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.
There are absolute ages and there are relative ages. We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.
Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. They are descriptions of how one rock or event is older or younger than another.
That's why geologic time is usually diagramed in tall columnar diagrams like this.
Just like a stack of sedimentary rocks, time is recorded in horizontal layers, with the oldest layer on the bottom, superposed by ever-younger layers, until you get to the most recent stuff on the tippy top.
We know that the curb was originally straight when it was first constructed. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
The fault cut the curb and is thus younger than the curb itself. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events? Finally, we note an erosional surface, I, at the top of the sequence (and immediately below the corn field) that cuts both A and G. Putting this all together, we can determine the relative ages of these rock layers and geological events: Given the information available, we cannot resolve whether H is older than A (or, vice versa).It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied.Through use of techniques (which were developed during the 20th century; see Section 2.2), they were able to later assign dates in years before the preset to important events in Earth's history.Use superposition to determine which is older: the road or the lava flow? states that a rock unit (or other geological feature, such as a fault) that is cut by another rock unit (or feature) must be older than the rock unit (or feature) that does the cutting.Imagine cutting a slice of bread from a whole loaf.The simplest is the law of superposition: if thing A is deposited on top of (or cuts across, or obliterates) thing B, then thing B must have been there already when thing A happened, so thing B is older than thing A.