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).
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.