The principle states that any geologic features that cut across strata must have formed after the rocks they cut through (Figures 2 and 3).According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited.

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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.

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.

For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.

By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.

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.

As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.

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.