I've never been to the Pikestones - I've found a similar one (perhaps) but with the mound still intact (and a closed up entranceway) - behind what-used-to-be Halliwell Bus Terminus. And I used to think the spiral carving was original - I couldn't understand why someone would go to that trouble to make that mark on the stone.
At Rivington the Blackamoor Head has been nicked from the Great Barn and is hypothesised as having been stolen to order. There should be another carved head very nearby, beside the restaurant, called Th'head'in'th'wall. It would be good to know that's still there - I tried to phone the warden, Hazel ?, but couldn't get through.
The newly found grooved rock, posted on the Pikestones page, is a great find and 'entirely consistent with similar specimens in Smithills'. (Allegedly). Dave Lane may have seen the vandalism occurring, from his microlight, and is worth an enquiry, perhaps. Better the rock getting bashed by a hammer than someone's face. I can use all this crazy energy at my worksite to move a wheelbarrow - theoretically !