Journey to the heart of the hill
The excavations to the central chamber are a critical phase of a £1million conservation project to stabilise Silbury Hill. The project has seen English Heritage archaeologists and Skanska engineers re-enter the Hill through the doorway sunk into its side in the 1960s so that they can empty the tunnel before repacking it with chalk, to stabilise the Hill for the long term.
The project has resulted in an exciting new theory being put forward about the use and design of Silbury Hill. The iconic shape that we recognise today may not have been how the Hill looked when it was first built. English Heritage archaeologists believe the summit may have changed from a domed shape when it was constructed 4,400 years ago into the current ‘flat top’ hundreds of years later, in Saxon or Norman times.
Archaeologists have discovered a series of medieval postholes, one of which was very large, on top of the Hill, indicating a possible huge military building there, during the Saxon or Norman periods. They have also discovered two iron arrow heads which suggest the building had a military/defensive function, such as a defended lookout post or signal station. They now also believe that the summit of Silbury Hill was also significantly modified and flattened during this period.
Jim Leary, English Heritage prehistorian and archaeologist, said: “We believe the top of the Hill was literally ‘lopped off’ around the time of the Battle of Hastings or even earlier when the Danes attacked in 1006 to create flat land for use as a military base. The absence of Roman deposits, and Professor Atkinson’s discovery of 11th and 12th century pottery in the side of the Hill, all appear to support the theory that there was a fortified Saxon or Norman building on the summit.”
While investigations continue to test this new theory, archaeologists are taking advantage of a rare opportunity for detailed recording and investigation along the 85 metre length of the 1960s tunnel. It reaches right to the heart of the Hill where archaeologists are working nearly 40 metres below the summit beneath thousands of tonnes of chalk.
The tunnel cuts through each of the Hill’s three main construction phases, which experts believe were built quite separately. The interface between each phase is clearly visible in the tunnel walls. Silbury I, the oldest part of the Hill, was constructed by its Neolithic builders as a stack of turf with a capping of clay. Silbury II was built of piled rubble chalk very soon afterwards in around 2400 BC. Archaeologists currently believe there was a gap of a few hundred years between the construction of Silbury II and Silbury III.
The part of the tunnel which cuts below the original Neolithic ground level reveals the earth’s natural geology with bright white Cretaceous chalk which is millions of years old. This is overlaid by clay with flints deposited during the Ice Ages, and then the Neolithic ground surface. During excavations, archaeologists have found parts of antler picks as well as animal bones and flint cutting tools. These remains will help to date the three phases of construction much more accurately and could radically alter our understanding of the Hill’s purpose.
Dr Simon Thurley, Chief Executive of English Heritage, explained: “Until now, the best guess was that Silbury Hill could have been constructed over a period of anything between 100 and 500 years. When our archaeologists and dating team have done more precise dating work we will be able to narrow this estimate and understand better how and why this monument was built. A shorter construction period over a couple of generations might indicate it was a heroic piece of work led by one or two charismatic individuals. If it stretches over hundreds of years and many generations we can conclude that it was integral to a much more longstanding set of spiritual beliefs.”
Organic remains such as molluscs and insects have also been found. The unusual burial conditions within the centre of the Hill mean that they have been excellently preserved. With analysis, English Heritage experts will be able to create a complete picture of the Neolithic landscape including the type of vegetation, the climate, and how the land was managed by prehistoric people for grazing, arable and woodland use.
A chance find for the archaeologists was a time capsule buried at the back of the tunnel by the BBC film crew who made a documentary about the Hill with Professor Atkinson in 1969. The capsule contains three reels of film of the documentary, two enamelled badges with the Silbury ‘S’ logo, a 50p coin and various pieces of paperwork. It is hoped these items will eventually be placed in the Alexander Keiller Museum in Avebury.
The current programme of works has been more complex than first envisaged and will now run until December. Heavy rainfall in the summer made tunnelling conditions more dangerous and the old Atkinson tunnels suffered collapse caused by the unexpected discovery of heavily saturated clay within the central chamber. In response, a new tunnelling method had to be devised by English Heritage and Skanska to continue the excavations to known voids in the centre of the Hill. This method has involved replacing the Atkinson arches with new, much more secure mining arches.
Mark Kirkbride, Skanska project manager, added: ”The engineering challenges have been much more demanding than originally anticipated but we have overcome these safely and the work achieved is a testimony to the unique partnership of engineering and archaeological disciplines. It also clearly demonstrates that the conditions we have found in the centre of the Hill prove that these works were essential to ensure the long-term conservation of this important monument.”
This week archaeologists will finish their recording of deposits from Professor Atkinson’s tunnel and the central chamber.
The next step will be backfilling the tunnel and the known voids with chalk, sealing and stabilising the Hill for future generations.
To find out more, visit English-heritage.org.uk.