Lidar and Field Survey of Warton Crag Hilltop Enclosure
In 2016, Historic England undertook aerial mapping and analytical field survey of the scheduled ‘hillfort’ that occupies the summit of Warton Crag in north Lancashire. Large parts of the site are heavily overgrown and to date have proved extremely difficult to investigate using traditional air- and ground-based survey techniques.
The recent aerial mapping, however, used specially commissioned lidar imagery. By emitting light pulses from a plane and measuring the time they take to return, lidar accurately records height differences on the ground, and is therefore capable of picking out archaeological earthworks.
If carried out at a high enough resolution, the technique can even ‘see’ through tree canopies to record the terrain beneath. The mapping of the new data, in combination with ground observation, has resulted in a much more detailed and nuanced understanding of the monument, and thrown in to question its hitherto accepted identification as a hillfort.
Warton Crag is a prominent limestone ridge, located within the Arnside and Silverdale Area of Outstanding Natural Beauty (AONB). It overlooks the eastern edge of Morecambe Bay and also has clear sightlines east towards the Yorkshire Dales. Three arcs of ruined stone walling isolate the ridge’s uplifted southern scarp-edge from the northern dip slope.
The walls have been recognised for over 200 years, and have been scheduled as the ramparts of an Iron Age hillfort for almost half that time, but have proved difficult to interpret because the thick tree and scrub cover makes planning – and even seeing and following – them on the ground extremely problematic. This vegetation is also a threat to the archaeology, and in consequence the monument was placed on Historic England’s Heritage at Risk Register in 2012.
The heritage interest of the ‘hillfort’, however, has to be weighed against competing environmental, geological and wildlife designations: besides being part of an AONB, the crag is a site of special scientific interest, a local nature reserve, and an area of ancient woodland, as well as being the subject of a Limestone Pavement Protection Order.
In 2016, Historic England and Morecambe Bay Partnership (the latter through the Heritage Lottery Fund-sponsored Headlands to Headspace Landscape Partnership Scheme) jointly commissioned new, high-resolution, lidar imagery of the crag from Bluesky International Ltd so as to facilitate detailed aerial mapping of the enigmatic monument that lies on the summit. The mapping was followed by detailed observation on the ground to interpret and refine the lidar plot, all as the first step towards drawing up a management plan for the long-term conservation of the site.
The existence of three walled circuits on Warton Crag was first recorded by William Hutchinson in a letter communicated to the Society of Antiquaries of London in 1788 (Hutchinson 1789).
Hutchinson was a solicitor, but also a keen antiquary and the author of historical accounts of Durham and Cumberland. The inner circuit was, and still is, the most substantial, described by Hutchinson as formed of facing stones 10 foot (about 3 metres) apart, set within a more extensive scatter of tumbled stones up to 10 paces (about 8–9 metres) wide.
He described the outer circuit as less massive, and the middle as less massive again. He identified two entrances in both the inner and middle circuits, and three in the outer; he also mentioned the ruins of a small square hut within the interior where ‘a beacon used to be fired’, and a circular depression which he thought might be a reservoir for water (although he did not specify whether he thought this contemporary with any of the other structures).
Hutchinson interpreted the site as a defensive encampment associated with native British resistance to the Roman conquest of northern England. This view is in line with the orthodox paradigm of the 18th century, which interpreted field monuments within a historical narrative derived from the few surviving relevant works by Classical and early medieval authors. We now call this period the Late Iron Age.
Hutchinson’s dating has been followed uncritically by subsequent investigators, including the Ordnance Survey, the Victoria County History, and the archaeologist James Forde-Johnstone, all of whom have surveyed the site and interpreted it as a hillfort – although each has offered a somewhat different take on the form and extent of the earthworks.
Lidar and field survey
The high-resolution lidar data central to the new research was collected on a 0.25 metre grid and processed in-house to produce various visualisations of the terrain. These are striking, showing much of the extent and form of the enclosure as well as its topographical setting. The monument can be clearly seen to be defined by three irregular circuits, of which the inner – enclosing almost 3 hectare of the Crag summit and containing large areas of exposed limestone pavement – is the most massive. There is no evidence that this inner wall ever continued above the main, steep southern scarp-edge known as Beacon Breast.
The lidar survey revealed no surviving internal structures contemporary with the enclosure – indeed the fractured and irregular nature of the bare limestone pavement makes permanent occupation unlikely. The walls of a number of small ‘pens’ are, however, visible against a low limestone scar just back from Beacon Breast, and a circular depression can be seen towards the centre of the monument. These are almost certainly post-medieval stock enclosures and a dewpond (the latter probably to be equated with Hutchinson’s reservoir) for grazing cattle. The visualisations also suggested a number of gaps in each of the three circuits although from the lidar alone it was impossible to ascertain whether these are original entrances or later breaks.
The density of vegetation cover over parts of the site meant that even lidar as high-resolution as this could not pick out all the earthworks, however. The aerial mapping was therefore loaded onto a handheld (mapping-grade) Global Navigation Satellite System device, to be taken in to the field where interpretation could be checked and enhanced, and additional detail added. The ability to view the mapping and lidar visualisations at the same time as having a good fix of one’s ground position was a great help; previously, it had proved difficult to follow the walls through the dense undergrowth, and almost impossible for a field investigator to know exactly where they were when examining features on the ground.
The field survey demonstrated that the walls are all similar in build, consisting of rubble infill between faces of orthostatic construction (that is, formed of stones set on end); they are, therefore, as far as we can say based on surface evidence, probably all contemporary. Survey was even able to identify individual in-situ surviving orthostats and confirm that the walls were originally up to 3 metres wide, as reported by Hutchinson.
The survey also revealed that the southern end of the middle wall extended 30 metres further than plotted from the lidar, all the way to the crest of Beacon Breast, and identified a number of additional stock enclosures. These features were all invisible on the lidar because the vegetation was exceptionally dense in particular areas.
Finally, the field survey confirmed that a number of the breaks seen on the lidar are original entrances and – on the basis that entrances are likely to be distinguished by large flanking orthostats set at right angles to the line of the walls – was able to identify further examples.
‘The walled circuits are unlikely ever to have presented a serious obstacle to determined attackers’
These observations all cast doubt on the traditional interpretation of the monument as a hillfort. Hillforts typically have massive defences and few entrances. The walled circuits on Warton Crag are extremely ruinous, but from what we now understand of their construction it is difficult to see how they could ever have stood much more than a couple of stone courses high; this, combined with the fact they lack accompanying ditches, suggests they are unlikely ever to have presented a serious obstacle to determined attackers. In addition, whereas hillforts normally only have one or two entrances, Warton has at least two in the inner circuit, three in the middle circuit, and possibly as many as four in the outer one.
Rather, the site seems to have more in common with a class of upland enclosure which in recent years has been recognised in increasing numbers across northern England. All examples of the class are defined by stone walls, though these vary in size and construction technique. Few have yet been extensively investigated, but several – particularly in Cheshire and Derbyshire – have seen small-scale excavation which has dated them to the middle and later Bronze Age.
Their function is not well understood – indeed they may not all have been used in the same way –- but the excavators of the Gardom’s Edge enclosure in the Derbyshire Peak District suggested that site may have acted as a central place for pastoralist farmers to gather at certain times of the year. The aim may have been to sort, exchange and serve animals, settle feuds, and simply re-affirm communal bonds (Barnatt et al 2017). Only excavation will confirm whether Warton Crag is another example of this class of Bronze Age enclosure, but we can be confident it is not a hillfort. Undoubtedly more such enclosures will be identified over the coming years, perhaps including a number that, as at Warton, are hiding as it were in plain sight.
The results of our research have been published as an Historic England Research Report (Evans et al 2017). They will be used to inform the future management and conservation of this important site for future generations to enjoy.
Marcus Jecock FSA
Senior Archaeological Investigator, Historic England.
Marcus has been an archaeological investigator for over 30 years, initially with the Royal Commission on the Historical Monuments of England (RCHME) and since 1999 with English Heritage/Historic England. He is a landscape archaeologist specialising in the survey and interpretation of landscapes and field monuments, and has worked in many different parts of England on sites of all periods from prehistory through to the 20th century. He is currently Historic England’s coastal survey Lead.
Aerial Investigator, Historic England
Sally is an Aerial Investigator in Historic England’s Historic Places Investigation Team (North & East). She has over a decade’s experience working on large-scale aerial investigation and mapping projects, focussing on the identification of previously unknown archaeological landscapes. She is interested in how analysis of project results can be used to identify threats to archaeological monument survival and inform future management and protection of archaeological sites.
Barnatt, J, Bevan, B and Edmonds, M 2017 An Upland Biography: Landscape and Prehistory on Gardom's Edge, Derbyshire. Oxford: Windgather Press
Evans, S, Jecock, M and Oakey, M 2017 Warton Crag Hilltop Enclosure, Warton, Lancashire: Aerial Mapping and Analytical Field Survey. Historic England Research Report 33/2017
Hutchinson, W 1789 ‘Account of Antiquities in Lancashire in a Letter to George Allan, Esq, FAS from William Hutchinson, Esq, FAS’. Archaeologia, 9, 211–18
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