Silbury Hill: A last look inside

9 mins read

The inside of Silbury Hill before being sealedOn May 29, 2000, Amanda Chadburn, the English Heritage Inspector of Ancient Monuments for the South-Western Region, received an alarming phone call: Silbury Hill had collapsed…

A large hole had appeared in the top of the huge mound – the largest mound in northern Europe – and something needed to be done about it. At first Amanda was tempted to dismiss the report as being sheer alarmim, but it turned out that it was indeed true: a large hole had opened in the top of the monument and caving enthusiasts were already abseiling down and exploring the crumbling sides.

There have been three major excavations of Silbury in modern times, each contributing to the destabilisation of the mound that led to the recent project to restore it. In order to understand the current English Heritage work there, it is essential to note that it has been clear from the start of the project three years ago that it was intended as a conservation effort, rather than an excavation. English Heritage was on site at Silbury Hill not to dig, but principally to solve the issues caused by previous archaeologists. This priority has been apparent in every facet of the undertaking – in the methodology of intensive sampling, in the recording techniques, and most visibly in the plans to seal Silbury Hill forever. There has been much heated debate about the decision to close completely the mound in perpetuity, and as English Heritage site director Jim Leary says: ‘I feel the responsibility every day, when I’m in the tunnel. I realise that what we are doing inside is the last chance anyone will ever get and no one will ever go back in. It weighs heavily on my mind.’ Backfilling is scheduled to begin on 15 November 2007, when over 500,000 pounds of crushed and liquified native chalk will be pumped into the tunnels and void areas; Silbury Hill will be fully consolidated, never to collapse again.


Excavating Silbury

In 1776, the treasure-hunting Duke of Northumberland employed miners to dig a hole down from the top of the monument. He was disappointed in his quest, and it was the remains of his shaft that had reopened and caused the May 2000 cave-in. The second attempt was in 1849, when the newly formed Archaeological Institute engaged Dean Merewether to dig a shaft from the side. He also used miners to dig his tunnel, and though he reached the centre of the mound, he too failed to find any central burial. The third major attempt came in 1968-1969 when the young and enthusiastic Controller of BBC2, David Attenborough, approached Richard Atkinson (one of the foremost prehistorians of his day) to tunnel once again into the mound to reveal its mysteries. Atkinson reported that the mound was constructed in three stages: the first stage was a turf mound, which had been covered by a chalk mound, which in its turn was enlarged yet again to form the huge mound that we see today. He obtained five radiocarbon dates from the mound that suggested that it had been built in the Late Neolithic, between 2500 and 2000 BC. However, by the end of the second season, the programme came to a halt. What should be done with the tunnels that remained?

Atkinson believed that they should be left open for future generations to explore and revise his interpretations. Others were not quite so certain that the iron arches would stand up, so the Ministry of Works (the predecessor to English Heritage) decided that the main tunnel should be backfilled. The main length of the approach tunnel was backfilled with road stone, but the process was not completed throughout the tunnel system. The central portion, where there was a cruciform excavation, was left open. Eventually, the Ministry of Works was proven right when the iron arches buckled under the weight of the hill above and a complex system of fissures developed. In May 2000, the central part of the chamber collapsed and the earlier shaft fell into the central chamber and a hole appeared at the top. The hole was hastily covered, though this did not prevent various caving enthusiasts from breaking in and abseiling down the void – and thereby dislodging even more material from the sides. A blocking of white polystyrene blocks was then inserted, and Skanska Engineering were called in as consultants. Since then, with the expenditure of over £1 million, the interior of Silbury has once again been investigated.


English Heritage gets stuck in

The most recent work started in May 2007 with the opening of the original tunnels. The portal of the original Atkinson tunnel was soon discovered, with an inscription of 1968 on the lintel. Progress was slow to begin with as archaeologists excavated the ditch around the Phase 2 mound, which had been backfilled when the mound was enlarged in the final Phase 3. This went down some three metres below the tunnel floor, but since the tunnel was already below the old ground surface the ditch must have originally been a fairly massive six metres deep.The only significant discovery was of an antler fragment from the middle fills, which was avidly seized upon for radiocarbon dating. Soon, and alarmingly, they began to find voids in the roof that had been opened up when the tunnel began to fall in. To explore these they brought in a Rovver, which proved admirably suitable for exploring the voids, winding its way in to explore the extent of movement within the hill caused by the stresses of the deteriorating tunnel.

In the last week of July 2007, there was a major collapse. Heavy rains had percolated through the porous chalk and destabilised it, causing more voids to open. Already there had been two specific zones of voiding and when they reached the Atkinson chamber, a significant collapse zone of rubble chalk was encountered and tunnelling was temporarily suspended.

The collapse caused a pause while the engineers worked out what to do. When the tunnel reached the turf mound at the centre of the hill, it widened out to form a chamber. This had never been backfilled, and it was here that the pressures were greatest. It would appear that the original Merewether tunnel of 1849 was present in a void above the Atkinson chamber and that this was left unsupported in 1969 -1970. Thus a gap was formed. A method was devised of inserting a specialist foaming resin to stabilise the rubble chalk in the immediate roof area, to prevent any further collapse and allow the tunnelling to proceed in safety.

Eventually the tunnellers reached the heart of the original turf mound; the archaeologists marvelled as their predecessors had done in 1968 at the excellent state of preservation of the organic materials that made up the original monument. Owing to the pressure of the overlying mound, the vegetable matter was perfectly preserved and the grass was still green. The extremely high level of preservation in the centre from the Late Neolithic period is unique, and archaeologists were able to confirm the original observation that the turves were not just cut from the footprint of the monument but would have been cut from a wide area including chalk grassland.

There were few finds from the excavations; however, it is clear that the development of Silbury Hill was a complex process. Silbury Hill does not just fit into three simple phases; there are many phases, the complexities of which are only now beginning to be untangled. Pits, stakeholes, and even a small satellite mound have also been recorded within Silbury 1. The site report from English Heritage is eagerly awaited; though Atkinson cannot really be blamed for not having completed the backfilling o
f Silbury Hill as it was not truly his responsibility, he can be blamed for never having published a complete record of his excavations there (although Alasdair Whittle made a valiant attempt to publish his fragmentary archive in 1997). Because of this, there are many of eyes on the data that will be presented; English Heritage is conscious of the waiting public.

The 2007 Silbury Hill Conservation Project has quite literally recorded every single inch of the tunnels, and a fascinating ‘virtual reality’ reconstruction as well as extremely detailed 3D sections have been created using photogrammetry. Over 400 samples have been taken, including a series of cores from the very centre of the original mound, with many of them earmarked to be frozen for study by future generations and their more advanced technologies.


Dating Silbury Hill

The main new evidence we can expect from the recent work at Silbury Hill will be in the dating. Already the preliminary results, when combined with Bayesian statistics (CA 209), have enabled the dates to be made considerably more specific and firmly place Silbury in context with the surrounding UNESCO World Heritage site monuments.

The latest thinking is that the original turf ,mound was probably constructed within a century of 2400 BC. This was a pinch-point of prehistory, when the Beaker People were arriving and the first stone Stonehenge was being constructed. However, there continues to be some dispute as to when the third and final phase was constructed. One interpretation suggests that the third phase may have followed quickly after the second, and it could all have been completed by 2400 BC. However, a clearly apparent standstill phase after Silbury 2 indicates that there may have been a pause of perhaps a couple of hundred years, making the final phase of construction as late as 2100 to


The future of Silbury Hill

The 2007 Silbury Hill Conservation Project has come to an end; here, curatorial philosophy has gone from one extreme to the other. Whereas Atkinson left his arches in position and wanted to leave the central chamber open, the current plan is quite the reverse. Silbury Hill will remain a wondrous monument in the landscape, untrodden by human foot.

However, that does not mean that there is no further opportunity for study of the site. Geophysics instrumentation has been installed under the hill in the natural chalk, including a Time Domain Reflectometry sensor (TDR) and an Electric Resistivity Imaging Cable. The TDR is laid under the most sensitive part of Silbury 1; and transmits a pulse along the cable, determining the soil moisture from the travel time and amplitude of the reflected energy. The Electric Resistivity Imaging Cable has steel electrodes every 0.5m out to 64m from the centre of Silbury 1; an electric current may be sent from a portable resistivity meter brought to site for the measurements, through any combination of buried electrodes. This current passes through dissolved ions in water (not chalk itself) and thus provides another measure of the variations in the moisture content. More voiding inside Silbury would, presumably, create an air space – obviously, lacking water – which would in turn show up in the readings as a high resistance anomaly and enable staff to respond before further damage can occur. Neither instrument is preventative, but it is hoped that the measurable ,results (taken at regular intervals for the foreseeable future) will enable archaeologists to continue to ‘see’ into Silbury Hill; and, as technology moves on, it is anticipated that further functions will be developed for the electrode array that will make it possible to get a clearer picture of what is inside the hill. We will never know everything, but what we do not know should diminish as time goes on. 

Many questions remain to be explored. Foremost among them is: why? What was the purpose of Silbury Hill – what did it represent, what function did it perform, what was its meaning to the people who went to such great efforts to build it? Folklore places it as the burial mound of King Sil; other theories attach it to the Goddess, and still others propose that it was an observatory or sundial – by means of the shadows cast by the mound on the plain to the north towards Avebury – used to track the seasons. It is certainly tied to the other monuments nearby through sight lines and other alignments, and Silbury is an integral part of this constructed sacred landscape. Perhaps the mound was aping a natural hill: Jim Leary points to the similarity of the final phase of Silbury Hill with Picked Hill – a prominent hill in the nearby Vale of Pewsey. In a flat area like the Vale of Pewsey, a feature such as Picked Hill would have stood out and may have had symbolic value – was Silbury an attempt to emulate it in the Kennet Valley? Jim also points to the Hill’s lowland setting and proximity to a river and spring. These are important features in the landscape and may well have been sacred for generations of earlier inhabitants – was Silbury marking them for all to see and ensuring their sanctity was never forgotten? Was the mound a response to the influx of new Beaker ideologies, materials and know-how- a way for the local population to assert their identity? The concentration of different local materials – clay, gravel, chalk, turfs, topsoil, and even small sarsen boulders – found in Silbury 1 suggests that the builders were bringing their landscape into the construction; was this a way of focussing their landscape and creating a microcosm of their world in one place?

There are plans to venture beyond previous discussions of Silbury and consider the later uses of the hill. Atkinson reported finding pottery from the Late Saxon/Norman period on the hilltop, and the present excavation found further evidence of this activity. One of the most exciting new theories to have emerged from the project is that the hill’s iconic shape may be the result of significant modification by the placement of a large Saxon or Norman military structure on top; archaeologists believe it may have originally been domed, rather than flat-top as we see today. If so, where was the top of the hill dumped, and what could possibly be found in that material? There will be work done on locating the source of the materials used in the primary mound; were the Pleistocene gravels dug up from deep underground, or exposed in a river valley? Additionally, no evidence of the large workforce that would have been needed to construct Silbury Hill has yet been found; was it a shared workforce with the other monuments in the surrounding area? It is interesting to consider a segregated population, with lumberjacks cutting the forest of trees needed to construct the West Kennet Palisaded Enclosure while stonemasons banged away at Stonehenge and Avebury and chalk quarriers got down to business at Silbury Hill. Certainly, better dating will start reducing the possibilities, especially as more information trickles out of the Durrington Walls excavation. ‘Eureka’ moments tend to happen in post-excavation, so it is possible that stunning results could be just around the corner.


Source: Jim Leary at English Heritage


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