http://www.themodernantiquarian.com/post/58001/avebury.html
is the picture that came to my mind.

Also I seem to recall someone (ah one of those antiquarians) saying it was 'hac pen' meaning head of a snake, because they were determined it was druidic and connected with snakes?? (or am I misremembering) and the snakey shape of the avenue would make the circles on the hill the head. 'Pen' means head / top in welsh. Not quite sure what they were intending the hack bit to mean though.