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Reference fossils are the remnants of dead animals and plants that we know existed at a particular time in history.
If we see one of these fossils in a layer of rock, this tells us how old that rock must be.
They're useful because they give us a more exact date for a rock, and they can be used to date more rocks since they're common.
Ammonites are a great example of an especially useful index fossil.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
We know the amount of radioactive substance when the rock was formed, and we know how fast it decays into other non-radioactive substances, so we can look at the amounts in each rock and use it to calculate how old the rock is.
Radiometric dating and index fossils are two methods we can use to figure out how old fossil-containing chunks and layers of rock are.
Sedimentary rock forms layers as particles settle on sea beds over millions of years.
The majority of the time fossils are dated using relative dating techniques.
Using relative dating the fossil is compared to something for which an age is already known.