1. Ch.3, Sec.2 – Relative Dating: Which Came First?
- determining whether an object or event is older or younger than other objects or events is called relative dating
The Principle of Superposition
- the idea that younger rocks lie above older rocks in undisturbed sequences is called superposition
- unfortunately, not all rock layers are in this position, so aging them can be challenging!

2. Ch.3, Sec.2 – Relative Dating: Which Came First?
The Geologic Column
- geologists use the geologic column to help them age rocks, which may not be in the correct order because of rock disruptions like earthquakes & volcanoes

3. Ch.3, Sec.2 – Relative Dating: Which Came First?
Disturbed Rock Layers
- if rock layers are not horizontal, then the faults, folds, tilts, and intrusions must have come after the layers were created
intrusion
fault
fold
tilt

4. Ch.3, Sec.2 – Relative Dating: Which Came First?
Gaps in the Record - Unconformities
- missing rock layers create breaks in rock-layer sequences called unconformities
- an unconformity is a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time

5. Ch.3, Sec.2 – Earth’s Story & Those Who First Listened