Relative dating utilizes six fundamental principles to determine the relative age of a formation or event.
The first principle is the Principle of Superposition which states that in an undisturbed succession of sedimentary rock, the oldest layers are on the bottom.
Pretty obvious that the dike came after the rocks it cuts through, right?
The Principle of Original Horizontality states that due to the influence of gravity all sediment is originally deposited horizontally.
In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon. Using this principle we can than assume that sedimentary layers which have been deformed/folded must have been deformed after all affected layers have been deposited.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
Much like the principle of cross-cutting, the Principle of Inclusions relatively dates objects based on their placement within other earth materials.
Once a rock is lithified no other material can be incorporated within its internal structure.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
No bones about it, fossils are important age markers.
In order for any material to be included within in the rock it must have been present at the time the rock was lithified.