Microbes aggregate sediment: blue-green algae
 

Cyanobacteria grow large layered clumps of rock and algae, called stromatolites, by trapping sediment in mucus and filaments.

   
  "Half dead, half alive, stromatolites represent a partnership between microorganisms and rock. The spongy coating is made of cyanobacterial filaments that secrete a sticky mucus. Grains of sediment get trapped in the mucus and stick together to form a crust of rock. As the filaments grow longer, they trap more sediment and add a new layer to the exterior. What’s left on the inside is dead zone." (Monastersky 1998:74)
  Learn more about this functional adaptation.