Glass Castles: The Mystery of Scotland's Vitrified Forts

Across Scotland, there are a number of truly mysterious monuments, vitrified forts whose stone walls have been turned to glass by some unknown method. These structures date back to the Iron Age, around 2,000 years ago. If these forts were found in a more remote part of the world, say in the jungles of Central America or high on the Tibetan plateau, it's likely that their mysterious properties would be attributed to aliens or extraterrestrial technology. But these hill forts were built by locals, considered barbarians by invading Romans, so how could they have harnessed such a powerful force to turn stone walls into glass? I first became aware of Scotland's vitrified forts in 1980, thanks to the TV series Arthur C. Clarke's Mysterious World. The episode discussed the mystery behind these sites, showing their strange and enigmatic nature. How did men find the technology 2,500 years ago to melt rock and turn their hilltop fortresses into impregnable glass? This sparked my interest, and I made a point to visit as many of them as possible. There's a list of about 16 in Wikipedia. Located near Inverness, Ord Hill is the first site I visited. According to George Anderson, who wrote about the site in 1824, small fragments of vitrification were reported along the line of the wall. However, the site's never been fully investigated, and it's presumed to be of Pictish origin, dating back to the late Pictish Iron Age, around 1,700 years ago. While I couldn't find small fragments of vitrification during my visit, the walk through the pleasant woodland was worth it, offering a glimpse through the trees of the surrounding landscape. I did notice the local sandstone, which is different from the fine-grained masonry stone found elsewhere in the area that this rock is a form of conglomerate. It's a sandstone, a concrete-like mixture of gravel and stones, which could be mistaken for fused rock. Craig Ferrick, if I'm pronouncing that correctly, is overlooking the beautiful earth just above Inverness. It's been extensively investigated. It is said to have been the meeting place of Brut, king of the Picts, and St Columba in the 6th century. The fort's main occupation and vitrification of its walls date from between 150 BCE and 400 CE, with the oldest parts dating back to around 400 BCE. There's also evidence that the fort remained in use until the 7th century, on the cusp of the early medieval period. Investigations over the last 70 years have shown both the lack and abundance of vitrification at the site. In 1971, excavation revealed that both ramparts were made of a timber-laced stonework, which had been burnt, and contained large masses of vitrified material. The local rock here is also a conglomerate, and while I did find some small traces of vitrification, they were less pronounced than I had hoped. These forts are not the glass castles I was expecting. In 1980, an experiment was carried out by archaeologists to recreate the construction methods of vitrified walls, as featured in Mysterious World series. The experiment demonstrated the method of timber-laced stonework, but the results were less than successful. The team used waste wood, particularly pine, but oak more readily available at the time of the construction would have generated greater heat. Professional drystone wallers toiled for days to build the wall of rocks laced with timber. For the wall itself to catch fire, tons of loose timber must have been stacked against the face. This, as regards authenticity, I think is a very good attempt at one of these walls. The experiment helped to shed light on the method used, but didn't really replicate the vitrification seen in the forts. Since the 1980s, the awareness of vitrified forts has expanded beyond Scotland, with similar structures being found across Northern Europe. This has prompted further experiments, such as a study in Sweden in 1996, where researchers attempted to melt rock near a vitrified fort dating from between the 3rd and 8th centuries. When charcoal, granite, gneiss, and amphipolite were heated in an open furnace with forced air, they did not melt. However, when insulated with turf, melting readily occurred with a forced draught. This experiment suggests that additional techniques, such as insulation and forced air, could have led to more vitrification. Some of the samples are here, little pieces of rock. We've no great chunks of vitrified material, but here, for example, one with a bit of granite where a lot of material is soldered onto it in this position, and this other one is one which has gone really glassy on the surface, the vitrified effects. You see perhaps when we move it around in the light that it catches it on this face. A good example of a vitrified fort is here at Knock Farrell near Dingwall. The site dates to the late Iron Age and is probably Pictish in origin. It's notable for being the site of the earliest known archaeological excavation in Scotland that occurred in the 18th century. At Knock Farrell, large sections of the walls are fused together, showing evidence of a timber formwork, which was likely made from oak given the fertility of the area. The formwork retained the irregular rocks as local drystone walls demonstrate. In my opinion, a lot of timber would have been used up here, similar to Roman and medieval timber stockades. The fine stonework and clever designs of coastal Iron Age rocks, which share architectural features with these inland forts, suggest skilled builders, and it's likely that inland fortresses were equally grand with timber ramparts and elaborate designs. Oak would have lasted for years, especially with airspace to dry out in strong winds, while turf could have been used to seal the elements out. Most medieval castles began as timber constructions with earthen ramparts, which were later carved in lime mortar for fire and water resistance. There is evidence that Roman forts used similar methods for their ramparts, including whitewashing to emphasise the status of the occupant. Given most of the vitrified forts are located in the northeast of Scotland, it's likely that they followed similar architectural fashions with each other. So, was vitrification deliberate? Vitrification weakened stonework, which suggests the process was likely deliberate, possibly as a means of destruction following an attack or disaster. These forts were built to withstand attack, so it stands to reason that for a victory to destroy them quickly, fire would have been the most effective method, rather than direct demolition. In my hypothesis, a small section of the wall could have been destroyed with the prevailing wind behind it, creating an airway to fuel the fire, and a small section on the opposite side could have been destroyed to create a kind of chimney effect, drawing the fire through the wall. The plastered walls and turf roof would have insulated fire, aiding in the vitrification process, and that is perhaps the mainstream opinion in that destruction was deliberate and vitrification was a by-product. Our final stop brings us here to Chapman North, one of Scotland's most impressive ancient settlements. It was featured in Arthur C. Clark's Mysterious Worlds. The Fort at Tapernoth is notable for its distinctive vitrified walls, set atop a steam hill. The site commands a stunning view of Aberdeenshire, and I can easily imagine how a Celtic chieftain would have chosen this strategic spot for their citadel. Recent excavations suggest that Tapernoth dates back to the big period around the 3rd century AD. Archaeologists from the University of Aberdeen have uncovered evidence that up to 4,000 people may have lived here, with over 800 huts dotting the hill. The size and complexity of the settlement indicates it could have rivaled the largest post-Roman settlements in Europe. The sheer scale of the site suggests that its fortifications were deliberately constructed, rather than being the result of a lost war. The most striking feature of Tapernoth is the vitrified walls. It is often argued the fused stone is weaker, a modern misconception based on our current understanding of materials, but here in reality the fused rock at Tapernoth has proven to be more enduring than the other walls, which have collapsed over time. Given the methods of attack used during this period, it seems that vitrified walls might have been more resilient to siege tactics than rubber walls within a timber frame, which would have been easier to destroy. The process of vitrification itself requires very specific conditions to be met, particularly in controlled and precise application of heat. While accidental fires could have led to some vitrification, the consistency of the fused walls of Tapernoth suggests a deliberate effort to achieve this result. The difficulty in achieving vitrification further suggests that it wasn't a haphazard event, but a planned technique, requiring careful preparation and the use of fuel, and possibly oak or even charcoal derived from oak woods, which would have been abundant in this region. The local rock at Tapernoth is granite, which isn't typically used for elegant stonework. This raises an intriguing question. Was the vitrification process employed intentionally for architectural purposes? The care with which the walls were fused indicates that this technique may not have been a product of destruction, but rather a deliberate construction method. The vitrification could have been used to display both architectural refinement and the power of the community. So an interesting thing is, for ice stone walling, you'd use a lot bigger pebbles than this. They'd be going right up to large rocks, whereas this is infill with a few brick-sized rocks. Picks may have used vitrification walls to demonstrate authority and sophistication, possibly emulating Roman stonework they may have come across, for both strength and aesthetic appeal. The fascination with well-crafted buildings is universal. Scar braids, neat stonework, dates back over 4,000 years. The brocks of Orkney, which are contemporary with these Pictish forts, reflect a lasting desire for visual appeal and craftsmanship, although Orkney and Caithness had the advantage of fine-grained sandstones. The picks' vitrified walls, like their stone carvings, likely serve both practical and symbolic purposes. Just as ancient Egyptians or Romans used architecture to symbolise power, the picks' vitrified walls may have reinforced their status within the region. The construction of vitrified forts raises questions about modern-age techniques. While often attributed to accidental fires, evidence from sites like Tappanoth suggests deliberate use of heat to enhance fortifications, but also requiring architects and designers. And we know that that was incorporated by the brock builders. These were more than defensive structures. These forts were statements of power and cultural refinement. As research continues, these picks' walls stand as a symbol of identity, endurance and mastery, and a bit of mystery. So, thanks for watching. I hope you enjoyed this. Do like, comment and subscribe.