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The most widely known form of radiometric dating is carbon-14 dating.
This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.
Each of them typically exists in igneous rock, or rock made from cooled magma.
Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.
Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.
These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit.
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification).
It is primarily used in the study of sedimentary and layered volcanic rocks.
The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century.
Known as the "Father of English geology", Smith recognized the significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England.