No Lancelot here, and technically this should be the 'sword ON the stone', since it is chiseled on the surface, not embedded. Anyway, I've passed this marker lots of times, but have always wondered what it is. I've googled, but to no avail. I have a reader who very, very often writes and enlightens me (Knez, are you there?), so maybe he can help if he's reading this.