Lichenometry
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In archeology and paleontology, lichenometry is the study of dating a surface using lichens as age markers: lichens 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. The use of lichnometry is of increased value for dating deposited surfaces over the past 500 years as radiocarbon dating techniques are less efficient over this period.
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.
It was first employed by Faegri in 1933, though the first exclusively lichenometric paper was not published until 1950, by Roland Beschel, considered widely to be the 'Father of Lichenometry'.
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