Well there is very little written about them and no serious study has taken place because no one has a clue what the hell they were for.
There is the old folktale that neolithic Shamans made them because they could see into the future to a distant time when a young man would arrive in Avebury and collect all the research material into one place and store it by means of magick on a silver disk (Priced £18)
:)
PeteG