Yellow map or world map lichen is frequently used in lichenometry, a geobotanical technique employed to estimate the age of exposure of a rock surface.
Since Rhizocarpon geographicum quickly colonizes newly-exposed rock surfaces and has a known rate of growth, geologists can, for example, examine glacially-deposited rocks and determine the rate of glacial retreat. For an excellent introduction to lichenometry, read Lichens, Lichenometry and Global Warming (PDF), a short and well-illustrated paper by Richard Armstrong in the September 2004 issue of Microbiologist.
Photography resource link: Photographer and writer Freeman Patterson. To see a sampling of his photographs, click on prints, and then the book covers.