Rhizocarpon geographicum, the map lichen. Growing on a rock face in Acadia National Park in Maine.

If you zoom in on the larger pictures, you can see the beautiful and intricate green and black pattern.

It turns out this species of lichen is common all over the globe on sheer rock faces and can be used to date events within 1000 years. The radius of an individual R. geographicum colony increases by about 0.5 to 0.8 mm/year (depending on the altitude)*. That would suggest that these colonies are at least over 100 years old (and hence are starting to fuse together).

• RK Chaujar in Current Science p 1552 (2006)
  map lichen