1. Determining a Sequence of Geologic Events

2. Principle of Uniformitarianism
“The present is the
key to the past.”
To understand geologic
processes of the past,
study geologic processes
of today.

3. Law of Original Horizontality
Sedimentary rock layers were originally deposited in horizontal layers

4. Law of Superposition
Oldest rock layers are on the bottom

5. Law of Continuity of the Layers
Ex. Layer C is the same rock unit separated by erosion

7. An exception to the Law of Superposition is a thrust fault
Here older rock is on
top of younger rock

8. Another exception is an
Overturned fold
OLD
YOUNG
OLD

9. Another exception: Igneous Intrusions
contact
metamorphism
on all sides
Another exception: Igneous Intrusions
The igneous rock is younger than the rock unit on
top of it

10. Igneous intrusion is the youngest rock unit here
contact
metamorphism
Igneous intrusion is the youngest
rock unit here

11. This is not an exception to the law of superposition, why?
No contact metamorphism
here
This is not an exception to the law of superposition, why?
There is no contact metamorphism on the top of the basalt,
Therefore this is a buried lava flow and not an intrusion

12. Later deposition may cover this lava
Lava flow seen here covering a road in Hawaii becomes
part of the rock layers and follows the law of
superposition

13. Law of Cross-Cutting Any geologic event that affects a rock
layer must be younger
than the rock layer it
affects

14. Layer X is an intrusion and is younger than any other rock unit.
This is an example of the Law of Cross Cutting

15. This fault is also younger than any other rock unit and is
Another example of the Law of Cross Cutting

16. Layers of volcanic ash also become part of the
geologic record, and serve as excellent time markers

17. UNCONFORMITY: ROCK LAYERS ARE MISSING DUE TO A PERIOD OF EROSION
Note- during deposition the
region is submerged underwater
During erosion periods
the region is dry land
unconformity

19. The Grand Canyon
has a complex
geologic history

21. Law of …?

22. Be able to explain these terms
Principle of Uniformitarianism
Law of Original Horizontality
Law of Superposition
Law of Continuity of Layers
Law of Cross-cutting
Unconformity