Difference between relative dating and radiometric dating
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.
Younger layers are deposited on top of older layers (principle of superposition).
The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).
These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.