The Geologic TimescaleA calendar of geologic time based

on Evolution

MillenniumCentury

Decade Year

Month Day

Subdivisions of Human Time

longer shorter

EonEra

Period Epoch

Stage Substage

Subdivisions of Geologic Time

longer shorter

Modern Geologic Time Scale

Hadean

Archean

Proterozoic

Phanerozoic

3800 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

January

Februa

ryMar

ch

Apr

il

MayJune

July

August O

ctoberNovember

December

2.5 Ga

540 Ma

4.6 Ga

Hadean Eon

Archean Eon 3.8 Ga

Sept

embe

r

Proterozoic Eon

Phanerozoic Eon

Modern Geologic Time Scale

Pale

ozoi

c

Ordovician

Cambrian

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Mes

ozoi

c

Triassic

Jurassic

Cretaceous

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene

Ceno

zoic

Terti

ary

Holocene

Qua

t.

Pale

ogen

eNe

ogen

e

540

510

439

408

360

323

290

245

208

146

57

35

23

5

1.6

.01

0

Carb

.

Quat. = QuaternaryCarb. = Carboniferous

Ma

65RIP

Hadean

Archean

Proterozoic

Phanerozoic

3800 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

Eons

Modern Geologic Time Scale

Pale

ozoi

c

Ordovician

Cambrian

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Mes

ozoi

cTriassic

Jurassic

Cretaceous

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene

Ceno

zoic

Terti

ary

Holocene

Qua

t.

Pale

ogen

eNe

ogen

e

540

510

439

408

360

323

290

245

208

146

57

35

23

5

1.6

.01

0

Carb

.

Quat. = QuaternaryCarb. = Carboniferous

Ma

65RIP

Hadean

Archean

Proterozoic

Phanerozoic

3800 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

Eras

Modern Geologic Time Scale

Pale

ozoi

c

Ordovician

Cambrian

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Mes

ozoi

c

Triassic

Jurassic

Cretaceous

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene

Ceno

zoic

Terti

ary

Holocene

Qua

t.

Pale

ogen

eNe

ogen

e

540

510

439

408

360

323

290

245

208

146

57

35

23

5

1.6

.01

0

Carb

.

Quat. = QuaternaryCarb. = Carboniferous

Ma

65RIP

Hadean

Archean

Proterozoic

Phanerozoic

3800 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

Periods

Modern Geologic Time Scale

Pale

ozoi

c

Ordovician

Cambrian

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Mes

ozoi

c

Triassic

Jurassic

Cretaceous

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene

Ceno

zoic

Terti

ary

Holocene

Qua

t.

Pale

ogen

eNe

ogen

e

540

510

439

408

360

323

290

245

208

146

57

35

23

5

1.6

.01

0

Carb

.

Quat. = QuaternaryCarb. = Carboniferous

Ma

65RIP

Hadean

Archean

Proterozoic

Phanerozoic

3800 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

Epochs

What records the passing of geologic time?

• Formation of rock layers• Sediments are deposited over time in

layers.• Each layer traps and records information

about the time during which it formed.• Sedimentary layers are analogous to the

pages that compose the book of Earth History.

East Devonshire

Problem - how do you determine the order in which rock layers formed?

At a single place, layers can be ordered using the law of Superposition.

oldest

less old

even less old

younger

youngest

time

Seco

ndar

y

Primary

Secondary

Transitional

Tertiary

Diluvialpost-Diluvial

circa 1790

Flood GravelsLayers composed of unconsolidated sediment

Hard rock layers with abundant fossils

Hard rock layers with sparse fossils

Crystalline rock

Earth History, 1700’s

Tertiary

Trans.

Primary

Primary

Secondary

Transitional

Tertiary

Diluvialpost-Diluvial

circa 1790

Parisian gypsum beds

London clay

alluvium

Sicilian strata

English chalk

OolitesLias

New Red Sandstone

Muschelkalk - TriasMagnesian Limestone

Coal Measures

Mountain Limestone

Old Red SandstoneDevonshire strata

Wenlock Limestone

Welsh Greywackes

Parisian chalk

Jura Mt. strataPerm strata

circa 1820

gravels

Crystalline (metamorphic) strata

British Isles Continental Europe

GeologicSystems

Cretaceous SystemD’Omalius d’Halloy, 1822

East Devonshire

Within a local region, rock layers can be correlated on the basis of their lithology (physical characteristics) to define a geologic system.

West Devonshire

Correlation - the matching-up of rock layers between different places.

• We can put local rock layers in the correct time order because we can see how they are stacked on each other.

• We can use the physical features of rock layers to correlate them into a regional system.

• The Problem: How can we correlate different regional systems so that they are in the correct time order if we can’t directly match their layers?

Great Britain Continental Europe

?

?

How can we correlate different systems if the layers cannot be correlated based on their physical features?

William Smith (1769-1839)surveyor, civil engineer

Smith made the first large scale geologic map showing the distribution and order of rock layers in Great Britain.

In his work as a surveyor, Smith noticed that the rock layers seemed to contain a unique sequence of fossil species that appear and disappear through time.

Even when the rocks look different, the sequence of fossils is always the same.

TI

ME

Evolves

Goes Extinct

Unique interval of time

Exists

TI

ME

Evolves

Exists

Goes Extinct

TI

ME

TI

ME

Great Britain Continental Europe

Fossils are the key to correlating regional systems

•Once a particular regional system was formally named and its fossils described, other regional systems with the same fossils were correlated to it and given the same name.

•The original system names thus came to stand for particular intervals of geologic time.

•The fossil species associated with each system are the only way to recognize other rock layers that formed at the same time.

Geologic Systems and Geologic Time

•For example, the Jurassic System was originally named for the rocks and fossils of the Jura Mountains between France and Switzerland.

•Now the Jurassic Period refers to the time interval during which the fossil species of the Jurassic System lived.

•Any rock layers with these fossils can be identified as Jurassic in age.

Geologic Systems and Geologic Time

Primary

Secondary

Transitional

Tertiary

Diluvialpost-Diluvial

Parisian gypsum bedsLondon clay

alluvium

Sicilian strata

English chalk

OolitesLias

New Red Sandstone Muschelkalk - Trias

Magnesian Limestone

Coal MeasuresMountain Limestone

Old Red Sandstone Devonshire strata

Wenlock Limestone

Welsh Greywackes

Parisian chalk

Jura Mt. strata

Perm strata

circa 1790 circa 1870

Modern Time Scale

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Precambrian

Quaternarygravels

Crystalline (metamorphic) strata

British Isles Continental Europe

How do we assign sedimentary layers to their correct place in time?

• fossils• each time interval in Earth history is defined by a unique set of species that existed at that time.

•Species evolve, live for a short time, and go extinct.

•The same species never evolves twice (extinction is forever).

•Evolution provides a “biological calendar” that geologists use to keep track of time.

• Fossils allow us to put the individual scenes from Earth history into the correct order to tell the full story.