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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).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.Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).Younger layers are deposited on top of older layers (principle of superposition).
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
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.
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.