Lichenometry is a method of numerical dating that uses the size of lichen colonies on a rock surface to determine the surface's age. Lichenometry is used for rock surfaces less than about 10,000 years old.
The basic premise of lichenometry is that the diameter of the largest lichen thallus growing on a moraine, or other surface, is proportional to the length of time that the surface has been exposed to colonisation and growth. Data on lichen growth rates can enable estimates of both the age of the thallus and the period of exposure of a rock surface to be made.
As a field technique it has the advantage that measurements are relatively simple and easy to obtain. Several factors however limit the application of the technique. The need for local date calibration is paramount. Lichens provide a minimum value of how long the rock surface has remained immobile and undisturbed. Lichen growth may be interrupted or terminated by sudden slope movements, or the encroachment of vegetation. There is also the problem of actually finding the biggest one. Studies have shown how it is invaluable for dating closely spaced recent events (Beschel 1973).
Two lichenometric techniques can be used in the Vall de Nuria:
maximum lichen diameter
lichen diameters measured on randomly selected boulders.