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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

Geo Talk: One of the youngest EGU 2016 General Assembly delegates sends sensor to space

Geo Talk: One of the youngest EGU 2016 General Assembly delegates sends sensor to space

Presenting at an international conference is daunting, even for the most seasoned of scientists; not so for Thomas Maier (a second year university student) who took his research (co-authored by  Lukas Kamm, a high-school student) to the EGU 2016 General Assembly! Not only was their work on developing a moisture sensor impressive, so was Thomas’ enthusiasm and confidence when presenting his research. Hazel Gibson and Kai Boggild, EGU Press Assistants at the conference, caught up with the budding researcher to learn more about the pair’s work. Scroll down to the end of this post for a full video interview with Thomas. 

Thomas Maier might seem like your average bright and enthusiastic EGU delegate, but together with his co-author Lukas Kamm, he has invented a water sensor that very well might help change the way astronauts live in space. Not only is their invention helping to revolutionise aerospace, but they are also the youngest delegates at the conference, Thomas is a second year university student at Friedrich-Alexander Universität Erlangen-Nürnberg and Lukas is attending high school at Werner-von-Siemens Gymnasium. We caught up with Thomas to speak with him about his invention.

Could you explain to us what led you to develop this water sensor?

We started this project four years ago for a contest called Jugend Forscht, a German youth sciences competition in Germany and the project we came up with was about giving plants demand driven watering. After we built our first sensor, we continued our work until it was possible to send the sensor into space, for a project called EU:CROPIS.

Can you tell us how your sensor works?

The sensor is based on a capacitive measuring method. So, you have two electrodes close to each other, which have an electrical capacitance (or ability to store an electrical charge) between them. The change in water content close to the electrodes changes the capacity of the sensor. Then we measure the capacity of the electrodes by measuring the time constant of the capacitor over time.

The greenhouse which forms part of the EU:CROPIS project. The greenhouse is home to Thomas and Lukas' water sensor. (Credit: Kai Boggild/EGU)

The greenhouse which forms part of the EU:CROPIS project. The greenhouse is home to Thomas and Lukas’ water sensor. (Credit: Kai Boggild/EGU)

Can you tell us more about the EU:CROPIS project?

The EU:CROPIS is mainly about this here [indicates greenhouse model], and this is a greenhouse which will go into space, July next year. The greenhouse will rotate and will generate different gravitational forces that may impact the amount of water available to plants which will be grown in here. And now, after a lot of work, our sensor will be placed on the very right [hand side] of the greenhouse and will measure the soil moisture for the plants.

What are you plans for this project into the future?

Our plans for the future are in taking part in the EDEN-ISS project, this is a project on the International Space Station, that is looking into planting 20 square meters of plants in the ISS and our sensor would be used too. So that is the next aim of this project.

Thanks Thomas for showing us your invention, and good luck to Lukas, who couldn’t attend the conference this year as he is busy with his high-school exams!

Interview by Hazel Gibson, video interview by Kai Boggild, EGU Press Assistants

 

The final days of the mountain glaciers

The final days of the mountain glaciers

In 1896 British lawyer, mountaineer and author Douglas Freshfield climbed an obscure mountain in the Caucasus called Kasbek and in his book detailing his adventures he described the mountain:

“From this point the view of Kasbek is superb: its whole north-eastern face is a sheet of snow and ice, broken by the steepness of the slope into magnificent towers, and seamed by enormous blue chasms.”

D Freshfield (1986) The Exploration of the Caucasus, page 93

A photo of the mountain taken at the same time highlights the Gergeti glacier (called at the time the Ortsveri glacier) running down the centre of the image. In 2015 Levan Tielidze took another photo of the same view which highlighted a shocking change.

The photo above echoes photos taken of mountain glaciers from across the globe over the last 100 years. All these photos, when compared with their older counterparts, show the comprehensive retreat of mountain glaciers in every country. The retreat of mountain glaciers was called a ‘canary in the coal mine’ along with other indicators of global climate change. But new data presented this month at the European Geosciences Union General Assembly, shows us that that canary is now past saving.

Ben Marzeion, professor of Geography and Climate Science at the University of Bremen, has found that mountain glacier retreat (and eventual disappearance) is now inevitable. In a session relating to the Paris agreements (where 195 countries from around the globe agreed to work towards limiting global temperature change to two degrees) Marzeion presented evidence that indicates that even if that ambitious target were achieved, 60% of current mountain glaciers will still melt away. In fact the impacts of climate change are more severe than even this number suggests. Prof Marzeion explains:

‘Even if climate warming were to stop today – which is physically impossible – about one third of the glacial ice in the world would still melt in the long term.’

Is this a farewell to the mountain glacier? (meeting of the penitents credit Simon Gascoin)

Is this a farewell to the mountain glacier? Credit: Simon Gascoin (distributed via imaggeo.egu.eu)

This is because the ice in the mountain glaciers responds to climate change with a time delay. Mountain ice is not sustainable and more than half of the ice in a mountain glacier is responding to temperature change that has already happened. The ice of the mountain glaciers has been melting for decades and once that process begins, it is very difficult to stop. Smaller glaciers and those at lower altitudes are at greater risk of complete loss, but Professor Marzeion says that any glacier where the summer snowline rises above the mountain peak will not survive. This has wider implications for water use in mountainous areas.

‘Saving the glaciers is an illusion in many mountain ranges,’ says Marzeion. ‘We will have to adapt to the consequences of glacier melt. This will affect the coastal regions of the world, but also populations in the mountainous regions, who will have one fewer source of water at their disposal in summer.’

Although the question of global temperature change is still one that we have to solve, it seems that our desire to take responsibility for our actions comes too late for those ‘magnificent towers’ and ‘blue chasms’ that Douglas Freshfield described over 100 years ago.

By Hazel Gibson, EGU General Assembly Press Assistant and Plymouth University PhD student.

Hazel is a science communicator and PhD student researching the public understanding of the geological subsurface at Plymouth University using a blend of cognitive psychology and geology, and was one of our Press Assistants during the week of the 2016 General Assembly.

GeoTalk: A smart way to map earthquake impact

GeoTalk: A smart way to map earthquake impact

Last week at the 2016 General Assembly Sara, one of the EGU’s press assistants, had the opportunity to speak to Koen Van Noten about his research into how crowdsourcing can be used to find out more about where earthquakes have the biggest impact at the surface.

Firstly, can you tell me a little about yourself?

I did a PhD in structural geology at KULeuven and, after I finished, I started to work at the Royal Observatory of Belgium. What I do now is try to understand when people feel an earthquake, why they can feel it, how far away from the source they can feel it, if local geology affects the way people feel it and what the dynamics behind it all are.

How do you gather this information?

People can go online and fill in a ‘Did You Feel It?’ questionnaire about their experience. In the US it’s well organised because the USGS manages this system in whole of the US. In Europe we have so many institutions, so many countries, so many languages that almost every nation has its own questionnaire and sometimes there are many inquiries in only one country. This is good locally because information about a local earthquake is provided in the language of that country, but if you have a larger one that crosses all the borders of different countries then you have a problem. Earthquakes don’t stop at political borders; you have to somehow merge all the enquiries. That’s what I’m trying to do now.

European institutes that provide an online "Did You Feel the Earthquake?" inquiry. (Credit: Koen Van Noten)

European institutes that provide an online “Did You Feel the Earthquake?” inquiry. (Credit: Koen Van Noten)

There are lots of these databases around the world, how do you combine them to create something meaningful?

You first have to ask the different institutions if you can use their datasets, that’s crucial – am I allowed to work on it? And then find a method to merge all this information so that you can do science with it.

You have institutions that capture global data and also local networks. They have slightly different questions but the science behind them is very similar. The questions are quite specific, for instance “were you in a moving vehicle?” If you answer yes then of course the intensity of the earthquake has to be larger than one felt by somebody who was just standing outside doing nothing and barely felt the earthquake. You can work out that the first guy was really close to the epicentre and the other guy was probably very far, or that the earthquake wasn’t very big.

Usually intensities are shown in community maps. To merge all answers of all institutes, I avoid the inhomogeneous community maps. Instead I use 100 km2 grid cell maps and assign an intensity to every grid cell.. This makes the felt effect easy to read and allows you to plot data without giving away personal details of any people that responded. Institutes do not always provide a detailed location, but in a grid cell the precise location doesn’t matter. It’s a solution to the problem of merging databases within Europe and also globally.

Underlying geology can have a huge impact on how an earthquake is felt.  Credit: Koen Van Noten.

Underlying geology can have a huge impact on how an earthquake is felt. 2011 Goch ML 4.3 earthquake.  Credit: Koen Van Noten.

What information can you gain from using these devices?

If you make this graph for a few earthquakes, you can map the decay in shaking intensity in a certain region. I’m trying to understand how the local geology affects these kinds of maps. Somebody living on thick pile of sands, several kilometres above the hypocentre, won’t feel it because the sands will attenuate the earthquake. They are safe from it. However, if they’re directly on the bedrock, but further from the epicentre, they may still feel it because the seismic waves propagate fast through bedrock and aren’t attenuated.

What’s more, you can compare recent earthquakes with ones that happened 200 years ago at the same place. Historical seismologists map earthquake effects that happened years ago from a time when no instrumentation existed, purely based on old personal reports and journal papers. Of course the amount of data points isn’t as dense as now, but even that works.

Can questionnaires be used as a substitute for more advanced methods in areas that are poorly monitored?

Every person is a seismometer. In poorly instrumented regions it’s the perfect way to map an earthquake. The only thing it depends on is population density. For Europe it’s fine, you have a lot of cities, but you can have problems in places that aren’t so densely populated.

Can you use your method to disseminate information as well as gather it, say for education?

The more answers you get, the better the map will be. Intensity maps are easier to understand by communities and the media because they show the distribution of how people felt it, rather than a seismogram, which can be difficult to interpret.

What advice would you give to another researcher wanting to use crowd-sourced information in their research?

First get the word out. Because it’s crowd-sourced, they need to be warned that it does exist. Test your system before you go online, make sure you know what’s out there first and collaborate. Collaborating across borders is the most important thing to do.

Interview by Sara Mynott, EGU Press Assistant and PhD student at Plymouth University.

Koen presented his work at the EGU General Assembly in Vienna. Find out more about it here.