Rock markings

I've recently returned from a short break in Northumberland where there are huge numbers of cup and ring rock markings. I looked at the Ordnance Survey map (Berwick on Tweed, can't remember the number) and there were hundreds within a few miles of where we were staying near Wooler.
Like a lot of low-key prehistory they tend to be in wild and wonderful places. There doesn't seem to be a consensus view on their purpose (it seems improbable that they are the results of idle doodling, given how long they must have taken). I quite like the idea of a kind of neolithic graffiti artist, leaving his (probably) tag in the local area, buidling up respect, notoriety and kudos. I doubt if there's any evidence to support this.
There are some fab photos at Julian Cope''s brilliant Modern Antiquarian site.