In reply to Resonox's post , but I can't access to quote from ,so apologies .

The markings are much closer to the earlier style found in the Neolithic -Bronze age in Orkney than the much later Ogham . These markings are to be found on the "side" of slabs e.g. Brodgar Farm and lintels . We don't know the audience , if any , they were intended for . The markings could be apotropaic , intended as a communication between the engraver and stone or spirits .No need for the most obvious surface to be used , it is often the case in rock art that what appears to be a perfect surface to moderns is avoided in favour of something that to us appears much less useful . In this case , maybe the intended audience was a relatively contemporary one .