In geology, “Cen” is almost always shorthand for the Cenozoic Era, the current stretch of Earth’s history that began 66 million years ago and continues today. You’ll see it on geological maps, stratigraphic charts, and field notes as a quick label for rocks and deposits from this era. Less commonly, “Cen” can abbreviate the Cenomanian Stage, a specific slice of the Late Cretaceous Period that ran from about 100.5 to 93.9 million years ago.
The Cenozoic Era: Earth’s Current Chapter
The Cenozoic Era started exactly where the age of dinosaurs ended. Its base is set at 66 million years ago on the International Chronostratigraphic Chart, matching the mass extinction event that wiped out non-avian dinosaurs. Everything that has happened on Earth since then, from the rise of grasslands to the evolution of humans, falls within the Cenozoic.
The word itself comes from Greek: “kainos” meaning new and “zoion” meaning life. So Cenozoic literally translates to “new life,” reflecting the wave of animal and plant evolution that followed the end-Cretaceous extinction.
How the Cenozoic Is Divided
The Cenozoic breaks into three periods, each containing smaller units called epochs:
- Paleogene Period (66 to 23 million years ago): Includes the Paleocene (66–56 mya), Eocene (56–34 mya), and Oligocene (34–23 mya) epochs. This is when mammals diversified rapidly to fill ecological roles left empty by the dinosaurs.
- Neogene Period (23 to 2.6 million years ago): Includes the Miocene (23–5 mya) and Pliocene (5–2.6 mya) epochs. Grasslands spread across continents, and many modern mammal families appeared.
- Quaternary Period (2.6 million years ago to present): Includes the Pleistocene (2.6 million to about 10,000 years ago) and the Holocene, which covers the last 10,000 or so years. This period is defined by repeated ice ages and the emergence of modern humans.
Why the Cenozoic Matters
The Cenozoic is often called the Age of Mammals because mammals became the largest land animals during this time. But that nickname undersells the era. As paleontologists at UC Berkeley’s Museum of Paleontology have pointed out, it could just as accurately be called the Age of Flowering Plants, the Age of Insects, the Age of Birds, or the Age of Bony Fish. All of these groups exploded in diversity during the Cenozoic.
The era also reshaped the planet’s geography. The collision of tectonic plates built the Himalayas and the Alps as the ancient Tethys Ocean closed. India plowed into Asia, Africa pushed into Europe, and the resulting mountain-building events continued from the late Eocene through the Quaternary. These shifts altered ocean currents, weather patterns, and habitats worldwide, driving much of the climate change and evolution that defines the last 66 million years.
The Other “Cen”: The Cenomanian Stage
In some contexts, particularly when reading about Cretaceous stratigraphy, “Cen” refers to the Cenomanian Stage. This is the first and oldest division of the Upper (Late) Cretaceous, spanning from 100.5 to 93.9 million years ago. It sits above rocks of the Albian Stage and below those of the Turonian Stage. The name comes from Cenomanum, the Latin name for the city of Le Mans in France, where these rocks were first described.
The Cenomanian is geologically significant because of what happened at its end. The boundary between the Cenomanian and Turonian stages marks one of the most severe oceanic crises of the past 100 million years. Known as Oceanic Anoxic Event 2, this was a period when large portions of the world’s oceans lost their dissolved oxygen. Rising atmospheric CO2 drove temperatures higher, which slowed ocean circulation. Increased rainfall washed nutrients into the seas, triggering massive algal blooms. As those organisms died and decayed, they consumed oxygen faster than it could be replenished, creating vast dead zones. More than a quarter of marine invertebrate species went extinct during this event.
How to Tell Which “Cen” You’re Looking At
Context usually makes it clear. On a standard geological map or timescale chart, “Cen” with no additional qualifiers refers to the Cenozoic Era. If you see it on a map legend next to rock units colored in the warm yellows and oranges typically used for Cenozoic deposits, that’s your answer. The Cenomanian abbreviation, by contrast, typically appears in detailed stratigraphic columns focused on the Cretaceous Period, and it’s often written as “Cen.” alongside other Upper Cretaceous stage abbreviations like “Tur” (Turonian) or “Alb” (Albian).
If you’re reading a geological map of a region with exposed Cretaceous rocks, such as parts of the U.S. Western Interior, northern France, or the Middle East, look at the surrounding labels and the map’s legend to confirm which meaning applies. In the vast majority of general geology materials, “Cen” means the Cenozoic.