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Submitted by Andy B on Wednesday, 20 February 2002 Page Views: 2760
Neolithic and Bronze AgeSurveys on Silbury Hill, the prehistoric monument near Avebury in Wiltshire, where the partial collapse of an 18th century mining shaft led to a hole appearing in 2000 at its summit, have revealed that the hill's structure appears to be stable. But more work must be done before this enigmatic chalk mound, built by our Neolithic ancestors 4,500 years ago, finally gives up all its secrets.Some fascinating details have already come to light. Archaeologists from English Heritage, the hill's guardian, have for the first time been able to make a digital model of the mound, the biggest of its kind in Western Europe. This reveals the possibility that it was built in a spiral fashion with a spiral processional way for ceremonial purposes. Chalk cores taken from the mound reveal evidence of the Neolithic 'building site' at its base.
A 3D seismic survey and other archaeological work were commissioned in the autumn last year by English Heritage to assess the condition of the hill and its interior. Several tunnels have been dug into it over the centuries, and English Heritage needed to plot them and any other holes before remedial work was contemplated. The collapsed 18th century shaft has already been filled in to protect the monument from further damage.
Dr Kevin Brown, Regional Director of English Heritage, said: "The results of the seismic survey are very encouraging as they have shown that the hill's structure appears stable. The survey has revealed, however, that a small part of a tunnel constructed near the base of the hill in 1969 has suffered a roof fall. It has confirmed evidence from surface surveys that there is a depression on the north flank of the hill which may relate to an area where the chalk has been disturbed. We are carrying out further tests on this area as soon as possible to discover the exact nature of this anomaly, and a section of the central area where the results have been inconclusive."
The seismic survey was carried out by Cementation Skanska, a leading international engineering company. Small diameter boreholes were drilled vertically into the hill and sound waves used to scan its interior for cavities and loose areas of chalk. The results were then evaluated by Cementation Skanska, English Heritage and independent experts. A team of experts will shortly be returning to the hill to gather more detailed information prior to preparing a full assessment of the hill's future maintenance.
Research has thrown light on how and when the mound was constructed and its subsequent history. Chalk cores taken from boreholes drilled down through the hill for the seismic survey and analysed at English Heritage's Centre for Archaeology at Fort Cumberland, Portsmouth, show what conditions were like when the mound was first built.
English Heritage archaeologist Fachtna McAvoy said: "We can see what is effectively a Neolithic building site at the base of the mound. The workmen were evidently struggling with wet ground conditions and churned up the land surface into a mixed layer of chalk and mud. We have also discovered that the mound when it was built was 31 metres high and that there were no long layoff periods during its construction."
Members of the public will be able to see the contents of the cores themselves as they have been digitally recorded. The pictures will be made available through the Internet after analysis and interpretation has been completed.
Using geophysical and surface survey techniques archaeologists have discovered that the mound could have been built in a spiral fashion, rather than terraced, as had been previously thought. While this may have been to aid construction it could also have provided a processional way to the summit. Neolithic art is characterised by its preoccupation with spiral forms. It also appears that the hill is not circular but has radial spines linked by straight lines rather like a spider's web. Platforms cut into the sides could date from the period when the Romans settled at the base of the mound and may have been used for monuments or altars.
David Field of English Heritage's Archaeological Field Investigation Unit said: "The digital model of the hill places it within its landscape. From this we can see how its lowland setting emphasises its enormous size and also how it has been placed on the very edge of dry chalk immediately next to water. We had indications of how important water was to the construction and meaning of the hill when the ditch at its base dried out and we could see signs of a substantial linear feature under its surface, extending for 50 metres. Silbury clearly holds many surprises yet."
A fragment of an antler pick found during excavations on the summit preparatory to the seismic survey provided the first secure radiocarbon dates for the hill itself of about 2490-2340 BC.
Background:
Silbury Hill, the largest artificial prehistoric mound in Western Europe, is from the late Neolithic period. Its height is 39 metres above present ground level, 28 metres wide at the top and 167 metres wide at its base. It is surrounded by a ditch from which chalk was dug for its construction.
No-one has ever been able to discover exactly why such an enormous monument was built (it has been calculated that it would have taken 3,000,000 man hours to complete it). No tomb has been found, though William Stukely wrote in 1743 that a skeleton had been discovered during tree-planting on the summit. The mound presumably has some kind of religious significance and forms part of the complex of ritual, funerary and ceremonial monuments which cluster round the village of Avebury and together form part of the Avebury and Stonehenge World Heritage site.
Silbury Hill was first given legal protection in 1883. It is in the Guardianship of the Secretary of State for Culture, Media and Sport and is managed by English Heritage. It is owned by Lord Avebury. It is also a Site of Special Scientific Interest because of its chalk flora.
In 1776-7 the Duke of Northumberland used miners to sink a vertical shaft down through the middle of the hill. Another horizontal shaft was excavated from the base of the hill to the centre in 1849. Further excavations took place in 1867, 1886, 1922 and 1968-70.
More: English Heritage (choose Heritage Briefing section)
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