Rock carving facts
tanumworldheritage.se
tanumworldheritage.se
Growing up on the countryside during the 80s meant spending a lot of time outdoors, in my case climbing the mountains surrounding our village, looking for adventures.
During one of those climbs, around the age of 10, me and a friend actually found some rock carvings that hadn't been recorded before. Which was kind of a big deal, especially since no carvings had been discovered in our part of southern Tanum.
We ran home and told our parents, who in turn called the municipality. When they came down to inspect, we were told the carvings weren't authentic, as in not from the bronze age. I don't remember any tests being conducted, I think it was mostly judged by the style.
My archaeology career peaked, and ended, at age 10.
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.