2 types of radiometric dating
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 principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).
Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
Third, magnetism in rocks can be used to estimate the age of a fossil site.
This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.