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.
The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).
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.
This is the principle of cross-cutting relationships.
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.
In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).
Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.
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.