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Stratigraphic Superposition - Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.
Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layers will be on the bottom.
With this discovery, Radiometric dating techniques became possible, and gave us a means of measuring absolute geologic time.
Radiometric Dating Radiometric dating relies on the fact that there are different types of isotopes.
When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks.
Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface.