Stromatolite

A stromatolite fossil specimen with a concentric circle pattern, collected in 2001 by Dave Smith at Utsingi Point on the East Arm, Great Slave Lake. Stromatolites are the oldest record of life on Earth, dating to billions of years ago. They are formed by colonies of blue-green algae or cyanobacteria excreting lime and trapping it in microbial mats. Commonly in the shape of mounds, they may also form in columns or sheets.