Map Lichen

Map lichen, or yellow map lichen, is the common name of Rhizocarpon geographicum, a lichen species that grows on rocks in mountainous areas with low air pollution. It forms flat, yellow-green patches outlined by dark, roughly circular borders, giving the rock surface a map-like pattern. Because its roughly circular growth spreads outward at a fairly constant, slow rate, the diameter of a map lichen colony is widely used in lichenometry, a technique for estimating the age of exposed rock surfaces such as glacial moraines.