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Determining Relative Ages

Using certain principles it is possible to determine the relative ages of rock units and reveal the geologic history of the region.

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Determining Relative Ages

What clues indicate a person's age?

There are ways to tell the ages of people relative to each other. For children we use height, for adults we might use gray hair and wrinkles. There are also ways to tell the relative ages of rocks. We'll practice in this concept.

Determining the Relative Ages of Rocks

Steno’s and Smith’s principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you 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. Can you figure out what happened in what order in (Figure below)? Write it down and then check the following paragraphs.

Example of a geologic cross section

A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E).

The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the fault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger.

The full sequence of events is:

1. Layer C formed.

2. Layer B formed.

3. Layer A formed.

4. After layers A-B-C were present, intrusion D cut across all three.

5. Fault E formed, shifting rocks A through C and intrusion D.

6. Weathering and erosion created a layer of soil on top of layer A.


  • The oldest rock units lie beneath the younger ones.
  • By the principle of cross-cutting relationships (and common sense) we know that something must exist before something else can cut across it.
  • The history of a section of rocks can be deciphered using the principles outlined in this Concept.


  1. What is relative age? How does it differ from absolute age?
  2. Why do the principles of relative dating not indicate the absolute age of a rock unit?
  3. Under what circumstances would a rock unit with an older fossil be above a rock until with a younger fossil?

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Use the resource below to answer the questions that follow.

  1. What does relative dating give you? What doesn't it give you?
  2. What is the order of rock layers and events in the first rock section shown?
  3. What is the order of rock layers and events in the second rock section shown?
  4. What is the order of rock layers and events in the third rock section shown?
  5. What is the order of rock layers and events in the fourth rock section shown?
  6. What is the order of rock layers and events in the fifth rock section shown?
  7. What is the order of rock layers and events in the sixth rock section shown?

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relative age The age of an object in comparison with the age of other objects.

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