I've said before that, just because something seems impossible by human means, even primitive humans, it doesn't mean we need to resort to alien visitation, the modern scapegoat for a number of hard-to-explain phenomena.
I found this video today. While it's not Stonehenge, it's impressive, and with more experience, and more folks, it just might be Stonehenge.
Stop underestimating your clever, clever species.
Druids and aliens don't mix, anyway.