Rhizocarpon alpicola

Rhizocarpon alpicola is a species of saxicolous (rock-dwelling), crustose lichen in the family Rhizocarpaceae. The lichen grows as a patch-like crust up to 15 cm across, composed of small yellowish-green tiles that develop black, button-like reproductive structures. A member of the Rhizocarpon geographicum species complex, R. alpicola can usually be distinguished from related species by its large and smooth areoles. R. alpicola is often utilised in lichenometry, a dating technique used to estimate the length of exposure of rock surfaces. Research on Norwegian glacier forelands has shown that R. alpicola typically establishes on rock surfaces after they have been exposed for about a century, with individual colonies growing slowly over many decades....read more on Wikipedia.