Cosmogenic dating

What's this?  A picture of an exploding galaxy a thousand light years away, taken by the Hubble telescope?  No, it's a close-up of the surface of an exposure of spotted dolerite at Carn Meini.  If you look closely at the rock surface you will see that there are at least four different surface textures.  In the centre of a slight hollow there is an iron-stained area, then outside that an area where the rock looks remarkably fresh and clean, then outside that a darker coloured weathered surface, and then finally patches of lichen-covered rock.  How do you sample for cosmogenic dating on rock surfaces such as this?  I was chatting to my old friend David Sugden about this a couple of weeks ago, and he confirmed how difficult it is to get reliable dates, given the huge variations in cosmic ray bombardment even within small areas such as this.  Standing water, temporary lichen growths, intermittent snow banks, intermittent peat or grass cover, spreading heather and gorse -- they can all protect a rock surface for decades, centuries or millennia from cosmic rays.  No wonder that the cosmogenic dates obtained by Danny McCarroll and colleagues for North Pembs seem to make no sense at all.......