On YouTube circulate many documentaries about megalithic monuments.
Stonehenge (GB) www.youtube.com/...stonehenge...
Avebury (GB) www.youtube.com/...avebury...
In the seek bar on the YouTube-website you can enter names of other monuments to discover other interesting documentaries...
Alas "A History of Ancient Britain" and "Standing with Stone" where remove from YouTube for reasons of copyrights. These interesting documentaries are available on DVD...
"A History of Ancient Britain"
(Very interesting and surprising series made by Neil Oliver)
"Standing with Stone"