Some of the most useful fossils for dating purposes are very small ones.For example, microscopic dinoflagellates have been studied and dated in great detail around the world.Learn how inclusions and unconformities can tell us stories about the geologic past.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!
Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.
In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.
It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.
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.