Harriet Tatton, Programme Coordinator, DigVentures looks at the community focused geophysical survey of Dunoon Castle in Scotland.
In summer 2023, a grant from the Castle Studies Trust facilitated an exciting collaborative venture to reveal some of the hidden mysteries of Dunoon Castle, Cowal. DigVentures, in partnership with Argyll and Bute Council, the Dunoon Area Alliance, and Castle House Museum, designed a community-orientated endeavour to learn more about the site of Dunoon Castle and raise awareness of the heritage of the Castle House Park and Gardens.
The medieval stronghold, a scheduled monument, has long been shrouded in the enigma of its past. Thanks to the support of the Castle Studies Trust, a comprehensive geophysical survey and community activity program were initiated, providing a fascinating glimpse into the castle’s rich history.
Professional archaeologists, building historians and amateur enthusiasts from the local community came together to conduct a non-invasive survey that aimed to unearth the secrets of Dunoon Castle and its surrounding parkland. The results not only contribute to the ongoing heritage management of the site but also enhance the narrative that can be shared by local heritage organisations such as the Castle House Museum. In total, just under 100 people were involved in the project, which included training workshops, a public talk and tours, and activities for children and young families.
Dunoon Castle has stood witness to centuries of history. Documentary evidence traces its roots back to the early 13th century AD, and today, it stands as a modified natural mound with a curtain wall, hinting at its medieval past. There is debate over the castle’s original purpose, with suggestions ranging from a motte-and-bailey castle to a ‘castle of enclosure.’ Recent interpretations lean toward the latter, aligning with a group of castles built during the 12th and 13th centuries by successive Scottish kings.
The castle’s complex history is evident in distinct phases. In the medieval and post-medieval era, Dunoon Castle played a pivotal role in Scottish history. From its Lamont origins, through conflicts and regencies, a visit from Mary Queen of Scots, to its eventual abandonment around AD1650, the castle witnessed tumultuous times. The 19th century marked a transformative phase as Dunoon Castle became a focal point for James Ewing’s architectural ambitions. His castellated mansion, set against landscaped gardens, turned Dunoon into a sought-after holiday destination.
As the 20th century unfolded, Dunoon Castle assumed new roles during both World Wars, serving as a strategic defence point with searchlights and coastal batteries. The aftermath of war saw the castle’s hill repurposed for military activities during the Cold War, reflecting the evolving strategic importance of the site.
During the weekend of activities, DigVentures’ small team of archaeologists and local people aimed to make sense of this historical jigsaw puzzle. The magnetometer survey, conducted in areas north and south of the tennis courts and south of the castle museum, uncovered intriguing details. Possible garden wall foundations around the museum, features related to landscaping of the castle gardens, and remnants of former garden plots and a fire station emerged from the data, making sense of the castle’s historical evolution.
Simultaneously, the earth resistance survey corroborated these findings across the same areas, providing a comprehensive dataset for analysis. The revelation of garden features, potential gateposts, and additional elements related to the castle’s history further enriched the understanding of Dunoon Castle’s past.
Over the weekend, Dr. Louise Turner spearheaded a rapid historic buildings assessment of the castle remains, and although not an exhaustive survey, the assessment hinted at the Victorian origins of much of the visible walling within the castle ruins. Surprisingly, limited evidence was found for the earlier phases of the castle and raises intriguing questions about its earlier configurations; documentary evidence suggests that a castle has been present at the site since at least the 13th century, but very little of this phase appeared to be visible.
Complementing these efforts, historic building and aerial 3D photogrammetric drone surveys were conducted. These surveys not only highlighted obscured walls and earthworks but also uncovered hidden features concealed by vegetation overgrowth. A resulting topographic survey, compared against historical maps and surveys, revealed that a substantial portion of the castle walls, previously recorded, is now obscured or even considered submerged beneath the current ground level.
The findings underscore the importance of ongoing conservation efforts, as vegetation threatens to permeate the fabric of the surviving castle walls, potentially causing damage, in particular to the earlier, less visible phases of the castle’s structure. This collaborative project aligns with best practices outlined by Historic England, the Chartered Institute for Archaeologists, and the Europae Archaeologiae Consilium, ensuring the responsible and insightful exploration of Dunoon Castle’s captivating history.
As the dust settles on this archaeological venture, Dunoon Castle stands as a testament to the power of collaboration, community engagement, and cutting-edge survey techniques in unravelling the mysteries of the town’s history. DigVentures hopes to do more work at Dunoon Castle in the near future.
Steven Spencer and Elena Faraoni of the Hoghton Tower Preservation Trust, look at the results of their work, funded by the Castle Studies Trust in 2019, in trying to find out more about Hoghton Tower in Lancashire.
Hoghton Tower sits 650 ft above sea-level in the heart of the Lancashire countryside. The stories of its visitors and family members are documented and shared whether it be in books, portraits, family albums or documents in the Lancashire archives. But there is one story which has always intrigued us and that is: what was the first tower of Hoghton Tower and where was it? It is clear when looking at the building today that this, like many other historic houses, is a ‘patchwork’ of different projects by different generations interlaced and blended…but where did Hoghton Tower start? Where was the original tower?
There are some clues: the ageing of the stone, the position of the well house, family stories passed down the generations, the shape of the windows and a mysterious mound of stones on the north side of the buildings. One of these stones has an intriguing mason’s mark… Spurred on by the interest of a group of our amazing volunteers who had just finished some research into historic graffiti and masons’ marks it was time to do some investigation under the guidance of Dr Mike Nevell and his team at Salford University. We designed a research project based on archaeological digs, building recording, geophysics and archives research based on the key exam question “where was the great keep of the Hoghton Tower hill?”
North side of Hoghton Tower (copyright Hoghton Preservation Trust)
Thanks to the grant from the Castle Studies Trust, work quickly got underway. Through a series of Salford-led workshops, the team surveyed, recorded and reviewed old photographs and pictures.
Then there was the wonderful five-day archaeological dig.
As they passed through the perimeter fencing on to the dig site, the usually mild-mannered volunteers underwent personality transformations as pairs of friends and even married couples were ‘pitted’ against one another. Was this the site of a 14th century Pele Tower, a 1643 victim of the First Civil War, who would make the crucial find?
Under the patient guidance and control of the Salford team, the test pits were marked out and the excavations began, to many this was the chance of a lifetime and was eagerly embraced. Each find was announced with enthusiastic shouting from the discoverer and muted derision from those yet to make a meaningful contribution.
Hoghton Tower Gatehouse from inside the courtyard (copyright Hoghton Tower Preservation Trust)
As the week progressed, 14th to 19th century finds were unearthed, thankfully shared out between the eight test pits. Clay pipe bowls (1640 to 1680), a musket ball, heat affected glass, sherds of medieval pottery and fragments of medieval roof tiles. Below a stone rubble layer, evidence of a stone-built structure was revealed in the form of large dressed stone blocks, together with walls and a stone flagged floor.
Spurred on by the whole experience, and encouraged by the de Hoghton family, the volunteers have produced and presented an ‘Outdoor History’ tour which aims to share the latest thoughts and discoveries.
Was this the site of the Hoghton Tower? Did we find anything categorical? Well yes and no. The archaeology revealed previously unrecorded stone structures. These together with the artefactual evidence were able to confirm that this part of the hilltop was occupied during the late medieval/early post-medieval periods. The geophysics also gave us other areas that warrant more digging and researching. So, some confirmation but also a lot more to understand and discover on this windswept hill!
Hello! A personal introduction before we get down to the geophysics. I’m Kayt Armstrong, and I am an advisor to the Castle Studies Trust as a specialist in the use of geophysics in archaeology. I am also a member of the board of the International Society for Archaeological Prospection, and I represent the UK on a European research network about soils and geophysics in archaeology. I obtained my PhD in Archaeological Geophysics from Bournemouth University in 2010. I have worked in the UK and Europe (Greece and Italy) since that time, conducting archaeological geophysics in a variety of research and developer-led contexts. I help the CST evaluate funding applications that have geophysical elements, and also comment on the reports from any resulting work.
If you were a Time Team enthusiast, you probably already know the answer to this one!
Geophysics is the study of the physical properties of the earth (or other planets – you can do astrogeophysics!). It is an extensive term that encompasses whole planets, right down to understanding the microstructures of stone. Archaeological geophysics falls into ‘near-surface geophysics’, which studies the first 30m or so of the ground. In fact, commonly, archaeological geophysics is only really concerned with the top 2m or so; material in the topsoil, rather than the bedrock.
Geophysicists use a variety of methods and instruments to get information about the physical properties of the ground, such as its ability to conduct electricity, or its magnetic properties. Small variations in those properties can then be mapped. Buried archaeological material causes variations in the properties in predictable ways. This means we can map buried archaeology using these methods, without having to dig everything up.
When it comes to Castles, there are three main geophysical methods: earth resistance (‘resistivity’), magnetometry, and ground-penetrating radar (GPR). All three techniques look at slightly different aspects of the sub-surface, and all three have benefits and weaknesses. It is really best to combine methods to get as complete a picture as possible, as they will all tell you slightly different things.
Magnetometry
Magnetometry is probably the most commonly used technique in archaeology. It uses sensors to look at small variations in the strength of the earth’s magnetic field, to look for changes caused by buried material. The soil on sites where humans live becomes more magnetic over time, due to things like fires for cooking and warmth and the fermentation of waste material. This material becomes the fills of cut features like pits and ditches. These end up more magnetic than the soil they are cut into.
Structures used for processes involving heating, such as kilns, furnaces and ovens, become even more strongly magnetised and have a very characteristic appearance in the data. Similarly, fired ceramic building materials like brick or tile have a distinctive signal, as do igneous or metamorphic rocks (those modified by heating during their formation).
Magnetometry survey of Pembroke, funded by the CST in 2016
Magnetometry is very fast, covering upwards of 10ha a day if using the latest equipment. It is also relatively easy for community groups to employ. However, the pace will be somewhat slower using hand-carried single sensors. The plus-sides are the speed of survey and the wide variety of archaeological features that can be detected and mapped. The downsides are that this method is strongly disrupted by ferrous material in the survey environment, and has problems on igneous and metamorphic geologies as happened with the survey of Tibbers in 2014. It is also not very useful for mapping stone remains that are not strongly magnetic (such as some sandstones and most limestones). Modern infrastructure within or adjacent to the survey area has a far greater impact on the results than any buried archaeology (as happened in the Wressle survey of 2019), masking it from detection. It is practically not possible to use this method in urban areas. This method cannot detect smaller structures if they are buried more than about 2m below the ground surface. Features in the first 2m can usually be detected but the size of the anomalies that can be distinguished depends on the resolution of the readings taken. However, this method doesn’t let you understand the depths of the anomalies, and so isn’t as helpful on multi-period sites.
Earth Resistance
Earth resistance examines how easy it is to pass an electrical current through the ground. The resistivity of the subsurface varies in relation to several properties. Still, the most substantial effect is caused by variations in moisture content. The fills of cut features like pits and ditches (as witnessed in the 2018 survey at Laughton which showed a possible ditch, confirmed in the 2019 excavation and of Tibbers which lead to the discovery of a new inner bailey) tend to have a more open texture than natural soil. They usually also contain more organic matter. This means they are generally wetter than the ground they are dug into. Conversely, buried structures like walls and floors, are usually much drier than the material surrounding them, because they can’t absorb as much water.
Earth Resistence Survey at Tibbers, co-funded funded by the CST in 2016. Copyright HES
This technique can be applied in two ways. You can collect a grid of readings over a flat area to examine the first 2m or so, producing a plan view. You can also collect long lines of readings with increasingly wide measurement points. This is called ‘Electrical Resistance Tomography’ or ERT, and produces vertical pseudo-sections through the ground, and can reach greater depths, typically in archaeology 8-10m.
The plan-view method typically involves 2 probes on a mobile frame, and two remote probes connected by a cable. 2 of the probes inject a current, and two measure the resistance to it. It is especially useful for mapping buried stone structures. It is therefore handy on ‘Castle’ sites where multiple building phases can be expected. It is relatively slow, however, and relies on being able to insert probes into the ground to get the readings. This is fine on a lawn or field, and it can work on paths and gravel, but the results get very noisy, and it isn’t possible over flagstones or tarmac or concrete. You also need to be able to manoeuvre the cables and place the remote probes at an appropriate distance. This method also doesn’t let you understand the relative depths of various anomalies.
ERT is less commonly used in archaeology, but it has some specific applications in the study of large defended sites. Because it can resolve structures at a greater depth than the plan-mode, it can be used to examine the construction of large structures. This includes moats, earthen banks and buried fortification walls, and other such features. If multiple adjacent profiles are collected, the data can be combined into a 3D model of the subsurface, which can help resolve questions about the construction sequence of a site.
Twin-probe (plan-view) resistivity survey is relatively straightforward to carry out. It doesn’t require as much skill on behalf of the instrument operator as magnetometry does. It is however, slow and laborious. The equipment is relatively cheap, and data processing and visualisation are relatively simple. This method is rarely used in the commercial sector these days but is an ideal research tool. Community groups have produced excellent research using this technique. The ERT method requires specialised equipment and a trained collector. The background knowledge needed to correctly process and interpret the data is also high.
Ground Penetrating Radar
GPR only made the odd appearance on Time Team, but in the last decade or so advances in computing (mostly increasing miniaturisation of components, and improvements in battery life) have led to a new generation of GPR kit that is more flexible and affordable.
GPR works a lot like sonar or the sort of radar employed by aircraft. A transmitting antenna sends out radio waves focused into the ground. These propagate downwards and are reflected by abrupt changes in the material of the subsurface. For example, when the waves leave a stone ceiling and move into the vault, some of the waves will be reflected back up. Some will continue on, to encounter the floor of the vault, and anything below it. The reflected waves are collected by a receiving antenna (usually in the same ‘box’ as the transmitter, a fixed distance apart). The strength of the returned waves, along with the time (in nanoseconds!) it takes for them to return is logged and plotted. This is a single trace.
Ground Penetrating Radar Survey at Shrewsbury Castle 2019, funded by the CST
The antenna is dragged along a line, and a series of traces are collected at a small interval (usually every 5cm or 2cm), which are combined together to make a profile. This is effectively a vertical slice through the ground. These are a bit difficult to read because the radio waves emit in a curved shape, so they actually travel in front of and behind the antenna, not just directly under it. This creates distinctive hyperbolas in the data. The depth of signal penetration and the size of the objects you can detect varies with antenna frequency. Depending on the frequency of the antenna, you can look very shallowly and resolve things that are a centimetre (or less) across. Very high-frequency antennas are used to assess the structure of concrete in civil engineering or to image different layers in mosaics and floor coverings. Lower frequency antennas cannot resolve smaller anomalies but can penetrate 10m+ to resolve much larger objects, such as former river beds, large walls or banks and ditches.
Groups of profiles collected in parallel lines can be combined together to make a 3D block of data. This can be processed in a way that allows different horizontal depth slices to be examined (as done at Fotheringhays; see time slice). A new generation of GPR system uses lots of antennas in an array to collect very high- resolution datasets (8cm in all directions), or arrays of different frequencies to quickly collect data with good resolution at multiple depths.
GPR requires a skilled operator to plan the work, collect the data and the process and analyse the results, but it is arguably the best technique for investigating Castle sites. This is because it tends to be good at detecting the sorts of things we would expect to be looking for, for example, voids, buried walls, culverts and surfaces. It can also be deployed inside standing buildings, to look underneath floors or behind walls. It can be used over tarmac and concrete (providing the concrete is not re-inforced!) and does well on most geologies, except for certain kinds of clays, and saline environments like estuaries.
The other advantage of GPR is that the data is relatively fast to collect, compared to earth resistance, and a broader range of features can be detected with it. It is also an inherently 3D method and allows complicated below-ground sequences to be visualised and interpreted. I have seen examples of staircases being visible in the data from 3D GPR, for instance. This technique has made the headlines recently, with the publication of a study of an entire Roman city, Falerii Novi, just north of Rome, by colleagues of mine from Ghent University and Cambridge University.
Geophysics and the Castle Studies Trust
Geophysical approaches form an increasing component of research proposals put to the trust, which is excellent to see! Geophysics can help to answer both broad and specific questions about castle-sites, without the potentially destructive process of excavation. Geophysics also has applications for the conservation of sites and planning for their future management. For example, in mapping the integrity of standing walls using GPR, or understanding the soils and material within earthworks to protect them from erosion in extreme weather events. They can help site managers decide whether an intervention is necessary, and can inform the design of any needed work. Geophysics can also play an essential role in the continuing life of these sites as homes or places of historical interest by mapping areas where conservation or building work is planned to ensure nothing is damaged by the work.
William Wyeth, Properties Historian at English Heritage Trust and project lead on the Castle Studies Trust funded project to geophysically survey Warkworth Castle explains what he hopes the survey will achieve.
In 2019 English Heritage, a charity which looks after over 400 historic properties and sites across England, began a project to change the way in which the stories of the people and buildings at Warkworth Castle in Northumberland were told. The castle is a popular destination in the county, and is both connected with notorious figures from the past as well as featuring an iconic piece of medieval architecture and design in its late 14th-century Great Tower.
Figure 1. Warkworth Castle from the south-east. The Great Tower, in the background, sits atop a large motte. In the foreground, the curtain wall is punctuated by the Grey Mare’s Tail tower. Archaeologists from Archaeological Services – Durham University are surveying the unenclosed strip of bailey
Warkworth is located at the foot of a narrow loop in the River Coquet, in coastal north-eastern Northumberland, about 25 miles north of Newcastle-Upon-Tyne and 30 miles south of Berwick-Upon-Tweed, themselves both significant fixtures of the late medieval history of this area. Just north of the castle proper and nestled on three sides within the river loop is the small village of Warkworth, arrayed in quite typical medieval layout. At the north end of the high street, sitting on a rough north-south axis, is the parish church of St Lawrence, probably an early medieval foundation, as well as a bridge with a toll-collecting tower built in the later 14th century. South of the church, numerous narrow parallel plots of land spread out at right angles from the high street. The southern trajectory of the street is abruptly broken by the enormous motte (earthen mound) of the castle, which acts to physically separate the village from the land south of the river.
Among the most famous historical figures connected to the castle was Henry Percy, eldest son of the 1st Earl of Northumberland, though he is more familiar to us today as Harry Hotspur. The origin of his martial nickname is not certain, but is accounted for in several traditions, all of which confirm that they drew from his short-tempered and violent character. One later 16th-century source rhythmically noted “For his sharp quickness and speediness at need / Henry Hotspur he was called in very deed.”
Though the early history of the Percy Northumberland earls and associated figures will form a key part of the story of the castle when the interpretation project is completed in 2022, other questions about the castle, and especially its earlier history, remain as yet unresolved. Among these is the relationship of the earthworks – the motte and bailey – with the stone structures atop them, the oldest of which date to the later 12th century. The Castle Studies Trust has graciously agreed to fund a geophysical survey of much of the castle earthworks to resolve three big questions.
The first touches upon the motte, which features the Great Tower of the 1370s, but was probably topped by an earlier structure. By assessing buried deposits around the tower, we aim to reveal traces of this earlier structure. But we also want to establish evidence for the means by which the Great Tower may have been provisioned, via a secure door to the motte-top outside the enclosing curtain wall which gave access to storage areas for beer and food in the tower’s north-west segment.
The second question relates to the bailey. In common with other castles of this type, the bailey was filled with buildings, often (as at Warkworth) in their earliest phases arrayed along the inner face of the enclosing wall. But were there buildings here before, or were there also buildings here from later periods, but for which above-ground evidence has been lost? Findings from the survey here will greatly influence how we understand the formal approach to the bailey’s principal buildings – its Gatehouse, Great Hall and Chapel – but also the late medieval Great Tower. The results may also shed light on the peculiar overhauling of spatial arrangements in the bailey occasioned by the construction of a 15th-century Collegiate Church which straddled the span of the bailey, arguably fundamentally changing how the castle was to be experienced.
The last question also relates to the bailey, but here to a strip of the bailey which sits outside the embrace of the late 12th-early 13th-century stone curtain wall, on the eastern side of the castle. The omission of this area from enclosure is unusual, though it is not without analogies from elsewhere, which suggest areas like this could contain gardens. It may be instructive that just within the bailey and adjacent to this strip was the location of late medieval stables – perhaps this area came to be used for the grazing of horses, though whether this was its original intended purpose remains to be seen. In addition to all of this, however, is the possibility that when the motte-and-bailey was built, perhaps well before the earliest stone parts of the castle were erected, the earliest enclosing wall of the bailey also embraced this eastern strip, thereby creating a larger bailey than the present one.
We hope that the survey will allow us to answer at least some of these questions. Whatever the outcome, it is certain that the results will help change how we understand the story of Warkworth Castle and its previous inhabitants.
We have the results of the survey at Fotheringhay Castle. You can find out more about what we found in Steve Parry’s excellent blogpost, complete with the earliest depiction of the castle.
The castle is most famous as the place where Mary Queen of Scots was tried and executed. It was thoroughly dismantled in the first half of the 17th century, leaving the motte intact but little else above ground. Thanks to work by the Museum of London Archaeology and funded by the Castle Studies Trust, we now have a better idea of how the castle was arranged.
Some of the Castle Studies Trust’s projects have made innovative use of cutting edge technology. Nick Tarr explains how a new survey technique was used this year at Pembroke Castle.
Geophysical Survey Technologies (GST) was formed to improve survey equipment for archaeologists to use in all environments including equipment suitable for use in woodlands. The equipment, ideally, should be within the financial reach of amateur groups.
The prototype survey frame resulted from research into voltage surveys (commonly called resistance surveys) where geology or other conditions are unfavourable for conventional methodology. The frame uses a commercially available data logger and power supply but has all four electrodes on a compact mobile frame which is collapsible to fit in boot of a car.
The version used at Pembroke Castle was aimed at keeping the energy from the power supply within the archaeological layers so maximising any opportunity of detecting any archaeology present. A comparison with the conventional twin array in both parallel and zig-zag walking modes was made over a single grid which contained part of a building and a track. The existing twin array frame gave no clear signal for the building, the track was the only major feature seen.
The prototype frame gave much better results. A further test across a monastic site in west Wales has also shown improved results over the conventional twin array methodology. Development work continues.
