More than 70 examples of Vitrified Forts - defensive military structures which have been subjected to intense heat - have been found across Scotland.

More recently, further examples have been found in Ireland; Bohemia, Silesia, Saxony, Hungary; and in several places in France.

They date from the Iron Age to the Early Medieval periods.

The heat was so intense that the rocks which were used to build the forts has vitrified - i.e. partially melted and taken on a glassy appearance.

It's not known whether the forts were deliberately 'fired' in an attempt to strengthen them (though some researchers say this would not have worked), or whether they were burned after having been over-run. Or whether there was some as yet unexplained ritualistic reason for the firings.

Further information 1911 Encyclopædia Britannica, Volume 28