Because of the alternation between dark and light sediments we can use each dark band as a marker for one year.
For really old dates we need to use radioactive isotopes.
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.
Later in the year, by wintertime, the clay has had enough time to settle to the bottom and it forms a thin layer of dark colored rock. We actually went to dinner and talked the whole time. At the end of the evening I felt warmer in my mind and heart....I had the pleasure to meet Kristin for a non-rushed two hour cuddling appointment.The first method of finding the absolute age of an object is by examining tree rings.If we looked at a cross-section ofa tree or log we would notice that all through it are concentric circles radiating out from the center to the bark. Each ring is also different, and the thickness of each ring is representative of the length of the growing season.