Unfortunately I don't have any pictures. I should take some next time I go back, at the end of this year. Might be worth re-evaluating. Having that said, I do recall several of the carvings well, and they had a distinctly different style compared to others in Tanum.

As for lichen growth, I couldn't differentiate the growth inside the carvings from that around them, so to 10-year-old me the carvings certainly appeared old.

But as an adult, I'd guesstimate that in that area, only ~100 years of lichen growth is enough for carvings to blend into the surrounding rock.

Edit: I am not sure if an lichen study was conducted.

Oh, that is fascinating, I'd love to hear you went back and got a bit more of a serious response from archeologists than your 10-year old version did. ;)