Answer:
The Law of Superposition in geology states that "Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top."
Most sedimentary rocks show evidence of distinct layers or strata. If you find a fossil in a lower layer then generally it is older than one found in a higher layer.
A good demonstration of this principle is the KT-Boundary. It is believed that the KT-Boundary is a layer of rock and debris from a huge asteroid impact from 65.5 million years ago. Because of high concentrations of a rare element, Iridium, this layer is recognisable all over the world. All non-avian dinosaurs fossils are found below this level and none are found above. This has lead scientist to believe that all non-avian dinosaurs became extinct at the end of the cretaceous.