Exercise 3.2. Reconstructing Relative Geological Time

What To Do

Figure 3.3 is a geologic cross-section where each rock unit is assigned a letter as a label. Use the labels in Figure 3.3 and the stratigraphic principles to answer the questions that follow. These questions are included in the Lab 3 worksheets.

All rocks are sedimentary except for the following:

  • H: metamorphic
  • P and S: intrusive igneous rocks
In the questions below, the term “geological event” could mean the formation of a particular rock unit (e.g., bed X is a geological event) or the development of a geological structure such as a fault.



Figure 3.3 | Geological cross-section for reconstructing relative time.


1. Which is the oldest geologic event?

2. Which geologic events are definitely older than the intrusion S?

3. What stratigraphic principle is used to date Fault B on the right side of the figure?

4. Which pairs of rock layers define unconformities?

Note: Once you find the unconformities, mark each by making the contacts thicker, or by using a coloured pencil. This will help you to answer question 6.

5. Which is the most recent (youngest) geological event in the cross section above?

6. List the rock units and the faults in order from the oldest to the youngest. Use the letter A to refer to Fault A, and B to refer to Fault B.


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