Personally I suspect that upturned turves were used continually as an outer skin throughout the building process, mainly to stop Silbury becoming a quagmire and melting like an ice cream when it rained. Something that only came to light recently, although clearly found in 1968, was mention by one of Atkinson's workers of basket weave marks in the chalk on the slopes at the base of Silbury. At first I thought this might be imprints left from carrying baskets, but I have decided to track down the gent concerned as there are no photos of this in the At files, and these marks if longitudinal, could be thatch matting. On completion the turves were turned over so in all I wonder if Silbury was ever white as such because it may never have been seen without its coat.

What suggests this more than anything is the way Silbury deals with weather. Not only quite remarkably it has survived in shape despite all that nature over millennia and archaeos have thrown at it, it still dealt with the weather even when EH made cock-up after cock-up in dealing with the cover of the open shaft.
The flora and fauna monitored over 150 years show that Silbury takes on water but filters and dispenses it, and collects it in the ditch, not as efficiently as it once did perhaps. The thing is is that today this water at the base is just part of the table, but it is plausible that the ditch was once puddled to keepthe water from Silbury entirely seperate from Swallowhead. Silbury being an island and without EH sheep, the water would be free from cryptosporidium. So not only when the winterbournes and the Kennet dry-up did Silbury have water, but it didn't back then give humans the shitty-pukes.

Can't recall where I got it from, but I was interested to learn that Swallow is a term adopted for springs where the water hasn't sufficiently penetrated the ground for long enough to be cleansed of bugs. Especially in sheep country such as the downs.

VBB