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This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface.
Therefore newer sediment is continually deposited on top of previously deposited or older sediment.
It settles out of the air and forms a layer over wide regions at the same time. The remians of the impact site in Mexico as seen by satellite imagery.
Example: the layer that marks the extinction of dinosaurs has characteristics of an asteroid impact.
If conditions are right the remains of the dying organisms can then be preserved as fossils within the rock that formed from sediments that covered the remains.
Since, all sedimentary rock is formed through the gradual accumulation of sediment at the surface over time, and since the principle of superposition tells us that newer sediment is deposited on top of older sediment, the same must also be true for fossils contained within the sediment.
The Principle of Fossil Succession states that groups of fossils (called fossil assemblages) proceed one another in a regular and determinable manner.
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When a violent eruption of a volcano occurs it may send dust high into the atmosphere where it can spread over the entire planet.For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.Then, by applying the Principle of Cross-Cutting we are able to relatively date those processes.In order for any material to be included within in the rock it must have been present at the time the rock was lithified.For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface.