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Vitrified fort

Stone enclosure with vitrified walls

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Vitrified forts 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 (3.7 m) 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. The walls themselves are termed vitrified ramparts.

Vitrification process

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 or how 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 are thought to have been subjected to carefully maintained fires to ensure they were hot enough for vitrification to take place.

The expert consensus explains vitrified forts as the product of deliberate destruction either following the capture of the site by an enemy force or by the occupants at the end of its active life as an act of ritual closure. The process has no chronological significance and is found during both Iron Age and early medieval forts in Scotland.

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