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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).
Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.
For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.
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.
" First, the relative age of a fossil can be determined.
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
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.
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.
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.Second, it is possible to determine the numerical age for fossils or earth materials.