Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.Much like the principle of cross-cutting, the Principle of Inclusions relatively dates objects based on their placement within other earth materials.This concept known as the Principle of Lateral Continuity allows us to assume that similar layers of rock or sediment that are separated by a valley or other erosional feature were once continuos.For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.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.As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type.
Many of these organisms have left their remains as fossils in sedimentary rocks.
Suppose you find a fossil at one place that cannot be dated using absolute methods.