Absolute dating and relative dating
Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.The majority of the time fossils are dated using relative dating techniques.Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.The absolute age of these events need not inevitably be known.Grand Canyon is a gorge located in Northern Arizona, USA. While some of the layers are uplifted, most of the landform is left undisturbed by nature.
By measuring the ratio of the amount of the original (parent) isotope to the amount of the (daughter) isotopes that it breaks down into an age can be determined.
Using relative dating the fossil is compared to something for which an age is already known.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
It is composed of rocks and sediments deposited over millions of years. It is the evidence of Earth's history over such a long span of time.
It is a perfect example of superposition (layers deposited one above the other) and lateral continuity (undisturbed and covering large distances).