The map lichen is a species of lichen, Rhizocarpon geographicum. It grows on rocks in areas of low air pollution. Each lichen is a flat patch bordered by a black line of spores. These patches grow next to each other. This makes the lichen look like a map or a patchwork field.
Map lichen is a lichen widely used by climatologists to determine the relative age of deposits. As an example, they might use it in a moraine systems, to tell how long a rock has been exposed. With this they can also tell about glacial advances. The process is called 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.
Growth rates for different areas and species can be obtained by measuring maximum lichen sizes on substrates of known age, such as gravestones, historic or prehistoric rock buildings, or moraines of known age (e.g. those deposited during the Little Ice Age).