Rules and principles of relative dating
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.The method of reading the order is called stratigraphy (layers of rock are called strata).Relative time places events or formations in order based on their position within the rock record relative to one another using six principles of relative dating.Relative time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.
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.
With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?