Stromatolitic limestones in the Cambrian of Utah, USA. (cross-section view; geology hammer for scale)

Stromatolites (e.g., see right above the yellow paint stripe on the outcrop - click on the photo to zoom in) 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 along coastal Western Australia).

Stratigraphy: Big Horse Limestone Member, Orr Formation, upper Marjuman Stage; almost uppermost Middle Cambrian