One of the first British geologists was John Williams, author of Natural History of the Mineral Kingdom. It was he who first described some strange ruins in 1777, of which more than a hundred examples have subsequently been found throughout Europe, mainly in Scotland. He called them vitrified fortresses , because its appearance is that of mounds surrounded by a compact stone wall. But with the peculiarity that these stones are cast.
All of them are located on hills that look like defensive positions and their shape is determined by the outline of the flat summits they enclose. Its walls vary in size, some are 3.7 meters high and in some parts they seem to be fortified with double or triple walls. In other parts there are large canvases of walls with blocks of uncut and unvitrified stone, which surround the vetrified center at a certain distance. The whole complex gives the impression of being an embankment.
Archaeologists have not found neither lime nor cement in none of these structures, being all consolidated and compacted by the fusion of the rocks that form them, which indicates that they had to be subjected, to a greater or lesser degree, to the action of heat.
These structures have amazed geologists for centuries, because there is no scientific explanation for how the rocks came together. The temperatures to which they had to be subjected for vitrification to occur are comparable to the detonation of an atomic bomb . And we are not talking about one or two, but hundreds of examples spread throughout Europe, as we said with 70 in Scotland alone. This is where they were discovered, and for a long time they were thought to be unique to there. However, examples have appeared in places such as Bohemia, Silesia, Thuringia, the Rhine provinces, Hungary, Turkey, Iran, Portugal, France and Sweden, among others.
In Scotland the most famous are at Dun Mac Sniachan, Benderloch, Craig Phadraig, Ord Hill, Dun Deardail, Knock Farril, Dun Creich, Finavon, Barryhill, Laws, Dun Gall, Anwoth, Tap o'North, Dunnider and Cowdenknowes. 
However, the vitrification is not complete in all the fortresses , nor homogeneous in the walls of the same site. In some cases the stones appear partially calcined and fused, while in others they are covered by a layer of vitreous enamel, and sometimes, although rarely, the entire length of the wall presents a solid mass of vitreous substance.
No one knows how these walls came to be vitrified. Some scholars believe it was intentional , to strengthen the defenses, but in reality this would have weakened them. Nor is it likely that it is due to war damage, the result of a siege, because to reach vitrification it would be necessary to have patiently maintained the fires for days at a temperature between 1050 and 1235 degrees Celsius , something certainly complicated, although not impossible.
Most archaeologists consider them to be the product of deliberate destruction by the attackers after they captured the position, or by their own occupants as a ritual closing act.
The dating of the fortresses covers a wide range of dates. The oldest date from the Iron Age , but there are also some from Roman times , and the last ones correspond to the Middle Ages .
The latest studies suggest that they were created by massive plasma events like solar flares. These occur when ionized gas in the atmosphere takes the form of giant electrical bursts, which can melt and vitrify rocks.
In the 1930s, the famous Australian archaeologist Vere Gordon Childe and Wallace Thorneycroft carried out an experiment with a gigantic bonfire on a stone wall, an experiment that was repeated in 1980 by archaeologist Ian Ralston. In both cases the experiments produced partial vitrification of some of the stones, but failed to explain how this could have occurred on such a large scale as in the vitrified fortresses .
In the absence of a definitive theory or conclusive evidence, the vitrified fortresses remain one of the strangest geological and archaeological anomalies in the world.