Basically the law of Superposition is applied in this regard. The law states that "the oldest layer is on the bottom and the youngest layer is on top".
When we find fossils at a particular depth we can relatively date a rock based on the strata we find them. A fossil in place in bottom beds will be older than the one in the topmost layer. Fossils also succeed one another in a definite pattern according to the principle of fossil and fauna succession.
