Silbury Hill is Europe's largest prehistoric artificial mound and a key feature of the Avebury World Heritage Area.
A brief introduction
Silbury Hill is probably the world's largest man-made prehistoric mound. It is one of the most intriguing sites in the prehistoric landscape of the World Heritage Site, especially as we do not know its purpose or meaning.
We know only that Silbury Hill was built in the late Neolithic period, around 2,400 BC. It stands 30 metres high and 160 metres wide and its construction would have involved around four million man-hours of work. Half a million tonnes of material - mostly chalk quarried from the surrounding area - were used to create it.
The enduring presence of Silbury Hill in its landscape has inspired myth and legend as people sought to explain it. In one such story, Silbury Hill is the burial mound of a mythical King Zel and his horse. The mound is also associated with pagan beliefs and mysteries.
While we don't know why it was built, we do know it was during a time of great change in the prehistory of Britain, with new forms of pottery and the first metal-working.
Silbury Hill was not conceived as one monument. It was built up over time to form the structure we see today and would have been a special gathering place for the local inhabitants.
Recent geophysical survey work has shown that Silbury Hill was at the centre of a Roman settlement, straddling the now nearby A4. In the medieval period, the top of the hill was probably flattened and a building (possibly defensive) was constructed on the summit.
The full story of Silbury Hill and its astonishing archaeology is still being discovered.
The need for conservation
In June 2000, a large hole suddenly appeared on the top of Silbury Hill.
It was soon clear what had happened: the head of an old excavation shaft, dug in 1776 to penetrate to the heart of the monument, had begun to collapse. English Heritage immediately embarked on a programme of research to understand why this had taken place and find out if the situation was likely to get any worse. As soon as the nature of the collapse was understood, work began to arrest it and return the ancient monument to its former glory.
Archaeological examination of the freshly exposed deposits showed that the top of the shaft had slumped and been filled up again several times before. In other words, the collapse was a recurring problem. English Heritage began a multi-disciplinary programme of investigation to determine the reasons for the collapse and to test the overall stability of the Hill. The investigations included documentary research, detailed surface mapping and analysis, geophysical survey (seismic topography) and geotechnical investigation (coring), as well as excavation-style recording.
We discovered that Silbury Hill is a very robust structure with no major defects that might threaten its overall stability, due largely to the way it was originally constructed 4,400 years ago. Two problems were, however, identified.
Firstly, subsidence within the 1776 shaft had occurred because it was only partly backfilled, with the lower part being left open. Secondly, tunnels excavated from the base of the mound to the centre of the hill had also not been properly backfilled.
It was known that the tunnel excavated by the Reverend Merewether in 1849 had never been backfilled. A new discovery was the fact that, contrary to assurances given at the time, a tunnel excavated in 1968 by Professor Atkinson and the BBC was inadequately filled and consolidated.
What we discovered during this process
Shortly after the collapse at the top of the shaft in June 2000, English Heritage's Landscape Investigation Team undertook an analytical survey of the earthworks of the mound and its surrounding area and made some intriguing discoveries.
The first recorded archaeological investigation of Silbury Hill commenced on 31 October 1776. Identical accounts in at least three journals of that date suggest that the 'excavations' began in a blaze of publicity, although somewhat overshadowed by events across the Atlantic in the American War of Independence:
'Silbury-Hill, the largest tumulus or artificial mound of earth in this kingdom, supposed to be of between 3 and 4,000 years duration, was begun to be opened by the miners of Mendip, on Thursday last. They have made a hole at top of eight feet square. The Antiquarians promise to themselves wonders from the bowels of this mountain!'
The statement did not name the protagonists, nor was there any further correspondence from them, but an account written 17 years after the event by the Reverend James Douglas names those in charge as the Duke of Northumberland and Colonel Drax. Miners were employed to dig a shaft through the centre of the mound, but this shed little new light on its function. Like most mineshafts of the time, the 'excavation' was almost certainly not filled in properly when work was completed, leading eventually to the collapse in 2000.
Observations in 1849 by John Merewether, the Dean of Hereford, indicate that the spoilheap was still visible then. Almost 75 years later, an early air photograph taken by Major Allen reveals that the depression left by the shaft was still visible in the late 1920s.
Careful examination of the surface of the mound revealed that traces of the spoilheap surrounding the 1776 shaft are still visible. Traces of later excavations can also be seen, including the position of the well-documented tunnel dug along the old ground surface to the centre of the mound in 1849 by Merewether and others, and subsequently re-investigated by Richard Atkinson and the BBC during the late 1960s.
The tunnel appears to have collapsed and its position is now marked by a linear depression on the surface. More depressions and scars mark the location of excavations carried out by the Wiltshire Archaeological and Natural History Society in 1866 to demonstrate whether the nearby Roman Road formerly ran beneath the mound, and where Flinders Petrie investigated the possibility of an entrance existing opposite the south-east causeway. The position of Atkinson's 1960's excavations on the uppermost ledges can still be detected as slight depressions - the first time that these excavations have been accurately located.
Modelling of the surface earthworks using Global Positioning System satellite mapping equipment together with more traditional survey techniques, resulted in a series of new plans of the monument. These not only allow the mound and its surrounding area to be digitally modelled, but also ensure greater understanding of the development of the monument in its landscape context. The survey suggests, for example, that the mound is not in fact truly circular: on the summit it appears to be more angular than circular, while at the base it is almost octagonal in form.
The Landscape Investigation Team's new analytical survey suggests that Atkinson's theory of tiered construction need to be modified and that the so-called 'steps' could in fact have been a spiralling ledge. If one follows the surviving uppermost 'step', one actually returns to a point a few metres lower than the starting point.
Caution is required with this interpretation, but if correct, it might make good sense in terms of access routes during the construction of the monument, and perhaps ritual processions to the summit. It might also link in to the preoccupation with spirals in Neolithic art. Another ramp leads from the next-to-highest ledge almost to the summit. While this could have been an original feature, caution is required here too, since it could equally have been a later construction: perhaps a ramp providing access for the miners in 1776.
After the Neolithic, the mound evidently continued to be regarded as an awesome monument, though it would be long before it became the subject of 'scientific' study by archaeologists. There were attempts to harness its gigantic size for ornamental purposes. A low bank around the edge of the summit is a tree planting ring and a fairly deep depression within it could be the result of the planting of a tree known to have taken place in 1723. This episode was reported by the great early archaeologist William Stukeley as the place where 'workmen dug up the body of the great king there buried in the centre'.
A number of deliberately constructed platforms were discovered around the lower slopes. These cut into the original profile of the mound and therefore post-date its construction. One particularly prominent example faces north up the valley towards Avebury. A second platform is situated on the south-east slope in a position where, if Silbury was comparable to other large monuments of this period, such as the tombs at New Grange in Ireland or Maes Howe in the Orkneys, one might expect an entrance.
However, the proximity of a Romano-British settlement immediately east and south of the monument, together with the Roman road that is aligned on it, indicate considerable activity at that time. Much pottery and other artefacts of this period have been found on and around the mound in the past. Indeed, the monument itself may have acted as a focus for the settlement and it could have been considered as sacred in the Roman period as when originally constructed.
It is almost inconceivable that the Romans did not leave their mark on the monument, and these platforms around its fringes could easily be be attributed to this period. The presence of considerable amounts of early medieval pottery in the trenches excavated by Atkinson on the upper ledges indicates that there was significant activity during that period too. A newly identified enclosure situated to the south of the A4 might date to either of these later periods.
The present pathway up the mound is a useful archaeological indicator, for its position has not changed at all since John Aubrey - often regarded as the founding father of analytical field survey - illustrated it during the 1660s. All features that the path overlies or cuts through must therefore be earlier. One earthwork that the path post-dates is a broad but low bank that rises from the terminal of the western causeway almost to the summit. It is conceivable that this marks the line of a hedge that once subdivided the mound, but it could represent an ancient route to the summit - perhaps even the remains of a flight of stairs.
Around much of the circumference of the mound, the original Later Neolithic ground surface is visible as a slight change of slope, now standing several metres above the present base of the ditch. Beyond the ditch, on all sides of the mound, there are remnants of the ancient chalk ground surface.
Our analytical earthwork survey and investigation of Silbury Hill and its surrounding landscape has for the first time placed the monument within its landscape context. Digital modelling of the surrounding valley floor not only serves to emphasise the enormous size of the mound, but also its lowland setting, on the very edge of dry chalk and immediately adjacent to water.
Whether this water flowed freely or intermittently during the period of the monument's construction and use is not known, but the importance of the drainage system to the construction and symbolic meaning of Silbury is undeniable. Given this, it is perhaps surprising that there has been so little investigation of the ditch. For just three days in early summer as the water-filled ditch dried out a huge vegetation mark, some ten metres wide, and indicative of a substantial subsurface feature, appeared extending across the floor for some 50 metres towards the mound. Clearly Silbury still holds many secrets in store for future generations.
The conservation project
English Heritage commissioned Skanska Civil Engineering to undertake works to stabilise Silbury Hill.
The archaeological element of the Silbury Hill Conservation Project was managed by Fachtna McAvoy between 2000 and 13th September 2007. From the 15th of June 2007 the archaeological work was directed by Jim Leary. Sarah May was the Project Manager between September 2007 and November 2008, and Brian Kerr was the Project Executive during this period. After November 2008 Jim Leary took over Project Management and Sarah May became the Project Executive. The Silbury Hill Conservation Project was under the overall project management of Rob Harding during the whole period and Amanda Chadburn was the Inspector for Ancient Monuments.
Atkinson's 1968 tunnel was re-opened, giving archaeologists a final opportunity to record the interior of the Hill. The sides of the tunnel were cleaned and recorded using high-resolution photographs. Environmental samples were taken from archaeological deposits and a remote-controlled filming vehicle was used to record inaccessible areas. Excavations also took place on the summit.
All the known voids inside Silbury Hill, and the crater on the summit, were then re-filled with 1,465 tonnes of chalk. The mound has now been restored to as near its original condition as possible.
With the stabilisation work complete, specialists from English Heritage and seven universities began analysing the excavated material from the Hill. This includes the study of flint and antler tools, and further analysis of biological remains such as insects, pollen and snails. Radiocarbon dating of the archaeological material will allow the various episodes of construction to be specifically dated.