If it were the same person perhaps you might expect them to bring a chisel or two, rather than just use a rock. There are a lot of people on the moors round there - it's a popular place to walk. It's a mystery and I've never even visited the site. There are a pair of cairns, the Two Lads, a couple of miles to the east, and they are Scheduled. The local paper had a competition, some time ago, over whether they should be left as they were or altered. The altered camp won and now there are three - and they look terrible. There seems little chance of them returning to their original intent. The fern - y design on the rock at Pikestones was very well done and 'congruous'. Late rock art ! It's not surprising someone wants to smash it.