Very true...

cupmarks are the hardest carvings to date, unless you find some other associated finds nearby. If you have a carved cup and ring stone inside a burial, then you can date it,but cup marks on exposed bedrock is obviously something that is a little more difficult to date, but you can still tell if made by man, especially when you see peck marks....

(let the comments by Hob, Fitz and Stonelifter commence)...