Relative age dating sequence
You can see that the curb is offset: the bottom half does not line up with the top half.
As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced.
But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: Relative age dating has to do with determining the temporal ordering of events in Earth's past.
Geologists employ a handful of simple principles in relative age dating; two of the most important of these are are the principles of Just as uniformitarianism is the key underlying assumption of geology, the science's most fundamental principle is superposition, developed by Danish anatomist Nicholas Steno (1638-1686) in the 17th century.
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?
Correlation can involve matching an undated rock with a dated one at another location.
Through use of techniques (which were developed during the 20th century; see Section 2), they were able to later assign dates in years before the preset to important events in Earth's history.
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.
The image below shows a sequence of Devonian-aged (~380 Ma) rocks exposed at the magnificent waterfall at Taughannock Falls State Park in central New York.
The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
An imaginary cross-section, showing a series of rock layers and geological events (A-I).