My guess is that many sites like Stonehenge are all much, much older than we now believe them to be.
"Excavation near Stonehenge found evidence of a settlement dating back to 7,500 BC, revealing the site was occupied some 5,000 years earlier than previously thought."
discovery.com/history/archaeology/stonehenge-settled-5000-years-earlier-than-thought-13041.htm#mkcpgn=rssnws1