The relative age of rocks and fossils can be determined using two basic methods: law of superposition and index fossils.
The Law of Superposition
Newer or younger rock layers are found on top. Older rock layers are found at the bottom.
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More Complex Rock Layering
Complex layering due to intrusions and extrusions, faults, or unconformities can make dating rocks and fossils challenging.
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Can you determine which occurred more recently E or A?
Can you determine which is layer is older D or B? |
Unconformity
The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.
Video Guide:
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Topic Tips
- The law of superposition takes advantage of the fact that older layers of rock tend to be found at the bottom and younger layers of rock tend to be found on top.