The principle of superposition builds on the principle of original horizontality.
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).
However, by itself a fossil has little meaning unless it is placed within some context.
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.
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.
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
Second, it is possible to determine the numerical age for fossils or earth materials.
Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.