Ah, nice one. I wouldn't have thought of that. Even the parallel/equal size groove thing would be accounted for by them taking up the same position further down the boulder.

I was just thinking though - if you look at the shot taken from directly above, the whole rectangle seems defined. Less emphatic along the edges of the stone, but there nonetheless. My original assumption (along Hob's lines below[thanks, Hob]) was a modification for Mass, perhaps to separate a 'sacred' area. I wasn't brought up in the tradition so I'm not even sure what equipment gets used, or how desirable it is to give it special placement.

So, I suppose, what I'm really wondering is; have you ever come across any monument, with a tradition of re-use as a mass-rock, that has been modified in any way in the process? You've probably seen more ancient sites than anyone else in the country... felt tired just typing that ;)