together; in many instances pieces of rock are enveloped in a glassy enamel-like coating which binds them into a uniform whole; and at times, though rarely, the entire length of the wall presents one solid mass of vitreous substance.

It is not clear why the walls were subjected to vitrification. Some antiquarians have argued that it was done to strengthen the wall, but the heating actually weakens the structure. Battle damage is also unlikely to be the cause, as the walls must have been subjected to carefully maintained fires to ensure they were hot enough for vitrification to take place. Currently, the most popular suggestion is that the process was a symbolic demonstration of power, spectacularly removing the seats of a warrior aristocracy.

Since John Williams, one of the earliest of British geologists, and author of The Natural History of the Mineral Kingdom, first described these singular ruins in 1777, about fifty examples have been discovered in Scotland. The most remarkable are:

• Dun Mac Sniachan (Dun Mac Uisneachain or Dun Macsnoichan), the ancient Beregoiium, Benderloch, north of Oban, ;

Part of the vitrified wall at Saint Suzanne (Mayenne)

Fragment of vitrified wall at Saint Suzanne (Mayenne)

• Craig Phadraig, or Phadrick, near Inverness;
• Ord Hill, Kessock, near Inverness
• Dun Dhardhail (Dunjardil) in Glen Nevis;
• Knockfarrel (or Knockfarrail), near Strathpeffer, ;
• Dun Creich, in Sutherland, ;
• Finavon (or Finhaven), near Aberlemno ;
• Barryhill, in Perthshire;
• Laws, near Dundee, ;
• Dun Gall and Burnt Islands, in Buteshire;
• Anwoth, near Gatehouse of Fleet, ;
• Tap o' Noth, Aberdeenshire; and
• Cowdenknowes, in Berwickshire, .

Dun Mac Tjisneachain is the largest in area, being 250 yd. long by 50 yd. broad.

For a long time it was supposed that these forts were peculiar to Scotland; but they are found also in County Londonderry and County Cavan, in Ireland; in Upper Lusatia, Bohemia, Silesia, Saxony and Thuringia; in the provinces on the Rhine, especially in the neighbourhood of the Nahe; in the Ucker Lake, in Brandenburg, where the walls are formed of burnt and smelted bricks; in Hungary; and in several places in France, such as Châteauvieux, Péran, La Courbe, Sainte Suzanne, Puy de Gaudy and Thauron. They have not been found in England or Wales.

References

fr:Mur vitrifié