Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).The principle of superposition builds on the principle of original horizontality.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.
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.
The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).
The principle states that any geologic features that cut across strata must have formed after the rocks they cut through (Figures 2 and 3).