1. Relative Time: Ages of events are placed in order of occurrence.
No exact date is identified.
Ex. WWI and WWII
Ex. I am the second child in my family.

2. Absolute Time: Identifies the exact date of an event.
Ex. 65 Million Years Ago
Ex. 1990

3. Let’s practice with a sequencing example…

4. A B C D I E H

5. Finding age with relative time:
Principles used to determine relative order of geological events:
1. The Law of Superposition In a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom.
Picture the pile of clothes in your room. Where is the first thing that you put on the floor now?

6. 2. The law of cross-cutting relationships – anything that cuts across layers of rock is younger than the rocks that it has intruded into. This applies to faults and igneous intrusions.
Igneous
rock cuts
earlier layering
Layered rock
formed first

7. Which statement is true?
D crosscuts B
2. D crosscuts C
3. C crosscuts D
4. B was deposited
before E A B C E D

8. 3. The law of included fragments - the fragments that make up a rock are older than the entire sample.

9. 4. Original Horizontality- Rocks are usually deposited flat and level.
Picture I took of Grand Canyon

10. 5. The Law of Folds or Tilts: tilts in rocks are younger than the rocks themselves.

11. Picture I took at Red Rocks Amphitheater (Colorado)

12. Other Guidelines for figuring out a sequence:
Sedimentary rocks are usually formed under water.
Weathering and erosion usually happen above water (on dry land).
Contact metamorphism (“igneous intrusion”) shows that the rock that was changed was there first when the intrusion happened.

13. Now sequence that stratigraphy!

14. Sequence 1: Uplift & Erosion
First limestone is deposited
Then sandstone is deposited
Then shale is deposited
Then the layers are uplifted
Lastly, erosion takes place

15. Sequence 2

16. Sequence 2: Faulting First, limestone is deposited
Then sandstone is deposited
Next shale is deposited
Then faulting occurs

17. Sequence 3

18. Sequence 3: Folding First limestone is deposited
Then sandstone is deposited
Next shale is deposited
Lastly folding occurs

19. Sequence 4

20. Sequence 4: Igneous Intrusion
Limestone deposited
Sandstone deposited
Shale deposited
Igneous Intrusion
Note: contact metamorphism is the same event as the intrusion.

22. What is an Unconformity?
A buried erosional surface.
Look for a “wavy” line that is buried beneath the surface (indicates that at one time this layer was at the surface and weathering and erosion took place).

23. Formation of an unconformity
Layers are formed according to superposition.

24. Something happens to uplift the area (folding, faulting, etc.)

25. Erosion wears away the uppermost layers

26. Area submerges and deposition begins again.
Here’s the
unconformity