The circle of bluepeeps dates, as shown in the photo above, by some estimations to the third millennium BC. It is thought that the altar-peep was put in place during this era.
Romantic persons are inclined to the belief that the Druids built PeepHenge, and despite this historical inaccuracy, sacrifices are carried out yearly at the summer solstice by modern-day impersonators and make full use of the altar-peep.
http://www.lordofthepeeps.com/peephenge/peephenge.html