GeoLog

GeoLog

Great walls of fire – Vitrification and thermal engineering in the British Iron Age

It’s long been recognised the peoples of European prehistory occasionally, and quite deliberately, melted the rocks from which their hilltop enclosures were made. But why did they do it? In today’s blog post Fabian Wadsworth and Rebecca Hearne explore this question.

Burning questions

Throughout the European Bronze and Iron Ages (spanning 2600 years from 3200 BC to 600 BC), people constructed stone-built, hilltop enclosures. In some cases, these stone walls were burned at high temperatures sufficient to partially melt them. These once-molten forts are called vitrified forts because today they preserve large amounts of glassy rock. First described in full in 1777, the origins and functions of these enigmatic features have been the subjects of centuries of debate.

Today, researchers generally agree that the glassy wall rocks are the result of in situ exposure to high temperatures in prehistory, similar in magnitude to those temperatures found in volcanoes on Earth. This was sufficient to partially or wholly melt the stonework, and the resulting melts are preserved as glass upon cooling.

There are still many outstanding questions concerning vitrified enclosures and forts, but the most immediate and arresting are: how and why were they burned?

Vitrified fort walls are mostly found in Scotland and are built from a diverse range of rock types

Vitrified enclosures occur throughout Europe but the best known examples are found in Scotland. Using the compilation created by Sanderson and co-workers (link provided below) we can map the distribution of forts, categorized by the rock-type from which they were built, and compare this with a simplified geological map of Scotland (from the British Geological Survey; Image 1). This shows that the building stone used in fort walls is not always the same and was more likely to be found locally.

Map of Scotland with simplified basement geology and cover-sediments marked. Vitrified fort positions are numbered such that 1- Finavon, 2- Craig Marloch Wood, 3- Tap O’North, 4- Dun Deardail, 5- Dunagoil, 6- Craig Phaidrig, 7- Laws of Monifieth, 8- Knockfarrell, 9- Dunskeig, 10-Dumbarton Rock, 11- Carradale, 12-Dun MacUisnichan, 13- Art Dun, 14- Mullach, 15- Trudernish Point, 16-Cumbrae, 17- Dun Lagaidh, 18- Sheep Hill, 19-Urquhart Castle, 20- Eilan-nan-Gobhar, 21- Eilan nan Ghoil, 22- Duntroon, 23- Torr Duin, 24- Trusty’s Hill, 25- Doon of May, 26- Castle Finlay, 27- Mote of Mark. (From the British Geological Survey).

Map of Scotland with simplified basement geology and cover-sediments marked. Vitrified fort positions are numbered such that 1- Finavon, 2- Craig Marloch Wood, 3- Tap O’North, 4- Dun Deardail, 5- Dunagoil, 6- Craig Phaidrig, 7- Laws of Monifieth, 8- Knockfarrell, 9- Dunskeig, 10-Dumbarton Rock, 11- Carradale, 12-Dun MacUisnichan, 13- Art Dun, 14- Mullach, 15- Trudernish Point, 16-Cumbrae, 17- Dun Lagaidh, 18- Sheep Hill, 19-Urquhart Castle, 20- Eilan-nan-Gobhar, 21- Eilan nan Ghoil, 22- Duntroon, 23- Torr Duin, 24- Trusty’s Hill, 25- Doon of May, 26- Castle Finlay, 27- Mote of Mark. (From the British Geological Survey).

How hot? How long?

A key question surrounding the melting and glass-formation processes that occur to form vitrified fort walls is what temperature was required and how long must the fires have burned? As we know, the required minimum temperature for melting rocks is the solidus, which varies by hundreds of degrees from rock-type to rock-type. Above the solidus, the partial melt fraction will increase until the material liquidus, above which all components of the rock are molten. In mineralogically diverse rocks, the partial melt fraction between the solidus and liquidus not only increases, but changes composition. Pioneering investigations of vitrified fort wall materials (for example, by Youngblood and co-workers) and others have explored this by looking at the composition of the glass that is formed when the partially molten fort walls are cooled. The composition of these materials and an understanding of the thermodynamics of the melting process yields temperatures of the prehistoric fires in question. Youngblood and colleagues found that temperatures were likely to be, on average, 900-1150 ºC.

In a recently published study, we used a different technique in an attempt to answer the same question. Samples of fort walls from Wincobank vitrified hillfort, Sheffield, UK, were used in high-temperature tests to measure the melting process in situ. We measured the amount of each mineral phase as it decreased above the solidus. This technique allowed us to extract additional information that previous investigators have not been able to probe. As well as suggesting a temperature window within which melting occurs, we were able to find the timescale of burning required to achieve the degrees of partial melting seen within Wincobank’s vitrified enclosure wall. Wincobank was constructed from a local sandstone; we found that the quartz in this sandstone was steadily removed upon experimental heating, matching the final quartz content of the enclosure wall rocks at a temperature window of 1050-1250 ºC for burning events of more than 10 hours.

When taken together, such investigations of the conditions required to form glass in prehistoric enclosure walls can more reliably inform the debate about why the fires were set in the first place. If glass is consistently found around a fort’s circumference (as is often the case, particularly in Scotland), and its particular building stone type dictates a heating event requiring a duration of 10 hours or more at peak temperature, it seems unlikely to us that such walls were burned accidentally or during periods of conflict or events of warfare (see below). In that case, if the enclosure walls were burned deliberately by the occupants, the outstanding question remains: why?

Why set fire to a stone wall?

In any archaeological investigation, a key goal is to attempt to explore and extrapolate the beliefs, motives, and desires of people in antiquity from their material culture. For practitioners of any discipline this is no easy task, with subjective interpretation of evidence and difference of opinion often resulting in vibrant discussion!

Consequently, numerous possible prehistoric motives for burning a fort wall to the point of melting have been posited. First is the possibility that the fires were lit during enemy attack or some other act of violence (mentioned above). Second, there’s the possibility that the fires were a product of deconstruction of the fort walls at the end of occupancy. Third, the conflagration was part of a ritual or display of prestige. Finally, there is the possibility that the fires were set with the intention of strengthening the stonework during construction. Each of these explanations has received attention; however, the last option had, until recently, been dismissed by previous researchers as largely unlikely, once it was recognised that heating rocks in general weakens them by the proliferation of microcracks from thermal stresses.

We revisited the idea that fort walls could have been burned during the construction in order to strengthen them. In our latest work we explored this in a simple way by showing that while the blocks in a fort wall will get weaker during high temperature burning, the more fine-grained rubble interstitial to these blocks will get much stronger. This strengthening occurs simply because the fine grained materials between larger blocks can fuse together by sintering when they are partially molten. And indeed, it is so often reported that large blocks are surrounded by a glassy mass that is fused to them (we show this in Image 2 from Wincobank fort, Sheffield, U.K.). We pointed out that this is contrary to the conventional view that fort walls must be weakened by the fires.

A block from the Wincobank enclosure wall in Sheffield, UK. This piece shows the typical feature where fine grained glassy material is welded to the larger, less altered blocks. In detail, this demonstrates that thermal gradients resulting from heating blocks of different sizes play an important role in determining which blocks melt and weld, and which blocks do not. (Credit: Fabian Wadsworth)

A block from the Wincobank enclosure wall in Sheffield, UK. This piece shows the typical feature where fine grained glassy material is welded to the larger, less altered blocks. In detail, this demonstrates that thermal gradients resulting from heating blocks of different sizes play an important role in determining which blocks melt and weld, and which blocks do not. (Credit: Fabian Wadsworth)

The debate rages on

We acknowledge that the strengthening effect does not rule out other motives. Indeed, the strengthening may be incidental to the true motive for the wall burning. It is also important to take into account the fact that, in many of the known examples of vitrified enclosures, where dated, the burning event takes place, in some cases, many hundreds of years after the fort’s initial construction.

A little-discussed possibility which is gaining momentum is that some of these forts may not have been forts at all. The very term “fort” is loaded and implies inherent military purpose, which remains a hypothesis with little solid evidence to its claim. Rather, they may have been monuments that were built and burned as displays of power and prestige or in some ritual event. In periods of our history where only the stones upon which we can base our suppositions remain, it is difficult to differentiate between these possibilities.

These acknowledgements highlight not only that there are outstanding questions requiring future investigation, but that each fort is different and there need not be a common explanation for them all.

The debate continues and, although the evidence sheds new light on the possible truths, we still do not know why Iron Age peoples throughout Europe set fire to stone enclosures and stoked those fires to volcanic temperatures. Rocks melt and crystallize and re-melt in volcanoes frequently and as a matter of natural process. To combine our understanding of these rock-forming materials and Earth processes as they are melted in anthropogenic conflagrations is essential to understand these curiosities of our Iron Age.

By Fabian Wadsworth, PhD student Ludwig Maximilian University of Munich, and Rebecca Hearne, Department of Archaeology, University of Sheffield

The Wincobank hillfort is in Sheffield in South Yorkshire, U.K. If the stone walls of its  enclosure were deliberately burned   this feature potentially extends Sheffield’s heritage of high temperature expertise – exemplified by the once-prolific steel works of the city – much farther into the region’s past than hitherto imagined.

Further reading

Imaggeo on Mondays: Moving images – Photo Contest 2016

Since 2010, the European Geosciences Union (EGU) has been holding an annual photo competition and exhibit in association with its General Assembly and with Imaggeo – the EGU’s open access image repository.

In addition to the still photographs, imaggeo also accepts moving images – short videos – which are also a part of the annual photo contest. However, 20 or more images have to be submitted to the moving image competition for an award to be granted by the judges.

This year saw seven interesting, beautiful and informative moving images entered into the competition. Despite the entries not meeting the required number of submissions for the best moving image prize to be awarded, three were highly ranked by the photo contest judges. We showcase them in today’s imaggeo on Mondays post and hope they serves as inspiration to encourage you to take short clips for submission to the imaggeo database in the future!


Aerial footage of an explosion at Santiaguito volcano, Guatemala. Credit: Felix von Aulock (distributed via imaggeo.egu.eu)

During a flight over the Caliente dome of Santiaguito volcano to collect images for photogrammetry, this explosion happened. At this distance, you can clearly see the faults along which the explosion initiates, although the little unmanned aerial vehicle is shaken quite a bit by the blast.


Undulatus asperitus clouds over Disko Bay, West Greenland. Credit: Laurence Dyke(distributed via imaggeo.egu.eu)

Timelapse video of Undulatus asperitus clouds over Disko Bay, West Greenland. This rare formation appeared in mid-August at the tail end of a large storm system that brought strong winds and exceptional rainfall. The texture of the cloud base is caused by turbulence as the storm passed over the Greenland Ice Sheet. The status of Undulatus asperitus is currently being reviewed by the World Meteorological Organisation. If accepted, it will be the first new cloud type since 1951. Camera and settings: Sony PMW-EX1, interval recording mode, 1 fps, 1080p. Music: Tycho – A Walk.

Lahar front at Semeru volcano, Indonesia. Credit: Franck Lavigne (distributed via imaggeo.egu.eu)

Progression of the 19 January 2002 lahar front in the Curah Lengkong river, Semeru volcano, Indonesia. Channel is 25 m across. For further information, please contact me (franck.lavigne@univ-paris1.fr)

 

Livers, guts and gills: understanding how organisms become fossils

Livers, guts and gills: understanding how organisms become fossils

It’s 10am and Thomas Clements, a 3rd year palaeobiology PhD Student, is getting ready to check on his latest experiment. Full kited up in what can only be described as a space suit, Thomas carefully approaches the fume cupboard home to his latest specimen: a decaying seabass, balanced on a specially designed ‘hammock’ in a tank of salty water. Opening the lid to check on the rotting fish, Thomas is hit by the smell, so atrocious the fume hood and protective overalls only go some way toward shielding his sense of smell. It might be a bleak start to the day, but it is central to Thomas’ research.

While the fossil record is dominated by the hard mineralised parts of organisms such as shells, teeth and bones, in the past few years palaentologists have started to rely on different fossilised remains and techniques to discover more about extinct animals.

“In fact, soft-bodied fossils are much more informative about the anatomy, physiology, ecology and behaviour of ancient organisms. They also give scientists a much better idea as to the type of conditions, ecosystems and environments in which the organisms lived,” explains Thomas.

However, before the vital clues held in the remains of soft-bodied fossils can be accurately interpreted by researchers, the processes which cause them to be preserved in the first instance must be fully understood too. This is where Thomas’s smelly study of decaying fish carcases comes in. Using seabass, because its genealogy can be traced back in time (organisms related to the blue fish are known to exist in the ancient fossil record), Thomas aims to better understand how decay processes affect the fossilisation potential of soft-tissues – especially of internal anatomy.

Thomas injects the silica gel around the probes to make sure the incisions are sealed. You can see the fish in it’s hammock. (Credit: Thomas Clemens)

Thomas injects the silica gel around the probes to make sure the incisions are sealed. You can see the fish in it’s hammock. (Credit: Thomas Clemens)

Along with the Palaeobiology Lab Group at Leicester University, Thomas has devised a series of novel experiments to investigate the process. Because he is particularly interested in how internal organs decay, Thomas cuts millimetre sized incisions into the fish, sourced from a local fishmongers, to place probes into liver, guts, stomach and even kidneys. The chemical data measured by the probes allows him to unravel both the timing and sequence of anatomical decay of the different organs.

“One of the most important parts of the experiment is to accurately recreate the natural death of the animal,” describes Thomas.

That is why it is vitally important that the wounds caused by the incisions are fully sealed with inert silica gel so as not to speed up the decay process. This also means that before any experiment, he painstakingly practices cutting and sealing for hours. By the end of his practice runs he is intimately familiar with the exact location of each internal organ and able to perform only the smallest cuts required to insert his probes.

Recreating the conditions under which an ancient fish would decay is also important. Surgical incision complete, probes inserted ready to acquire data, the fish is gently placed on a hammock of inert plastic netting (again, so that no chemicals plastic may give off will interfere with the natural break down of the body parts) and lowered into an aquarium of salty water.

Thomas’ experiments are sustainable and environmentally friendly too! Rather than placing the hammock directly on the tank floor, it is suspended in the water, by way of plastic ropes attached to the corners of the aquarium. This means that as the fish is left to decompose over time, most fish parts sink towards the base of the tank an eventually dissolve in the water – making it extremely foul-smelling, as you might imagine! Once the experiment is complete, whatever fish parts may remain on the hammock can be simply discarded and the (washed) plastic used in a new experiment.

Thomas teaches visiting PhD student, Yujing Li, about the anatomy of a Seabass. (Credit: Thomas Clemens)

Thomas teaches visiting PhD student, Yujing Li, about the anatomy of a Seabass. (Credit: Thomas Clemens)

The experiments performed so far show that the decay process is actually very quick. After 60 days, the majority of the fish has fully decomposed, with only fins and very small tissue parts remaining. It takes no more than 20 days for muscle fibres to disappear and as little as five days for ultra-structures to break down. Through his work, Thomas now knows that the preservation of soft tissues during fossilisation has to happen very quickly or conditions have to be just right.

Thomas thinks that “slowing down the decay process is what gives soft-bodied parts a better chance of preservation.”

This is why during the experiments he has been testing how changes in the conditions, from lowering the water temperature, reducing agitation of the tank, changing salinity or even reducing bioturbation (the disturbance of sediment caused by sea floor dwelling critters),  affect how likely it is for tissues to be preserved.

Despite the advances and better understanding gained through the experiments, enigmatic questions still remain: why are some organs, such as guts, often found preserved in the fossil record, but why are others, such as eyes, so much rarer? And so, Thomas’ work in the lab, complete with rotting fish, surgical gloves, spacesuit-like protective equipment and stomach turning smells continues.

By Laura Roberts Artal, EGU Communications Officer

Thomas Clements presented his work at the 2016 EGU General Assembly, at a press conference entitled: How ancient organisms moved and fed: finding out more from fossils. The full press conference can be streamed here. In addition, the work was presented in session SSP4.2: Experimental solutions to deep time problems in palaeontology. Thomas’ abstract can be found here.

Share the work you presented at EGU 2016: upload your presentations for online publication

Share the work you presented at EGU 2016: upload your presentations for online publication

This year it is, once again, possible to upload your oral presentations, PICO presentations and posters from EGU 2016 for online publication alongside your abstract, giving all participants a chance to revisit your contribution  hurrah for open science!

Files can be in either PowerPoint or PDF format. Note that presentations will be distributed under the Creative Commons Attribution 3.0 Licence. Uploading your presentation is free of charge and is not followed by a review process. The upload form for your presentation, together with further information on the licence it will be distributed under, is available here. You will need to log in using your Copernicus Office User ID (using the ID of the Corresponding Author) to upload your presentation.

Presentations and posters will be linked to from their corresponding abstracts. If your presentation didn’t have an abstract (this is the case for Short Courses and others), but you still want to share it with the wider community you can consider uploading your presentation to slideshare or figshare as a PDF to share it instead.

All legal and technical information, as well as the upload form, is available until 19 June 2016 at: http://meetingorganizer.copernicus.org/egu2016/abstractpresentation

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