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However, by itself a fossil has little meaning unless it is placed within some context.

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.

The age of the fossil must be determined so it can be compared to other fossil species from the same time period.

Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.

Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).

Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.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.

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.This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top (principle of superposition).In the Grand Canyon, the layers of strata are nearly horizontal.A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.