There are only a few stones left at Avebury now compared to its time of construction.  After doing a little research I have found out the missing stones were either broken up by local Christian authorities in the 14th century as part of their effort to remove vestiges of pagan religious practices, or they were removed and broken up to use in the construction of houses and the cleared area used for crops. Where the stones once stood there are now markers to show where they were positioned. It is rather sad so many were removed.