You have to date the volcanic layer above and below the fossil layer.

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Relative age dating diagrams video

By doing this repeatedly, for many locations around the world, 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.

Global Boundary Stratotype Sections and Points have been established for the time scale.

This means that many localities around the world were examined for each dated geologic boundary, and the best was selected to be the worldwide "standard reference section." A "golden spike" has been driven at the point that represents the partcular age on the geologic time scale.Igneous rocks can be used to provide absolute dates for sedimentary rocks.In example A, the age of the shale is bracketed between two lava flows, one older than the shale, and one younger than the shale.Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age, (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.