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Geologic Time
Earth’s History
The present is the key to the past
Uniformitarianism
 Processes
that happen today are the same processes
that happened in ancient times
Fossilized Ripples
Mudcracks
Modern
Ripples
Modern
Mudcracks
Dating – 2 types
• Relative Dating
– Know order of events
but not dates
• Absolute Dating
–Know exact age
Relative Dating – Key Concepts
• Law of Superposition
• Principle of Original Horizontality
• Principle of Cross-Cutting Relationships
• Principle of Inclusions
Law of Superposition

Each bed is younger than the bed below it.
Principle of Original Horizontality

Sedimentary layers are deposited horizontally
Principle of Cross-Cutting
Relationships
• Faults and intrusions are younger than the rocks they
cut through
Inclusions
• Fragments of one rock unit that have been enclosed
within another are older
Relative Dating
Youngest 
Oldest 
G
A
B
C
F
D
E
E is younger than D. Which Principle used?
Law of Superposition
G is younger than F. Which Principle is used?
Principle of Cross-Cutting Relationships
Principle of Inclusion
A

B
Using the Principle of Inclusions, which picture shows the
granite intrusion to be the youngest event?
B
Conformable Beds
• Beds that have not been disturbed
Uncomformities
• Substantial break or gap in the rock record
• 3 types
– Angular
– Disconformity
– Nonconformity
Angular Unconformity
• Older layers have been deformed, eroded,
and younger layers deposited above
Disconformity
• Beds are parallel relative
to each other but there is
a break in sequence
• Implies a cessation of
sedimentation or
erosion, but no tilting
• Hard to recognize
Nonconformity

Sedimentary layers overlie igneous or
metamorphic layers
Unconformities
1.
2.
3.
4.
Question:
Surface 1 is an example of ?
Surface 2 is an example of ?
Surface 3, 4 and 5?
Granite is _________ than the Schist.
Answer:
Nonconformity
Angular unconformity
Disconformity
Younger
Correlating Rocks

Matching rocks of the
same age in different
locations
How do we correlate?


Walking out the contact
Matching fossils
Fossils

The remains or traces of prehistoric life
Types of Fossils

Body fossils
–

Trace fossils
–
Record movement
Bone, teeth, shells
Fossilization

Hard parts
–
Unaltered
Fossilization - Petrified

Turned into stone
Petrified Forest, Arizona
Fossilization - Replacement

Minerals replace organic matter
Fossilization – Molds & Casts
Mold
Cast
 Mold – impression or cavity
 Cast – deposited material that fills cavity
Fossilization - Amber

Hardened Resin
Conditions favoring Preservation


Dies
Rapid burial
Possession of hard parts
Buried
Eroded
Fossils and Correlation

William Smith
–
Each rock formation contained
fossils unlike the ones above
& below
–
Sedimentary strata in widely
separated areas could be
identified & correlated by their
fossil content
Principle of Fossil Succession

Fossils occur in a definite, invariable sequence
in the geologic record
–
Apply with Law of Superposition and can document
evolution of life through time
Index Fossils


Widespread geographically
Limited to a short span of geologic time
Correlating Rocks & Fossils
 Igneous rocks are
dated radiometrically
 Ages of fossils can be
bracketed
 Fossils in another area
can be correlated
containing same fossils
 Age in another area
determined indirectly
 Doing this repeatedly the
geologic time scale
established
Geologic Time Scale