Sedimentary rocks form by the accumulation of layers in a variety of environments such as a sea floor, lake or desert. The sediment will eventually consolidate to become rock strata (layers).

Generally, the lowest layers are older than the upper layers and any plant or animal remains they contain will be older, as will any minerals that were formed during or soon after the time of deposition.

There are some situations, however, where the Principle of Superposition will not apply such as when molten magma intrudes underneath (older) surrounding rock or when rock sequences are pushed over by folding and faulting.

Palaeontologists apply the Principle of Superposition to determine the order in time of fossils found within rock layers. In turn, particular fossils help indicate the time at which individual rock layers were deposited. The use of fossils to date rocks - biostratigraphy - provides the basis for the divisions of the Geological Timescale.

Fossils provide the main means for subdividing the Geological Timescale, and they also allow correlation —establishing which rock units in different places are the same ages.