Stromatolite (cut slice; field of view ~12 mm across) from the Fort Laclede Bed, Laney Member, Green River Formation (Eocene) of southwestern Wyoming, USA.

This is a sediment filling between two stromatolite columns. The grains are mostly a mixture of ostracod shells, oolites, composite oolites, and peloids. Sediment pouch is ~8 mm across.

Stromatolites are layered structures built up by mats of cyanobacteria (= blue-green bacteria, often incorrectly called “blue-green algae”). Stromatolites are fairly common in portions of the shallow-water marine & nonmarine fossil record, but they also occur in a few modern localities (e.g., Shark Bay, coastal Western Australia).