Answer:
A sedimentary rock's age can be estimated (relative age) by index fossils that may be present in the rock. Certain fossils only appear in rocks from a certain geologic age. This method is known as 'relative dating' and the age is called the 'relative age'. The 'absolute' geologic age of the strata which bear the index fossils has been determined by the radiometric age of igneous intrusions which are relative to the layers of sedimentary rock, according to the Law of Superposition and the Law of Horizontality. For igneous and metamorphic rock, age is determined primarily by radiometric dating based on decay of isotopes found within the rock. For some igneous rocks, the age of the rock is known precisely from historical records correlating to its formation.