∞ generated and posted on 2016.11.05 ∞
Determination of the age of fossils based upon their position within geological strata.
The higher up things are, generally the younger they are, and the lower the older (that is, except under those relatively rare circumstances when strata are flipped upside down by geological processes). A lower fossil thus tends to be older than a fossil that is found somewhere above it.
These ideas are codified in what is known as the 'Law of Superposition', which states: "Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top."
This is just as newspapers found in accumulated trash tends to be older on the bottom than they are on the top. See by contrast the complementary suite of techniques known as absolute dating, i.e., the equivalent of looking at the dates stamped by the publisher onto those newspapers!
It is also possible to determine the relative position of fossils not just in one location but across the globe. In fact, the geological record was initially worked out (by geologists!) based upon the relative position of fossils within strata as found worldwide. See also stratigraphy.