Rhizocarpon geographicum (L.) DC.
Yellow Map Lichen, World Map Lichen
This is one of several “map lichens,” so-called because at close range, each tiny lichen is bordered by black spores that resemble borders on a map. Yellow map lichen is common in cool forests, such as the Sierra Nevada mountains, northern Maine, Canada, Greenland, Iceland, and Siberia. Closer to the equator, it is found at progressively higher elevations. Although it can withstand two weeks of exposure to space, it is sensitive to pollution, and fares best in low pollution environments.
Identification: The pale yellow patches with black spore-lined edges do not closely resemble other lichens. Yellow map lichen is crustose, forming a thin, tightly attached surface on rocks.
Rhizocarpon geographicum on Wikipedia
Rhizocarpon geographicum at Stephen and Sylvia Sharnoff's lichen.com site
Rhizocarpon geographicum at the University of British Columbia Botanical Garden Center for Plant Research
Rhizocarpon geographicum at Alan Silverside's Lichen Pages on LastDragon.org
Rhizocarpon geographicum on the Toolik↔Arctic Geobotanical Atlas
Rhizocarpon geographicum description by Thomas H. Kent, last updated 2 Oct 2013.