Stromatolites by mark i geo on Flickr.
Stromatolites are structures formed by cyanobacteria. As cyanobateria go through chemical routines, the cyanobacteria became very slightly tacky, and that tackiness trapped microparticles of dust and sand, which became bound together to form weird but solid structures through many, many years
Source: “A short history of nearly everything” by Bill Bryson.
