Relative age dating principles
Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.
Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
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.
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 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.
Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.