aye, you're definitely not alone in your thoughts Mr H. Where you have just peckmarkless cups it’s almost impossible to tell, unless they're forming a lovely pattern of course. Theres enough panels about where natural fissures & planar erosion have been incorporated into the design, so why not natural cuplike depressions also. Such features may have been the precise reason why they chose the rocks, or decided to carve close by in the 1st place.
Which epoch they belong to is an interesting question, as wolfy says the general feeling is they belong to late neo – early BA, because of the position of some panels next to dateable evidence found in some burials. You also find cup marked rocks incorporated into walling (bronze age, IA etc), so from this its presumed it belongs to an earlier period. Theres allsorts of legends attached to cup marks going back to at least mediaeval times. So some may well have been carved then, or even by naughty Victorians trying to exploit the new fashion of tourism.
I also happen to think that some carvings go further back than the periods designated. No real evidence for it, other than a feeling I get from the erosion patterns of certain designs.
