Stromatolites are trace fossils of the biofilms of blue-green algae that grow around an object such as a shell or oyster. As the algal filaments grow outwards, sediment is continually deposited and accreted onto the surface of the algal mat. The stromatolite increases in size and forms distinctive concentric rings if you break the stromatolite in cross section.
The outer surface of the fossil stromatolites look quite knobbled in their appearance. These round boulders can sometimes be quite large in the fossil record at Lake Turkana, sometimes reaching a meter wide.
Stromatolites grow in shallow, hypersaline pools in shore settings, inhospitable to molluscs and other invertebrates that might otherwise graze on the algae. There are no modern examples of stromatolites found today at Lake Turkana, however there are modern marine stromatolites that are in lagoons in Shark Bay in Australia. A 3D model of a stromatolite from 0.7 million year deposits at Lake Turkana can be seen here;