This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).

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Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.

This is the principle of cross-cutting relationships.

" 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.

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.Second, it is possible to determine the numerical age for fossils or earth materials.Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.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.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.