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In practice, both relative and absolute dating are combined, following a procedure like this:
1. Igneous rocks, such as lava flows, volcanic ash beds, and intrusions) are dated
radiometrically.
2. The dates of fossil-bearing sedimentary rocks are in a certain area are bracketed using the
dates of associated igneous rocks which have been dated radiometrically. Some
sedimentary rocks can be dated radiometrically, if they include datable minerals such as
glauconite.
3. The fossil-bearing sedimentary rocks are correlated with sedimentary rocks in other areas
which contain the same fossils.
4. The age of the rocks in other areas is determined indirectly, from the ages of the fossils
they contain..
By doing this repeatedly, the geologic time scale has been established. The geologic time scale
is a composite vertical sequence representing all known rock units and their fossils, worldwide,
placed into sequential order (oldest to youngest), based on relative dating and correlation. The
absolute ages of the rocks have been determined through radiometric dating where possible.
Hence, the geologic time scale provides a calibrated "yardstick" for determining the ages of
rocks worldwide through an examination of their fossils.