|I'd like to add my amateur opinion to the debate.
Like everyone else, I was quite excited by the idea of a newly discovered monument in the Avebury area. Having visited it though, I am convinced that it is purely a natural feature, formed by the Roman road cutting off a spur at the end of Waden Hill.
The roundness of the hill is unusual, as is the fact that the angle of its slope matches that of Silbury. However, both can be explained by its location.
The hill is on the edge of the flood plain of the River Kennet, as can be seen by the very flat ground at its base. This means that it has been eroded by the river over many, many years. This has had the effect of shortening the gradual slope of Waden Hill - the long tail of the hill has been washed away.
Any large scale erosion brought about by floodwater would cause a cliff of chalk to form, but this would fairly soon settle down into a slope. All soils have a natural angle at which they settle. This is called their 'angle of repose'. You can see this in action by pouring sugar onto a table - internal friction means that the pile will always settle into a slope of the same angle.
It so happens that the angle of slope of Silbury Hill is pretty close to the natural angle of repose of chalk soil. This is discussed in the English Heritage report on Silbury ('Silbury Hill', February 2005) which says that "Professor Chandler... considered the slope stability of the Hill and [noted that] it was close to the angle of repose".
In other words, whether due to design or because of erosion over the years, the slope of Silbury Hill is similar to that of a naturally eroded chalk cliff. That the slope of Silbaby is almost the same is no accident – it too follows the natural angle of repose for chalk – but due to the natural processes of erosion and slope formation rather than by the hand of man.
A similar angle of slope can be seen on the tree-lined banks of the Kennet by the Swallowhead spring, where the hill coming down from the West Kennet long barrow has been cut away by the stream.
Stukeley's print showing a 'barrow cut through by the Romans' is a bit of a red herring. Stukeley was clearly referring to a disc barrow – a small, banked Bronze Age mound – and not to a large hill. His engraving of the 'Avebury Serpent' shows this disc barrow clearly, together with the relationship between the Roman road and the tail of Waden Hill.
Overall, I'm sorry, but I'd say that Silbaby was a natural feature. Unusual, intriguing, and definitely worth protecting, but natural nonetheless.
On the other hand, I can't explain the alignment between Silbury, Silbaby and the Sanctuary…
Posted by Dunstan
17th March 2008ce
Edited 17th March 2008ce