Sediments Get Laid in Beds

One of the fundamental laws in geology is that sediments do, indeed get laid in beds. This fact long ago gave rise to what is refered to as the Principle of Superposition, the principle that, except in extremely deformed strata, a bed that overlies another bed is alway younger. Alongside the Principle of Superposition is the notion of Cross-cutting Relationships, the idea that in order for a rock to be cut by some intrusion, it must have been there before the intrusion occured, and therefore is younger than the intrusion.

Using these two principles, any geologist can establish a relative timeline for the deposition of an outcrop of rock. As an example, refer to the diagram below.

In this case, the first rock that was laid down as sediment were the beds labeled E, D, and C. Later, an intusion of igneous rock occured, denoted by bed B. Still later, bed A was laid down on top. Finally, the entire series of bed was cut by a fault.