Mega-henges: the evolution of Mount Pleasant’s monuments

It was long thought that huge and complex monuments like Mount Pleasant in Dorset had developed over many centuries – but new dating evidence suggests that the diverse elements of this site came together much faster than previously imagined, with intriguing implications for our understanding of these late Neolithic enclosures. Carly Hilts reports.


Close to the Dorset town of Dorchester, the rivers Frome and South Winterborne form a V-shape whose arms embrace a diverse scatter of Neolithic monuments. From the early Neolithic causewayed enclosure at Maiden Castle and middle Neolithic sites like the Alington Avenue long barrow and the 100m-wide early henge known as Flagstones, to late Neolithic constructions like the Maumbury Rings henge monument and Greyhound Yard’s 380m-wide palisaded enclosure, these sites form a rich and wide-ranging ceremonial landscape spanning hundreds of years. The largest and most complex feature of this enigmatic environment, though, is Mount Pleasant.

Unlike southern England’s other ‘mega-henges’, which have all undergone recent archaeological investigation, Mount Pleasant was only excavated in 1970-1971. This photograph shows one of the monument’s more enigmatic elements, Site IV, under excavation in 1971. Photo: Peter Sandiford ALL OTHER IMAGES: Susan Greaney, unless otherwise stated.

This impressive array is made up of multiple monumental elements, but the site is dominated by a huge henge enclosure – a circuit of bank and inner ditch – which forms a rough oval measuring c.370m east–west and c.320m north–south. The scale of this construction has led to Mount Pleasant being dubbed a ‘mega-henge’, one of a select group of broadly contemporary monuments in southern England whose other members are Knowlton, also in Dorset, and a Wiltshire trio represented by Avebury, Durrington Walls (see CA 5 and CA 334), and Marden (CA 17 and CA 316). These four enclosures have all seen extensive recent investigations, but the only major excavation at Mount Pleasant was Geoff Wainwright’s campaign in the 1970s (CA 23), whose findings have long shaped interpretations of the site.

Wainwright’s work documented not only the henge itself, but the fact that the monument had also once comprised a tall wooden palisade, a huge round mound, and a mysterious structure formed from concentric arrangements of timber and stone (all of which we will explore in greater detail below). Until recently, it was thought that these disparate elements had evolved over a long period of time stretching from the Neolithic into the Bronze Age – but now a new study headed by Susan Greaney at Cardiff University and recently published in the Proceedings of the Prehistoric Society (see ‘Further reading’ box on p.33) has proposed a new timeline that dramatically compresses their construction into the space of little more than a century.

left This map shows the wider prehistoric landscape around Mount Pleasant known as the ‘Dorchester complex’ showing its key Neolithic and early Bronze Age monuments.
This map shows the wider prehistoric landscape around Mount Pleasant known as the ‘Dorchester complex’ showing its key Neolithic and early Bronze Age monuments. Image: LiDAR imagery from the Environment Agency via Digimap. Contains public sector information licensed under the Open Government Licence v3.0

The key to this discovery lay in the archive from Wainwright’s excavations, which is stored at Dorset County Museum, and which includes a range of materials suitable for radiocarbon dating, including charcoal, fragments of human bone, and a number of antler picks that would have been used to create the enormous earthworks. Radiocarbon dating of this assemblage has now provided 59 new dates that have been subjected to Bayesian statistical analysis, and the results have been combined with context records from the excavation to produce a much more precise sequence for how – and, crucially, when – each element of the monument was built.

Key components

Unlike the other mega-henges, which occupy low-lying ground or surround valleys, the remains of Mount Pleasant enclose the highest point of the Alington ridge, offering a commanding view over the surrounding landscape and across a fording point on the river Frome. The site has been used for arable cultivation since at least the 1850s (and remains so today), meaning that its earthworks are damaged by ploughing, but it is still possible to unpick its key components.

It might be helpful to begin by summarising the features that Wainwright identified. Chief among these is the huge henge enclosure: today its bank (made from heaped chalk rubble) is poorly preserved, but it originally represented an imposing boundary some 4m tall and between 16m and 23m wide. Beyond the bank, a substantial inner ditch followed the same course, ranging in width from 9m to 17m – taken together, these would have created a formidable obstacle, and entry to the henge’s interior was only permitted through five gaps in its circuit.

Within the henge enclosure was another wide circular trench containing large, closely set post-holes, which Wainwright interpreted as the foundation of a towering timber palisade. Based on assumptions that a third of the length of each post would have been sunk into the ground, this fence would have stood some 6m high, and was made up of an estimated 1,600 timbers. Based on recovered charcoal, the posts were oak, which is the most-common choice of wood used in other known late Neolithic palisaded enclosures and timber circles recorded in Britain and Ireland – possibly because of the size and longevity of the timbers that such trees can produce, though there may also have been some kind of religious significance attached to this species.

left This plan shows the known details of the palisaded enclosure, including patterns of whether particular sections had been burned, removed, or left to decay.
This plan shows the known details of the palisaded enclosure, including patterns of whether particular sections had been burned, removed, or left to decay. Image: based on Wainwright 1979, Fig.99

Unlike the outer bank, this barrier only had two entrances through it, and they were remarkably narrow, barely 1m wide. Even more strikingly, though, these gaps were dwarfed by the posts that flanked each of them: enormous trunks some 1.8m in diameter, which would have weighed an estimated 17 tonnes. They would have required large numbers of people and a concerted effort to move them, and even the smaller posts making up the main palisade probably represent the substantial trunks of entire felled trees – this was no small construction project. Where, though, did all this wood come from? Analysis of snails from the ditches of Alington Avenue long barrow, Flagstones, and Mount Pleasant itself suggests that these monuments stood in open, intensively grazed land – a far cry from the established woodland required for trees of this type. Perhaps, Susan Greaney and her co-authors write, we might imagine the monument-builders searching far and wide for suitably tall and straight trees, which they then transported back to Alington ridge over land or by river.

Alternatively, they suggest, different communities could have contributed their own tree to the building project, a scenario which has previously been proposed for stone circles made up of different lithologies, such as the Ring of Brodgar in Orkney. Certainly, it appears that people were travelling long distances to gather at some of the mega-henges. Isotope analysis of animal bones suggests that huge feasts were being staged at these sites involving meat from animals raised on a variety of geologies – at Mount Pleasant, animals were being brought from at least 50km away, while remains from Marden and Durrington Walls speak of even longer journeys (see CA 334).

The other main components of Mount Pleasant are Conquer Barrow – a large earth mound, 30m in diameter at its base, that was built on top of part of the henge bank to create a vantage point 7m high (the mound itself is c.3.4m tall) – and an enigmatic concentric construction known as Site IV, which was built within the enclosure at the highest point of the site. This latter feature is formed of five concentric rings of post-holes, divided into quadrants by ‘aisles’ running roughly north–south and east–west. This was surrounded by a circular ditch 43m in diameter and accessed by a single causeway to the north, and at the heart of the arrangement there was another square setting, this time formed of posts and standing stones. Such ‘square in circle’ monuments are known from several other sites in Britain and Ireland, though for now their purpose remains obscure.

below Dr Peter Marshall taking dating samples from antler picks recovered from Mount Pleasant, in the stores of Dorset County Museum.
Dr Peter Marshall taking dating samples from antler picks recovered from Mount Pleasant, in the stores of Dorset County Museum.

Until recently, the relationship between these diverse elements and the order in which they were built were not particularly well understood – but the new study by Greaney and her colleagues is set to transform this picture.

Transforming the timeline

The project’s 59 radiocarbon dates place Mount Pleasant squarely in the mid-3rd millennium BC, making it broadly contemporary with the other mega-henges, as well as with the construction of the main phase of Stonehenge. As for a more detailed timeline, the first part of the monument to be built appears to have been the mighty henge enclosure. Charcoal samples taken from the soil beneath the enclosure and from the surrounding ditch, as well as antler picks recovered from the base of the ditch which were probably used in its construction, suggest that this feature was dug in 2610-2495 BC, and most likely in 2580-2530 BC.

The stratigraphic position of Conquer Barrow is not entirely clear, but an interpretation of it being built on top of the earlier bank is favoured by the authors, based on the surviving earthworks. The mound lies on private land and is very overgrown, limiting opportunities for investigation, but a single antler pick that is known to have come from the primary chalk rubble of its surrounding ditch (and is therefore thought to be associated with its construction) was dated to 2580-2460 BC, most probably 2525-2475 BC, suggesting that it was one of the next features to be added to the complex. This would mean that the mound is not a Bronze Age barrow, but late Neolithic in date – a kind of monument known as a ‘large round mound’, with the most-famous example being Silbury Hill near Avebury (see CA 293). There appears to have been some kind of relationship between such mounds and major enclosures – Hatfield Barrow stands at Marden, and Knowlton has the Great Barrow – and, if Conquer Barrow should be counted among their number, it is one of the earliest examples yet identified, the research team argues.

Simple antler picks like these were the main tools used by the Neolithic builders of Mount Pleasant. The pick comes from Geoff Wainwright’s excavations in the 1970s.

The palisade had been interpreted by Geoff Wainwright as a Bronze Age construction, but the new dating evidence suggests that it is broadly contemporary with Conquer Barrow, having been built in 2560-2440 BC, and probably in 2530-2465 BC. These results, based on samples of oak charcoal from the post-holes (some sections of the palisade appear to have been destroyed by fire, while others were left to rot in situ), together with a number of well-worn antler picks recovered from the chalk rubble packing material that had supported the timbers, place the palisade alongside other timber enclosures from across Britain and Ireland. Radiocarbon dates are available for 12 large late Neolithic timber palisades built within these islands, and it appears that the Mount Pleasant example was a member of the same family.

below Simple antler picks like these were the main tools used by the Neolithic builders of Mount Pleasant. The pick comes from Geoff Wainwright’s excavations in the 1970s. A packing label from this investigation’s archive is shown inset.
A packing label from this investigation’s archive.

The creation of the palisade within the henge enclosure is an intriguing decision by its builders, as it effectively made the monument much harder to access. True, even in its original incarnation you would still have had to walk to one of the five entrances around its circumference, which might have been quite a hike depending on where you were coming from, but the addition of so many tall, closely placed posts effectively screened off the monument’s interior from onlookers, while its few and very narrow entrances made physical access difficult. This effect was heightened by the fact that at least two of the earlier gaps through the ditch and bank were blocked off by the palisade’s construction; only the causeway to the north and the eastern entrance had corresponding (and much smaller) gaps through the timber fence. The south-west and south-east entrances were effectively put out of use (the fate of the fifth entrance, to the south, is inconclusive as the stretch of palisade nearest to it has not been excavated).

The way to and from the river was left unencumbered, perhaps indicating the waterway’s importance as a transport route (and perhaps its ceremonial significance at the site), but otherwise the construction of the palisade meant that physical and visual access to the henge’s interior was now very tightly controlled. Why this change took place remains obscure, but its impact on how the site was used must have been dramatic.

Finally, the last dated major construction activity at Mount Pleasant appears to have been the digging of the ditch surrounding Site IV’s concentric timber and stone monument (there was no suitable dating evidence for the timber structure within the ditch, so this remains undated and its place in the sequence unknown). The ditch appears to have been created c.2555-2400 BC, and most likely 2515-2440 BC. Taken together, all of these dates suggest that the entire process of constructing Mount Pleasant, from the formation of the henge enclosure to the digging of the ditch around Site IV, took no more than 35-125 years: a radically shorter period than has been suggested before.

Henge enclosures, palisaded enclosures, and late Neolithic round mounds that have been identified in Britain and Ireland. Open symbols denote probable sites.

A monumental undertakingThis remarkably fast construction indicates that the complex reached its final form in a matter of just two to five generations, the researchers suggest, meaning that its evolution could have taken place within a period of living memory. It is not impossible, they write, that the grandchildren of those who helped to excavate the enormous henge ditch could have been the people who cut down the trees and erected the palisade posts, and that their children in turn may have dug the ditch around Site IV. All of these undertakings would have required the input of huge numbers of people, both in terms of manual labour and logistical support, and it is possible that this task force was drawn from a wide area. They could have only dreamed of the technology that modern construction projects rely on – the same simple antler picks that provided some of the project’s vital dating evidence would have been their primary tools, making it all the more impressive that they achieved their aim in such a short period of time.

This truncated timeline has implications for how we should imagine Mount Pleasant’s place in the landscape, the researchers say. Instead of imagining a very gradual evolution over many centuries, with the site appearing largely static to prehistoric onlookers, we should instead visualise a very dynamic monument that was in a constant process of rapid and determined change. While this research has been able to draw together a likely sequence for the order in which the site was built, however, we do not know how long it remained in use in its final form. There are some clues from the monument that suggest that some parts of the site only stood for a few generations before at least part of it suffered a destructive attack.

Around one to two centuries after the ditch around Site IV was dug, this boundary had been allowed to silt up considerably – and within this fill a layer of burnt material together with chips of sarsen stone and pieces of Beaker period (early Bronze Age) pottery was deposited. The sarsen fragments most likely reflect stone-breaking rather than the uprights’ original erection, the team suggests – but what had happened? This layer dates to around 45-160 years after the estimated arrival of the Beaker cultural package from continental Europe, a shift indicated by the appearance in English graves of the distinctive pots that give this movement its name. This phenomenon did not only herald revolutionary technological changes, however; recent ancient DNA studies have indicated that the arrival of the Beaker people resulted in a dramatic genetic turnover, almost completely replacing Britain’s Neolithic population in just a few centuries (CA 338). Might the apparent destruction of the Site IV structure be linked to this transformative and turbulent period – the eradication of an important megalithic structure in a sacred space, as part of the huge cultural changes that were taking place?

This is not the only indication of Bronze Age interest in the site: Beaker pottery had also been deposited in the top of the palisade trench, and in the ditches surrounding Site IV and the main henge. Meanwhile, in the northern terminal of the henge ditch, close to the western entrance and overlooked by Conquer Barrow, someone had placed an early Bronze Age decorated bronze flat axe.

Yet the significance of the huge henge, even if its purpose had changed, still seems to have been recognised by later prehistoric communities, who wanted to be associated with its remains. The wide henge ditches were visible as distinct earthworks up to 2m deep at least into the Iron Age, and it was during this period that at least one (probably more) graves were dug into the ditch, containing the remains of children. Close-by, the outline of a roundhouse and associated pits were superimposed over the location of Site IV. Perhaps these people linked the silent earthworks to distant ancestors, or perhaps the site merely offered a useful viewpoint over their surroundings, but interest in the mega-henge evidently endured for centuries after it had ceased to be used as its builders had intended.

The wider view

This focus on Mount Pleasant has provided fascinating details about how a huge monument came into being, but the project has also added to an intriguing wider picture of late Neolithic monument-building across Britain and Ireland. The new dating evidence means that the various components of the site can be compared to similar structures excavated across the British Isles, and the resulting similarities add to a strong impression of contacts and/or movement between communities in widely scattered regions. That is not to suggest a ‘pan-British’ ideology, the researchers stress, as there are large parts of these islands where such forms of ceremonial enclosures are not found, but it does suggest that there may have been shared ideas about monument-construction, perhaps even a shared religion, that was transmitted across large areas at this time.

left This diagram summarises the construction sequence proposed by Susan Greaney and her fellow researchers. Each image represents the probability that a particular element of Mount Pleasant existed within any specific 25-year period (light colours represent lower probability, dark shading higher).
This diagram summarises the construction sequence proposed by Susan Greaney and her fellow researchers. Each image represents the probability that a particular element of Mount Pleasant existed within any specific 25-year period (light colours represent lower probability, dark shading higher).

The other wider point that this study eloquently expresses is the importance of the maintenance of archaeological archives, and their proper long-term storage. Improvements and advances in scientific research techniques – such as aDNA, isotope analysis, radiocarbon dating, and the ability to determine the contents of ancient pots by examining residues left on their surfaces – mean that even very old collections may still have new secrets to share, long after their original excavators have passed on.

Further reading
S Greaney et al., ‘Tempo of a mega-henge: a new chronology for Mount Pleasant, Dorchester, Dorset’, Proceedings of the Prehistoric Society 86, pp.199-236,