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By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared.
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).
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).
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
Second, it is possible to determine the numerical age for fossils or earth materials.
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.
In the Grand Canyon, the layers of strata are nearly horizontal.
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.
Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.