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Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed.
Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.
William Smith's collecting and cataloging fossil shells from rocks led to the discovery that certain layers contained fossils unlike those in other layers (see: fossil sorting).
For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.
For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.