1. The Law of Superposition
A sedimentary layer is more recent than the one it covers
It mean that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.
2. The Principle of Continuity
The principle of continuity, which is the basis for geological correlation, is defined as follows :
A sedimentary layer limited by a base and top and defined by a given facies has the same age throughout.
In other words, when a layer was laid down the same sedimentary processes occurred at the same time at numerous points in the sedimentary environment. In Practice, it is impossible to follow a layer or series of layers continuously except in places that lack vegetation. The principle of continuity must not be applied unless a facies has been completely defined and its paleontological characteristics are unambiguous. It will be shown later that it is always preferable to define the palaeological identity before asserting the continuity of layer or series of layers.