Superposition is a relative age dating principle which states
Law of Superposition: The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.
Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest.
If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers.
But if a layer of sediment buries a fault, the sediment must be younger than the fault.
Principle of baked contacts: The principle of baked contacts states that an igneous intrusion “bakes” (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion.
Principle of inclusions: The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the inclusion.
These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.
It provides the geologic underpinning for the theory of evolution.
Example: Bed 1 at the base contains fossil species A, Bed 2 contains fossil species A and B, Bed 3 contains B and C, Bed 4 contains C, and so on.
The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region.
We can use these principles to determine relative ages of the features.
Investors decided to construct a network of canals to transport coal and iron, and hired an engineer named William Smith (1769–1839) to survey some of the excavations.