(oops. forgot to include a link)
This article:
The souls of Silbury Hill are bared in burial mound dig
speculates on the origins of the hill:
" Silbury Hill, researchers believe, could well have been built as a sort of spiritual tomb, filled with spirits rather than skeletons."
However, the end of the article provides a clue that points to a different explanation:
"Until the 19th century, the linkage between the Kennet river and Silbury was reflected by an annual local ritual in which water was collected from the main source of the river – the Swallowhead Spring, 200 metres from the monument – before being taken to the top of Silbury where it was mixed with sugar and then drunk. "
Water (as we are being reminded here in Atlanta) is a precious resource. While primitive cultures wouldn't do anything as stupid as waste billions of gallons of water on some unwanted mussels (/rant), building reservoirs or water towers would have been difficult. If your water source is subect to seasonal variation, how can you use it to even out sporadic rainfall?
The Wikipedia article on Silbury refers to Atkinson:
" He argued that the hill was constructed in steps, each tier being filled in with packed chalk, and then smoothed off or weathered into a slope."
The Wikipedia article on chalk contains this interesting bit:
"Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons. ."
If you can't build a large hollow container to hold water, maybe the next best thing is a pile of chalk. The Sarsen stones might have been just for structural reinforcement.
It is difficult to completely justify this in economic terms without a better picture of the climate at the time. However, the modern day British apparently have a reservoir about 1k to the south of the mound, and another to the northeast. The area appears to be farmland. An irrigation ditch could have been used to bring water to the base of the mound from the spring (the upper levels of which appear to be at a higher elevation than the mound). Lacking pumps, a bucket brigade would have been needed to carry water to the top of the mound (as depicted in the ritual).
Water seeping from the mound would have provided some ground water for the surrounding area. Irrigation ditches could also have been run from the main ditch around the mound. The ancient builders at Silbury piled 248,000 cubic meters of chalk on top of an existing hill. This translates to 65 million gallons of chalk, capable of holding a significant (unless you're a mussel) amount of water.
Interestingly, the modern day English are extracting water from chalk in the ground rather than build chalk mounds:
The Importance of WaterAccording to:
The Absorbency of Rock
Chalk can absorb 32% of its volume in water. So the 65 million gallons of chalk added to the original hill could hold about 20 million gallons of water.
There are 2 different ways the reservoir could be filled:
1. Precipitation
2. A bucket brigade from at least the bottom of the hill.
Precipitation seems the more likely. This suggests a climate with a wet season and a dry season (as opposed to a country that exports raincoats).
A mound such as this would not necessarily have had a 15 year plan. Rather, past experience would have led the village founders to look for a small hill with reservoir potential. Then a ditch would be dug to trap water seeping out of the hill during the dry season. Then the hill would be incrementally enlarged in hopes of increasing its capacity. At some point, diminishing returns or a change in climate would cause construction to stop.