We had a post in the winter from the blog of archaeologist Mike Pitts which showed the ditch around Silbury Hill filled with water following the high levels of rainfall in 2012 – and a replenishment of the chalk aquifer – see http://mikepitts.wordpress.com/2013/01/12/silbury-hill-flooded/
In the July/August 2013 issue of British Archaeology last year's wet weather at Silbury was a bit of an eye opener as the flooded fields and ditch allowed them to see how things may have looked when it was built. Modern water extraction from the chalk aquifer has tended to distort the flow of water from springs that feed the River Kennet – such as the Swallowhead. Other small streams flow into the Kennet or nearby and these have been largely dry for a number of years. Ground water, it is estimated, was at least 5m higher in the past, and this fact was brought home in 2012 as springs and streams that had not flowed for years burst into life – precipitation was so high in 2012 the aquifer was replenished and overflowing, and the ditch around Silbury Hill filled with water.
The Kennet is a canalised river, managed to stop it flooding. It was formerly a slow moving braided river system in the Avebury area, a complex of a number of monuments, Silbury Hill being one of them, but the henge being the dominating monument. Silbury Hill sits on a confluence of two streams, the Winterbourne (winter stream) and the Beckhampton Brook. The latter rises from springs that feed a pair of large ponds and only when these fill up does the water move on towards Silbury. However, springs actually feed the ditch independently of the stream – springs that had not bubbled for years. He recorded, in January last, the positions of 82 springs around the western ditch, and another 7 along the eastern ditch. The area, he notes, is literally a sponge. The inference to be drawn, it seems, is that the area around Silbury was chosen because of its very springs – and these were incorporated into the monument. Water seeping out of the ground could have made the site important, it is conjectured – and the term sacred is launched. However, chalk streams are not particularly novel, as it is the result of the geology, water seeping through the chalk until it meets another layer – such as clay, and coming to a stop, and emerging often along the scarp edge, but in reality at any geological impervious point in the chalk. I can think of several places in the Chilterns where this happens, and rivers form and springs bubble – but there is no sign of a Silbury Hill like structure.
The author of the piece then describes what happened in January when there was some snow and frost. After a couple of days all the areas associated with a spring cleared up as the spring water was warmer than the snow – as it had an underground origin. Grassy patches emerged within the snow and around the western ditch there was a green strip a metre or so wide – and the same grassy patches were a feature of the Swallowhead, Pan, and Woden springs which feed the Kennet. There has never been a hydrological survey of the Avebury area in spite of growing evidence of a connection between moving water and Neolithic monuments. Steve Marshall, the author, has a book out later in 2013, on Avebury.