A Little Bit of History…
• The history of geologic time begins with the human interest
in mining.
• Interests in rock units began to flourish with commercial
mining in the 1500s and 1600s
• People began to identify strata (rock layers) by the types of
fossils that were found within them.
• Two scientists were credited with contributing the most
research to this area -- Nicolaus Steno and James Hutton
.
Nicolaus Steno

Adapted two geologic principles
1.
Law of Superposition : sedimentary rocks are
found with the oldest on the bottom and
youngest on top.
E.x. Paint layers
2.
Principle of Original Horizontality: If left
undisturbed rock layers will stay in horizontal
order
James Hutton
18th-century Scottish
physician and farmer
 Studied his farm land for geologic changes
*The Present is the Key to the Past
 Principle of Uniformitarianism: geologic
processes from the past are the same as the
current geologic processes.
E.x. What happened then happens now
Relative Age: gives the age of a rock layer based on
the ages of layers around it (younger or older)
What sort of events can change the order of rock layers?
Law of Crosscutting Relationships
 Fault or body of rock is younger than any other body of rock
that it cuts through.
 Igneous intrusions that may be uplifted to the surface would
also be younger than the rock they push through
Unconformities- breaks
in geologic time where
rock layers erode away
or when sediment isn’t
deposited
Paleontology
 The study of fossils.
 Fossils: remains of plants
 and animals that lived in a previous
geologic time
Organisms have changed throughout the
geologic past.
 Fossils provide the clues behind
environmental changes of the past.
What kinds of rocks are fossils found in?

Fossilization


Organisms with hard
parts are typically
preserved.
Those without hard parts
are fossilized by quick
burial.
How are fossils formed?
 Mummification
 Amber
 Tar seeps
 Freezing
 Petrification
Mummification
Drying of organisms; found in deserts.
Amber
Hardened tree sap, insects get
stuck in sap and sap hardens
La Brea Tar Pits
•Thick petroleum
•Animals get stuck in tar
and it preserves them
Freezing
Animals preserved because bacteria
cannot survive in cold climate to
decay the bodies
Petrification
When organic material is
replaced with minerals
Types of fossils
 Trace Fossils
-Evidence that
an animal existed.
Ex. footprints
Types of Fossils continued…
 Molds: animal
remains dissolve
away, but the shape
of the animal
remains.
 Cast: when a mold
fills in with sediment
and becomes
hardened.
Imprints- carbonized imprints
Coprolites fossilized animal droppings
(learn eating habits)
Gastroliths- stones found in dinosaur
stomachs to help them digest food
(are generally round and polished)
Index fossil: Unique fossil that lived
during a specific time
Trilobites lived 750 million years ago
Specific index fossils for every time period
The Geologic Time Scale is a type
of calendar geologists use to put
events in their proper order
Calculating Earth's Age


Earth’s age was
estimated by measuring
the radioactive decay of
uranium to lead.
With improved
techniques and evidence
from tree rings and
glaciers, the age of
Earth is estimated to be
about
4.5 – 4.6 billion
years old.
Geologic Column
•
•
•
•
By applying relative ages to sedimentary
rocks we began to look at fossils to learn to
date them.
Not one place contains a complete record of
geologic time.
Geologic Column- ordered arrangement of
rock layers based on relative ages of the
rock
Once geologists have dated a lot of the rock
layers, they can fill in the gaps.
Divisions of Geologic Time
Geologic time is typically divided by
clear changes.
Ex. extinctions and dominant fossils
MYA- millions of years ago
 BYA- billions of years ago

Divisions of Geologic Time cont…
• Eons – largest unit of geologic time (4 total eons).
• We call the first three eons the Precambrian time
which has little fossil evidence.
Microbe-rich stromatolite reefs, like this one in Shark Bay,
Australia, were among the first sources of oxygen on Earth. They
flourished throughout the planet's shallow waters 3.5 billion years
ago, but are extremely rare today.
Continued…
Era- Unit of time that includes two or more periods.
•
There are 3 eras that the Phanerozoic Eon can be divided
into:
1.
Paleozoic
*Zoic = “animal life”
2. Mesozoic
3. Cenozoic.
•
Different fossils are represented in each of these eras:
marine life, birds and reptiles and mammals.
•
Paleozoic: explosion of diverse life
 Beginning
– Oceans
 Middle
to the End
– Forests and jungles
– Land animals evolved
Devonian Period: Age of Fishes
Mesozoic: Age of Reptiles
Jurassic Period:
Dinosaurs ruled the
Earth
Cenozoic: Age of Mammals
• Eras are divided into shorter time
units called Periods.
• Each period is characterized by
specific fossils and is named for
the location they were discovered.
• Epochs-when the fossil record is so
complete we can keep dividing
periods into epochs.
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