1. Geologic History
Relative Dating

2. Birth of Modern Geology
Began in the late 1700’s when James Hutton published his Theory of the Earth.
He was the first scientist to argue effectively that geologic processes proceed over long spans of time.

3. Siccar Point, Scotland

4. Hadrian’s Wall, Scottland Built c. 1500, Hutton worked in 1700s.

5. Relative Dating
Prior to the discovery of radioactivity, geologists had no reliable way of giving specific dates to geologic events and had to rely on relative dating techniques.
Relative dating means placing rocks or events in their proper order, based on a comparison to other rocks

6. Relative Dating Laws The Principle of Uniformitarianism states that
the physical, chemical, and biological processes
that operate today have also operated in the past.
“The present is the key to the past.”

7. Relative Dating Laws
2) Principle of Original Horizontality states that sediments are deposited in horizontal layers
Layer 3 – Sandstone
Layer 2 – Limestone
Layer 1 – Siltstone

8. What happened to these rock strata?

9. What happened to this rock strata?

10. Relative Dating Laws
3) Principle of Superposition states that in an undeformed sequence of strata, each bed is older than the one above it and younger than the one below it.
*The layers on top are youngest.
* The layers on the bottom are oldest.
Relative dating (nonconformity animation); (Angular Unconformity)

11. Which layers is youngest? Oldest?

12. Possible Exceptions to the Law of Superposition
Folding of Strata over upon itself.

13. Possible exceptions to the law of superposition
Strata is displaced by a Thrust Fault

14. Relative Dating Laws
4) Principle of Cross-Cutting Relationships states
that an intrusion or fault is younger than the layers
it cuts through.

15. Cross-Cutting Relationships
Fault

16. Cross-Cutting Relationships

17. Relative Dating Laws
5) Principle of Inclusions states that an inclusion is
older than the rock layer it is in.

18. Unconformities
Buried erosional surfaces that are preserved in the rock record
Unconformities represent gaps in the rock record.
Relative dating (nonconformity animation); (Angular Unconformity)
unconformity animation

20. Angular Unconformity

21. Put the rock layers in order from oldest to youngest.

22. 1) Rock layers A-E are deposited.

23. 2) Rock layer D (a sill) intrudes.

24. 3) Rock layer F (a dike) intrudes.

25. Volcanic Column with radiating dikes, exposed after erosion.

26. 4) Layers are tilted and eroded (an angular unconformity).

27. 5) Deposition of rock layers G-K .

28. 6) Upper surface is eroded.