Relative age dating sequence
Because of cross-cutting relationships, the cut that divides the slice from the rest of the loaf is younger than the loaf itself (the loaf had to exist before it could be cut).
When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks.
That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.
Some of the most useful fossils for dating purposes are very small ones.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.
Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.
Taughannock Falls near Trumansburg, New York, illustrating the Principle of Superposition. Superposition is observed not only in rocks, but also in our daily lives. The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older (and likely smellier! Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: most likely, the newspapers at the bottom of the pile have dates on them that are older than the newspapers at the top of the pile.
The photograph below was captured at Volcano National Park on the Big Island of Hawaii.
Relative dating does not provide actual numerical dates for the rocks.
An imaginary cross-section, showing a series of rock layers and geological events (A-I).
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.
A curb in Hollister, California that is offset by the San Andreas fault. The cartoon below shows an imaginary sequence of rocks and geological events labeled A-I. This problem could be resolved, however, if we were to observe A cutting across H (i.e., the fault displacing the igneous intrusion).
Using the principles of superposition and cross-cutting relationships, can you reconstruct the geological history of this place, at least based upon the information you have available?