Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.
Ever wondered how scientists know the age of old bones in an ancient site or how old a scrap of linen is?
Once an organism dies, it stops taking in carbon-14.
The carbon-14 it contained at the time of death decays over a long period of time, and the radioactivity of the material decreases.
By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is.
This technique works well for materials up to around 50,000 years old.