The TMA Eds have received an email from the owner of the land on which this rock art lies. It is not on the land of Argrennan Mains, as may have been suggested below. He seems to be a friendly and accommodating chap and says he is happy to show the rocks to anyone who is interested, and he will be happy to escort you to them; indeed he says that the marks recorded for Park of Tongland are in fact some distance from the ones these photos show.
He would like to stress that he does not want people entering the fields on their own and pulling the vegetation from the rocks, exposing the marks to the weather.
If you want to visit this site, please have the courtesy to call at the farm first. Indeed this method applies to all sites on private land.