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Units of time not to scale; not all units shown. For
the complete International Chronostratigraphic
Chart, visit stratigraphy.org.
A Rocky Start
In the 19th century, early geologists went on a naming
spree, labeling rock formations including strata. With
similar enthusiasm, they started arguing about them.
Several early attempts at standardizing the names failed.
It wasn’t until the 1960s that the International Union of
Geological Sciences created a system and nomenclature
that everyone could, more or less, agree on.
The current system organizes geologic time into five
kinds of units, nested like Russian dolls. Think of it as
the deep time version of a month being broken down
into seconds, minutes, hours, days and weeks. Despite
this orderly hierarchy, some confusion remains for the
non-scientist. Because the system evolved from using
both relative and absolute dating methods, there are
two coexisting names for each type of unit. You may
see, for example, the Cretaceous called either a system
(the chronostratigraphic unit, emphasizing the position
of rock strata relative to each other) or a period (the
geochronologic unit, emphasizing the actual, or absolute,
age of the rock). Either way, the Cretaceous began
145 million years ago and ended, along with all dinosaurs
except birds, 66 million years ago.
Divide and Conquer?
Boundaries between units are determined by events
such as changes in the fossil record, polarity reversals in
Earth’s magnetic field, significant climate shifts or large
global deposits of a particular mineral or type of rock. The
Carboniferous of 300 million to 360 million years ago, for
example, got its name from massive coal deposits that
formed during its span across much of the world.
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What’s in a Name?
Chronostratigraphy and closely related disciplines have
been evolving for more than 200 years, so perhaps it’s no
surprise that the names of units are a mishmash of the
scientific and the sentimental, the practical and the poetic.
Geographic: Many units are named for the geographic
location where rocks and fossils of that age were first
recorded, such as the Permian (Perm, a Russian city about
700 miles east of Moscow) and the Jurassic (after the Jura
Mountains of France and Switzerland).
Historical: In the mid-19th century, European geologists
often borrowed the names of ancient peoples, such as the
Silurian and Ordovician, after two ancient tribes of Wales.
Or they’d take inspiration from ancient towns, such as the
Zanclean, honoring the pre-Roman name of what’s now
the Italian city of Messina.
Sequential: Some periods (or systems) of the Paleozoic,
including the Devonian, are simply divided into upper,
middle and lower epochs (or series).
Geological: Similar to the Carboniferous, the Cretaceous is
named for global deposits of creta (Latin for “chalk”). The
origins of the Triassic’s name reflect a distinctive trilayer of
rocks first found in northern Europe.
Environmental: The Cryogenian, about 635 million to
720 million years ago, reflects the “snowball” state of
Earth during prolonged ice ages.
Alternate: While the International Commission on
Stratigraphy recognizes one global name for each unit, a
number of regional differences persist, usually for recent
ages, which have yielded the most detailed data. For
example, within the Quaternary, our current period, the
age before the Holocene is known as the Devensian in the
United Kingdom, the Weichselian in much of northwestern
Europe, and the Wisconsinan in the U.S.
Chronostratigraphic Geochronologic
Eonothem
Erathem
System
Series
Stage
Eon
Era
Period
Epoch
Age