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Map Lichen (Rhizocarpon geographicum), Setesdalheia, Norway
The map lichen is a species of lichen, which grows on rocks in mountainous areas of low air pollution. Each lichen is a flat patch bordered by a black line of spores. These patches grow adjacent to each other, leading to the appearance of a map or a patchwork field.
Map lichen is a lichen widely used by climatologists in determining the relative age of deposits, e.g. moraine systems, thus revealing evidence of glacial advances. The process is termed lichenometry.
Lichenometry is based on the assumption that the largest lichen growing on a rock is the oldest individual. If the growth rate is known, the maximum lichen size will give a minimum age for when this rock was deposited.