Great walls of fire – Vitrification and thermal engineering in the British Iron Age
Burning questions
Throughout the European Bronze and Iron Ages (spanning 2600 years from 3200 BC to 600 BC), people constructed stone-built, hilltop enclosures. In some cases, these stone walls were burned at high temperatures sufficient to partially melt them. These once-molten forts are called
vitrified forts because today they preserve
large amounts of glassy rock. First described in full in 1777, the origins and functions of these enigmatic features have been the subjects of centuries of debate.
Today, researchers generally agree that the glassy wall rocks are the result of
in situ exposure to high temperatures in prehistory,
similar in magnitude to those temperatures found in volcanoes on Earth. This was sufficient to partially or wholly melt the stonework, and the resulting melts are preserved as glass upon cooling.
There are still
many outstanding questions concerning vitrified enclosures and forts, but the most immediate and arresting are:
how and
why were they burned?
Vitrified fort walls are mostly found in Scotland and are built from a diverse range of rock types
Vitrified enclosures occur throughout Europe but the best known examples are found in Scotland. Using the compilation created by Sanderson and co-workers (link provided below) we can map the distribution of forts, categorized by the rock-type from which they were built, and compare this with a simplified geological map of Scotland (from the British Geological Survey; Image 1). This shows that the building stone used in fort walls is not always the same and was more likely to be found locally.
Map of Scotland with simplified basement geology and cover-sediments marked. Vitrified fort positions are numbered such that 1- Finavon, 2- Craig Marloch Wood, 3- Tap O’North, 4- Dun Deardail, 5- Dunagoil, 6- Craig Phaidrig, 7- Laws of Monifieth, 8- Knockfarrell, 9- Dunskeig, 10-Dumbarton Rock, 11- Carradale, 12-Dun MacUisnichan, 13- Art Dun, 14- Mullach, 15- Trudernish Point, 16-Cumbrae, 17- Dun Lagaidh, 18- Sheep Hill, 19-Urquhart Castle, 20- Eilan-nan-Gobhar, 21- Eilan nan Ghoil, 22- Duntroon, 23- Torr Duin, 24- Trusty’s Hill, 25- Doon of May, 26- Castle Finlay, 27- Mote of Mark. (From the British Geological Survey).