A gravestone with lichen growth. Source: designscience.com
I was reading the charming Bella Coola Blog the other day and came across an interesting post discussing how lichen had been used to date glacial features in the area. So-called lichenometry is a well-established dating technique which relies on the principle that lichen grows in roughly circular patterns, and that there is a linear relationship between the diameter of the circle and the age of the lichen patch. Now, such patches can be many hundreds of years old. So for an event where rock is newly exposed and then lichen grows on it, the lichen will give a minimum date on when the bare rock was first available as a lichen environment. Anytime you have a bare rock and a dating technique for when the rock became bare, the archaeology antennae rise.
Northwest Coast Archaeology 