The attached photo shows mid-20th century graffiti - heart with initials inside of it.
The lichen and moss cover was removed to show the graffiti better. The graffiti had a similar lichen & moss cover as the rest of the boulder. The graffiti is a maximum of 70 years old (post 1930's … probably 1950s or 1960s era). The other bare patches in the photo are other spots where the lichen cover was removed to reveal more graffiti. The photo was taken by Lisa Stone on the coast of Nova Scotia. The boulder is a type of soapstone. Below the graffiti is a crescent shaped groove. One expert I showed this to identified the groove as typical Native American soapstone quarrying technique. Note the coloration difference in the patina of the graffiti and and the groove. Surface patina is far more reliable for relative dating on this boulder than the lichen cover.
This classic example of rapid lichen and moss growth. It serves to illustrate some of the potential pitfalls of lichen based dating.
3 comments :
I was not talking about lichen-based dating. I was talking about lichen-based ordering of the relative ages of different rocks at one site.
I did some research on lichen dating some years ago, and found out that acid rain in the Northeast causes the lichens to grow at unusual rates, and thus this technique is not useful for dating here.
Nevertheless, I take your point, Peter, that lichen growth is helpful in a way. I think most of us notice it and have used it to judge tentatively what features at a site might be older than others. Our process at a site is of necessity one of speculation, and we should notice and even chronicle every aspect of what we see in case some ways of putting it all together eventually arise.
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