• The relative age tells whether a rock is older or younger than another rock without giving its age in years. It depends on:

• In an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top.
• (Each new layer is laid down on top of the previous layers.)

2. Index fossils

• Index fossils

• are fossils of organisms that lived for a short period of geologic time but were widely distributed geographically, then became extinct.
• Examples are trilobites and ammonites.
• How to use them:

• If the same index fossil is found in two rock layers far apart, the two layers have the same relative age, even if the rocks above and below them are different.
• The layer containing the index fossil is used as a reference the layer below it is older and the layer above is younger.
• Why are index fossils good for dating?
• Because their short existence limits the layer’s age to a narrow range of time, and their wide distribution allows matching of layers in different places.

• In rock sequence 1 (from bottom to top) the layers are A, B, C, D.
• Fossil X is found in layer B and fossil Y in layer D.
• Which organism appeared first?
• B is below D, so B is older, so organism X lived earlier than organism Y.

• In an area, layer P contains trilobite fossils; in a distant area, a layer Q contains the same trilobite species.
• We conclude that P and Q formed at the same time, even if P is limestone and Q is sandstone.
• A layer above P that holds fossils of a later species is younger than P.

• Relative dating tells us the order of events, not the number of years.
• It is used when rocks cannot be dated in years, such as when there are no suitable radioactive minerals in a sedimentary rock.