Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.
The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
Younger layers are deposited on top of older layers (principle of superposition).
Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).
Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
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.
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.