Stromatolites, colonial structures created by cyanobacteria (commonly called blue-green algae) are among the oldest fossils on earth, being found in rocks over 3 billion years old. Once the dominant life form on the planet, stromatolites are now found in only a few places, such as Hamelin Pool, near Shark Bay, Australia.
Cyanobacteria form these mound-like structures by growing through sediment, binding the particles together resulting in successive layers which, over very long time periods, may harden to form rocks. These structures may be from fractions of an inch to several feet in diameter.
Location in Minnesota
In Minnesota, stromatolites most commonly occur in the Biwabik Iron formation, the rocks of which have been mined for iron for over 100 years. The fossils are made of chert, a form of quartz, which has replaced the carbonate and other sedimentary material. Small amounts of hematite, an iron mineral, often give the rock a red color.