1. Geologic Time
Uniformitarianism vs. Catastrophism

2. Uniformitarianism vs. Catastrophism
James Hutton, 1788
Theory of the Earth
Charles Lyell, 1830’s
Principles of Geology
Gradual geologic processes
Erosion, deposition, mountain building
Earth must be billions of years old
Most everyone else
All geologic change occurs suddenly, during catastrophes
Earth only a few thousand years old

3. Today, a Little Bit of Both
Modern geology incorporates both ideas
Most geologic change is gradual and uniform
Catastrophes in Earth’s history have caused geologic change

4. Relative Dating: Which Came First?
Determining whether an object or event is older or younger than other objects or events (one object relative to another)
Stratigraphy
The branch of geology that studies rock layers/layering
Based on 5 Principles
Horizontal Origin
Superposition
Unconformities
Cross Cutting
Correlation (Rock Layers & Fossils)

5. 1) Horizontal Origin
• Principle states that most sediments were deposited in relatively
horizontal layers
• All sediments found otherwise must have been deformed by
some force

6. 2) Superposition
- Younger rocks lie above older rocks in undisturbed layers- layers that are horizontal

7. 3) Unconformities •Gaps in the Geologic Record •Types
–Product of erosion or non-deposition
•Types
–Non-conformity
–Disconformity
–Angular Unconformity

8. Disconformity
Resulting from EROSION of underlying beds. Part of the rock layer sequence is missing
Most common unconformity and represents 1000’s to 1,000,000’s of years of missing time.
Disconformity

9. Angular Unconformity
In this type, the older beds are tilted and eroded at the surface of unconformity.
Represents 1,000,000’s of years of missing time.
Angular Unconformity

10. Nonconformity
Occurs when sedimentary rock has been deposited above pre-existing (eroded) metamorphic or igneous rock
Represents 1,000,000’s of years of missing time.
Nonconformity
Igneous rock
Deposited sedimentary rock
Igneous rock

11. 4) Cross-cutting Relationships
•A rock is younger than any rock/event across which it cuts.
•Events include:
–Igneous Intrusions
–Faults

12. 5) Correlation
Key rock strata and index fossils found in one location will be dated during the same time as similar key rock strata and index fossils found in another location.

13. Identify the sequence of events in the diagram below.

14. E G L C H M D J A O N K B F O