How do sedimentary rock show earth's history?
The layers are time-based, and can show the development of different chemicals in the environment, and plant and animal life.
Because sedimentary rocks are formed by the compression of "sediment" - the accumulation of dirt - in large bodies of water, it can also trap and fossilize plant and animal remains. The strata of the rock is related to the timeline of its formation. Thus, a "history" of the earth at that place is recorded.