What circumstances geologists infer that two distant formations are about the same age

Determining the Relative Ages of Rocks

The relative age of a rock is its age in comparison with other rocks. If you know the relative ages of two rock layers, you know which is older and which is younger, but you do not know how old the layers are in years. In some cases, it is very tricky to determine the sequence of events that leads to a certain formation.

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