Earth and Space Science / Unit 5: Weathering, Erosion and Earth's History
5.5 Relative Dating
Notes
Relative dating puts events in order without exact ages.
- Law of superposition: in undisturbed layers, the oldest are at the bottom.
- Original horizontality: sediment is deposited in flat layers.
- Cross-cutting relationships: a fault or intrusion is younger than the rock it cuts.
- Inclusions: a fragment inside a rock is older than the rock around it.
- An unconformity is a gap in the rock record where erosion or no deposition occurred.
- Index fossils (widespread, short-lived species) help match layers across regions.
Practice
In undisturbed sedimentary layers, the oldest layer is usually:
Law of superposition.
A fault cuts through layers A, B and C. The fault is:
Cross-cutting relationships.
A good index fossil is:
Such fossils match layers of the same age.
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