Radiometric dating relies on the properties of isotopes.
These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.
Strata are differentiated from each other by their different colors or compositions and are exposed in cliffs, quarries, and river banks.
Today's knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating.The universe is full of naturally occurring radioactive elements.Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.If a fossil is found between two layers of rock whose ages are known, the fossil's age is thought to be between those two known ages.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.