Outline 5: The Geologic Time Scale
 How it was created.
A First Attempt, 1759
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Based on "appearance" of the rocks.
Latin terms:
Primary - crystalline rocks
Secondary - folded sedimentary rocks
Tertiary - undeformed sedimentary rocks
Quaternary - loose sediments
Work in the 1800s
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Based on relative ages of fossils rather than the appearance of rocks.
The Eras: (Greek terms)
Paleozoic: old life
Mesozoic: middle life
Cenozoic: recent life
Work in the 1800s, cont’d
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The periods of the eras were named after places or unique
characteristics.
Cambrian - Latin for Wales
Ordovician - the Ordovices a Welsh tribe
Silurian - the Silures, a Welsh tribe
Devonian - Devonshire, England
Work in the 1800s, cont’d
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Carboniferous - carbon in coal
Mississippian: rocks in Mississippi River Valley
Pennsylvanian: coals in Pennsylvania
Permian - the Perm Basin in Russia
Work in the 1800s, cont’d
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Triassic - a 3-part division of rocks in Germany
Jurassic - the Jura Mountains of Switzerland
Cretaceous - Latin for chalks of Paris Basin
Tertiary - left over from 1759
Quaternary - left over from 1759
Work in the 1800s, cont’d
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The Cenozoic epochs have Greek roots.
Paleocene - ancient recent
Eocene - dawn of the recent
Oligocene - little recent
Miocene - less recent
Pliocene - more recent
Pleistocene - most recent
Holocene - completely recent
Work in the 1800s, cont’d
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What about really old rocks with no fossils?
Geologists didn’t know how to divide these rocks.
All were called Precambrian, an informal term we still use today.
Work in the 1900s
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Precambrian rocks were divided into two eons:
Archean Eon: the beginning
Proterozoic Eon: earliest life
Younger rocks were grouped into the Phanerozoic Eon: visible life
Work in the 1900s
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Radiometric dating allowed the various eons, eras, periods, and
epochs to be dated.
It turned out that the subdivisions are of very unequal duration. For
instance, the Cretaceous Period at 79 MY is longer than the Cenozoic
Era at 65 MY.
Work in the 1900s, cont’d
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Although the subdivisions are of unequal length, they still are very
useful for talking about geologic time.
Geologists use the names of the Geologic Time Scale because it is a
type of "shorthand" easier to remember than dates in years.
A “Must Do” for You
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Memorize the names of the Geologic Time Scale in your book. Don’t
worry about dates in years. Do note that 88% of earth history is
Precambrian.
Use the pneumonic devices that I will give you in class (see below)
Archean, Proterozoic, Phanerozoic: All Purpose Pill
Paleozoic, Mesozoic, Cenozoic: Park My Car
Cambrian through Neogene: Can Ordinary Students Dupe Crabby Professors
to Judge Class Projects Nicely?
Cenozoic epochs: Paleocene, Eocene, Oligocene, Miocene, Pliocene,
Pleistocene, Recent: People Eat Only Mario's Pan Pizza, Right?