A Stonehenge mystery solved?
Last month we brought you the latest thinking on how scientific techniques are helping to pin down the origins of the Stonehenge bluestones. Now new research has located the most likely source of the monument’s larger sarsen stones. Carly Hilts reports.
Towering above modern visitors, the Stonehenge sarsens must be among the most immediately recognisable constructions in the world. Some 52 of an estimated 80 sarsens (a type of rock technically known as silcrete) remain in situ, making up most of the Neolithic monument’s key features: all 15 stones of the central horseshoe of trilithons, 33 uprights and lintels in the outer circle, and outlying elements such as the Heel Stone, the Slaughter Stone, and two of the Station Stones. Measuring up to 9m tall and with the heaviest weighing around 30 tonnes, the sarsens would have been an imposing addition to Salisbury Plain when they were erected c.2500 BC, during the second phase of Stonehenge’s development. Until recently, however, it was not known where these mighty megaliths had been sourced from.
That is not the case for all elements of Stonehenge: previous scientific analysis has established that the monument’s smaller ‘bluestones’ (all of them rock types that are ‘exotic’ to Salisbury Plain, such as rhyolites and dolerites) have impressively far-reaching origins, having been quarried at various locations in the Preseli Hills in west Wales, 200km away (see CA 366 and 345). The sandstone Altar Stone, meanwhile, has been linked to east Wales. The sarsens, however, have received far less geological attention – it has long been assumed that they had been sourced rather closer to home (not least due to the difficulty of transporting such massive stones long distances), but this theory has never been investigated in detail.
Since the 16th century the Marlborough Downs, 30km north of Stonehenge, have been mooted as a plausible location, but other areas have potential too: sarsen has also been used to construct megalithic monuments in Kent, Dorset, and Oxfordshire. But now a cutting-edge combination of geochemical and statistical analysis may have pinned down a much more precise location. Recently published in Scientific Advances, the British Academy-funded study was led by University of Brighton geomorphologist Professor David Nash, and co-authored by English Heritage prehistorian Susan Greaney and colleagues from UCL and the universities of Bournemouth, Brighton, and Reading. Their results have proven illuminating.
The researchers began by carrying out non-invasive X-ray fluorescence analysis on all of the 52 surviving sarsens, selecting their flattest, least lichen-covered surfaces to take five readings from each. (For more on how this and the other techniques used in the study work, see ‘Science Notes’ in CA 367) This generated 260 analyses for 34 chemical elements within the make-up of the stones, revealing that 50 of them shared a strikingly similar geochemistry. These results suggest that the vast majority of the sarsens, whether upright or lintel, and across the disparate structural parts of the monument, have a common origin. Intriguingly, though, two stones were distinctively different, both from each other and from the rest of the sarsens. These are upright 26 and lintel 160 – of which, more anon.
Other than the two outliers, then, where did the sarsens come from? In order to compare their geochemistry to individual outcrops, the team would need to examine the interior of a stone – and carrying out destructive testing on an upstanding part of Stonehenge was out of the question. But then came a surprising breakthrough.
THE CORE OF THE MATTER
It all began with a break-up, more than half a century ago. In 1958, ambitious plans were afoot to re-erect a toppled trilithon – uprights 57 and 58, and lintel 158 – which had collapsed in 1797. During this initiative, though, fractures were discovered running through Stone 58, and to fix the fragile sarsen and prevent future falls, three horizontal channels were carefully drilled through its entire thickness. The work was undertaken by Van Moppes, a Basingstoke-based company more usually engaged in cutting diamonds. Into the three channels went strong metal ties, secured with boltheads, and the surface holes were filled in with plugs of sarsen. The stone was secure – but what happened to the cores of sarsen that had been extracted during this process?
The cores were long presumed to have been lost – but, in 2018, one returned from an unexpected source, completing a journey that dwarfed the miles travelled by the bluestones: it had spent the intervening decades in the USA. It transpired that Robert Phillips, a former Van Moppes employee who had been involved in the 1958 restoration, had been given one of the cores he had drilled as a souvenir. The stone cylinder had been proudly displayed in his office until Robert retired and emigrated to America, after which it travelled with him in various moves to New York, Illinois, California, and Florida – but, with his 90th birthday approaching, Robert wanted to reunite the core with its mother monument. To fulfil this wish, one of his sons brought it thousands of miles back to the UK to present to English Heritage (see CA 352).
The publicity stemming from this donation led to the identification of a 0.18m section of a second, broken core rather closer to Stonehenge – in the collections of the Salisbury Museum. The location of the rest of this example and the third core remain unknown, but the return of the more-complete Phillips core represented a truly exciting prospect, offering a unique opportunity to examine the interior of a Stonehenge sarsen.
UP TO THE DOWNS
The second phase of research focused on the recovered core. This measured 1.08m long and 25mm in diameter, and was preserved in a plastic tube. Even with this protection, though, it had broken into six pieces, some of them probably reflecting the original cracks in the stone. Permission was given to sample the smallest of these pieces, and the stone was carefully cut in half lengthways. One of the resulting semicylinders was kept by English Heritage, while the other was cut into thirds, which were subjected to petrological, mineralogical, and geochemical analysis. Statistical analyses of the X-ray fluorescence studies at Stonehenge had already suggested that Stone 58 was chemically representative of the majority of sarsens at the monument. So where did it come from?
Sarsen is found in scatters across southern Britain, in the form of boulders resting mainly on underlying chalk. Today, no known boulders reach the size of the Stonehenge megaliths, but the deposits have been depleted over the centuries: the stone was not only favoured by prehistoric monument-builders, but also featured in the construction of Roman villas, medieval churches, and later roads and farm buildings.
To help pinpoint the Stonehenge source, the project team examined the chemical ‘fingerprints’ of boulders at 20 representative concentrations in a wide area that stretched from Norfolk in the east to Devon in the west. The study also took in sites in Suffolk, Essex, Kent, East Sussex, Hampshire, and Dorset, but the greatest concentration fell within Wiltshire, including six sites in the Marlborough Downs. Within these 20 locations, stones were selected at random and 100g samples taken (with the permission of the landowners) – these were then subjected to the same analysis as the Phillips core. The results were striking: all the sites outside the Marlborough Downs proved a poor match for the Stonehenge sarsens, and key trace elements also enabled the team to discount five of the locations within the Downs.
The remaining boulder concentration lay at West Woods, a 6km2 plateau in the south-east of the Marlborough Downs. Lying 220m above sea level and cut by two narrow valleys, this area was once thick with sarsen deposits, though most were removed from the mid-19th century onwards. Many large boulders remain, however, and the team’s samples yielded matches with all the immobile trace-element ratios from the Phillips core. It now appears that Stone 58 – and therefore the majority of the Stonehenge sarsens – comes from West Woods. The next step, the team suggests, would be to carry out archaeological investigations and more-detailed sampling in West Woods and the surrounding area, to narrow down precise source areas even more tightly – and perhaps even identify Neolithic extraction pits like the bluestone quarries that Mike Parker Pearson and his team have been investigating in Preseli (CA 313). Intriguingly, West Woods lies within an area of intense early Neolithic activity: it is close to Avebury, numerous long barrows, and the Knap Hill causewayed enclosure. Was it also an important extraction site?
ANSWERS AND QUESTIONS
If West Woods was the source of the Stonehenge sarsens, we do not yet know why this spot was chosen in an area with numerous sarsen clusters. Perhaps its proximity to Stonehenge played a role, though the far-flung bluestones demonstrate that practicality and ease of access were not the only considerations in supplying the monument’s stones. Might the West Wood boulders have been of particularly good size or quality?
Some aspects of the Stonehenge story are becoming clear, though. It now appears that the various sarsen elements of the monuments, whether the trilithon horseshoe, the outer circle, or the peripheral stones, were probably almost all sourced from the same area, and that they were perhaps erected at around the same time. Previous assumptions about the Heel Stone have also been revised thanks to the new study: due to its large size and undressed surface, it had been suggested that this stone had not been quarried and brought from elsewhere, but was a convenient natural sarsen in the immediate vicinity of Stonehenge that had been incorporated into the monument – in fact, the team’s analysis indicates that it, too, probably came from West Woods.
And what about the two anomalous sarsens, Stone 26 and Stone 160, which were found to have unique geochemistry within the monument? Interestingly, both stones occupy prominent positions at the northernmost points of their respective arrays – Stone 26 is the northernmost upright in the outer circle, while Stone 160 is the lintel of the northernmost trilithon. Is this a coincidence, or a sign of something more significant? In their closing discussion, the researchers wonder if multiple communities might have worked on the construction of Stonehenge – a similar theory has previously been suggested for the digging of the various segments of ditch surrounding the monument – perhaps each sourcing their materials from a different location. If this is the case, it is possible that some of the estimated 28 sarsens that are now missing may have shared a geochemical signature with the two outliers. For now, though, their source outcrops remain unknown – one more mystery for future Stonehenge studies to solve.
To read the full study, see ‘Origins of the sarsen megaliths at Stonehenge’ by D J Nash, T J R Ciborowski, J Stewart Ullyott, M Parker Pearson, T Darvill, S Greaney, G Maniatis, and K A Whitaker, Science Advances vol.6, no.31: https://advances.sciencemag.org/content/6/31/eabc0133.