Sequence of Events Lab
INTRODUCTION: The geologist works to develop a model of the earth’s history and
Attempts to put events in order of what happened first, what happened second, and so
on. When the relative age of a rock is determined, this information can be diagramed as
a cross-section showing the vertical distribution of geologic rock units.
OBJECTIVE: Using cross-sections you will infer a logical sequence of geologic events
and establish a probable relative age for a series of rock layers.
For each of the diagrams provided, determine which layer is the oldest and then tell what
events took place in proper order.
1. Use SEPERATRE LINED PAPER and title it “Report Sheet”.
2. Briefly describe the events that occurred in numbered steps. Start with the
formation or deposition of the oldest rock layer. Be sure to indicate when any
emergences or subsidences of the area must have occurred.
Discussion Questions: (answer in complete sentences)
1. Describe how the Principle of Superposition was used in determining the relative
age of the cross sections.
2. In a cross section suppose you find older rock layers on top of younger rock
layers. Explain a probable cause for this exception to the Principle of
3. Why is an igneous intrusion younger than the rocks in which it is found?
4. Explain why the age of a fault is younger than the rocks in which it is found
5. In Diagram 1, explain why there is a difference in the surface topography on
either side of the fault line.
6. In Diagram 2, how can you tell that the sandstone was not deposited on top of the
7. In Diagram 3, explain why the most recent unconformity indicates that a long
time has passed.
8. In Diagram 4, what evidence is there to suggest there was a surface basalt rather
than an intrusion?
CONCLUSION: What are the guidelines used to determine the relative ages of the
rocks events shown by a geologic cross-section?