Reconstructed setting· The columns of time
Archean world
4.03 Ga → 2.5 Ga
Shallow seas worked by waves, with low microbial mounds on the floor. It is the first backdrop with a horizon, and the first with anything alive in it.
What the evidence shows
The wave-worked shallows come from the Strelley Pool microbial mat facies, which preserves wind-blown crescentic ripples: a beach, in other words, and a way of knowing the sea was shallow. The mounds enter the scene only from 3.7 billion years ago, the age of the oldest reported stromatolites, at Isua in Greenland, and they are wide and low on purpose. Tall columnar stromatolites are a Proterozoic development, and drawing them here would import a later world into an earlier one.
Bounds and conventions
The date at which the mounds appear is contested: the Isua structures have been read as microbial and read as tectonic, and at that degree of metamorphism the two are hard to tell apart. The backdrop draws them from 3.7 billion years ago, which is the published claim, not a settled fact.