Geologic time scale relative dating
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.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.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).This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.
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.
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.
Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.
Third, magnetism in rocks can be used to estimate the age of a fossil site.
A geologic era is a subdivision of geologic time that divides an eon into smaller units of time.