Vitrified Forts: The Hillforts With Walls Melted Into Glass

Vitrified forts are stone hillforts, mostly Iron Age, whose walls were partly melted into glass. Experiments since 1937 show large fires in timber-laced walls can do it; why people burned them is still debated.

A vitrified fort is a stone enclosure, usually on a hilltop, whose walls have been partly melted and fused together into a glassy mass. Scotland has the largest number, around seventy, including Tap o' Noth in Aberdeenshire and Craig Phadrig near Inverness, and others are known in France, Ireland, Germany, Sweden and Portugal. They were first described scientifically in 1777 by the Welsh engineer John Williams.

To melt stone this way takes temperatures of roughly 850 to 1100 degrees Celsius sustained for hours, which raised the question of how, and why, Iron Age people did it. Many of the walls were originally built of stone laced with timber beams, which could act as fuel. Not every fort with a timber-laced wall became vitrified, which suggests special conditions were needed.

In 1937 the archaeologist V. Gordon Childe and Wallace Thorneycroft built a test wall of stone and timber and set it on fire. It produced true vitrification, showing that a large, well-fed fire in a timber-laced wall could do the job. Debate continues on whether the burning was an attack by enemies, a ritual act of destruction at the end of a fort's life, or an attempt to strengthen the walls, which vitrification actually weakens.

Because they look as if they were blasted by extreme heat, vitrified forts appear in ancient-weapon theories, sometimes linked to the Indian epics. The experiments show no exotic energy is needed, but the effort and intent behind the burning still puzzle archaeologists. Some sites, such as Tap o' Noth, were also huge settlements, so their destruction may have been a major political event.