A triangulation point, used by the Ordnance Survey for mapping. They stopped using them several years ago (using GPS mapping now), and many were removed, but a campaign to keep some as a reminder of times past, means that there's still a few dotted around the landscape. This is one of the remaining 'trig points', and they seem to draw walkers (seeking the high points), such that any visiting space alien might conclude that they held some religious significance for people.

Not exactly a rock, but a stony concretion nonetheless, so a bit of poetic licence with this one.