Lichenometry - a new term for me

In archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic dating that uses lichen growth to determine the age of exposed rock, based on a presumed specific rate of increase in radial size over time
The map lichen (Rhizocarpon geographicum), and Xanthoria are the lichens most used in lichenometry.
It has also been explored as a tool in assessing the speed of glacier retreat due to climate change.

From Lichenometry - Wikipedia