Lichens are a symbiotic relationship between algal and fungal communities and they increase in size radially as they grow. Measuring the diameter of the largest lichen on a rock surface can thus be used to determine the time the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years, providing the maximum age limit of the technique.
Lichenometry can provide dates for glacial deposits in tundra environments, lake level changes, trim lines, rockfalls, talus stabilisation and former extent of permafrost or very persistent snow cover
Among the potential problems of the technique are the difficulty of correctly identifying the species, delay between exposure and colonisation, varying growth rates from region to region as well as the fact that growth rates are not always constant over time, dependence of the rate of growth upon substrate texture and composition, climate and finally, actually finding the biggest one.--Bdewaele 11:01, 2 Jan 2005 (UTC)