Rise of the Mega-Henges

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Unpicking the evolution of Mount Pleasant’s monuments

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. CREDIT: Peter Sandiford

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.

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 has proposed a new timeline that dramatically compresses their construction into the space of little more than a century.

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.

This map shows the wider prehistoric landscape around Mount Pleasant – known as the ‘Dorchester complex’ – showing its key Neolithic and early Bronze Age monuments. CREDIT: Susan Greaney

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 postholes, 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.

This plan shows the known details of the palisaded enclosure, including patterns of whether particular sections had been burned, removed, or left to decay. CREDIT: Susan Greaney

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.

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.

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

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.

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.

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). CREDIT: Susan Greaney

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.  

This is an extract of an article that appeared in CA 371. Read on in the magazine. Click here to subscribe

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