Don't know about Avebury but I do remember speculation that there was a broad, shallow moat around Silbury Hill. The speculation concentrated on the reflection of the still white mound adding even further to its spectacle.

Just remembered. It was Adam Thorpe in On Silbury Hill. Which you are all now aware of.

Don't know if this is an accepted theory, something their is evidence of, or another example of Mr Thorpe's diverting speculation.