Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.
Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).
The principle of superposition builds on the principle of original horizontality.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.