Banded iron formations, a.k.a. "redbeds", appear in the rock record ranging from about 2 billion years ago. These wide layers, composed of fully-oxidized rust-colored iron compounds, are not found in geological formations older than 2 billion years. This suggests that oxygen was not available in large quantity anywhere on earth before then, which in turn suggests the rapid appearance of atmospheric oxygen, a milestone that defines the point in time when photosynthetic organisms began to dominate the Earth's biota.
In other words, "redbeds" are geologic evidence of a sudden transformation of the Earth's atmosphere, heralding the appearance of plant life as we know it.
A similar story can be told with the record of uranium salts in rocks.
(Image courtesy of the University of California Museum of Paleontology.)