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The Red Beds

Like the Banded Iron Formations before them, the red beds are huge deposits of iron-rich sediment. The difference is that the BIFs were formed underwater and in an oxygen-poor environment. This was the situation during the early stage of Earth history (more)

By contrast, the red beds were formed in an oxygen-rich environment, and are testimony to the widespread oxygenation of the planet by the time they first appeared. When did the transition come about? Opinions vary, but at least one influential source says some 2.2 thousand million years ago (around the Archaean/Proterozoic transition).

The red beds were formed on land, and not under the sea as the BIFs were. They form large wind-blown sand dunes. And you need quite a large land mass to allow large dunes to form. So the red beds are also taken as evidence that, after a couple of thousand million (2,000,000,000) years, our planet was seriously settling down into old age. And that reasonably large land masses were forming; and staying tolerably large throughout the supercontinent cycle.

It has settled down even further in the 2½ thousand million years since.