San Francisco Bay Region Geology and Geologic Hazards

About Quaternary Faults > What is the Quaternary?

What is the Quaternary?

Quaternary time is the current period of geologic time, which began about 1.8 million years ago.

We divide time into years, months, and days, and we give these divisions names, such as 1776, August, and Friday. Similarly, geologists have divided the time of Earth’s long history into eons, eras, periods, and epochs of geologic time, and they have given each division a unique name. When they first started using these names, nobody knew how old the Earth was, or how many years ago the time divisions were. Instead, they based the division of time on how the fossils found in rocks changed from the oldest layers to the youngest. The Quaternary period is the name for the time in which we live. It spans the two most recent geologic epochs, the Pleistocene and the Holocene. Fossils from the Holocene epoch are like the animals living today, whereas Pleistocene fossils are much like living animals but with some differences. Many observations show that Pleistocene time was characterized by long periods of arctic conditions that allowed ice and snow to cover vast areas of land and sea, and so it is sometimes called the Ice Age. Animals of that time, such as the mammoth and the saber-toothed cat, were equipped to deal with those conditions. Holocene time is the warmer epoch since the last time of widespread icy conditions.

In the 20th century, following the discovery of radioactivity, geologists developed a tool to assign numeric ages to the divisions of geologic time. By observing the rate of decay of radioactive elements, and then measuring the amounts of both the radioactive elements and their decay products in rocks and minerals, geologists can calculate a numeric age for certain rocks and deposits, including those with bits of charcoal for carbon-dating. By careful study of the relations between the fossil-bearing sedimentary rocks and the rocks that have numeric ages, geologists have calculated the ages of the divisions of geologic time. The beginning of the Holocene was about 11,500 years ago, and the beginning of the Pleistocene was about 1,806,000 years ago.

Illustration - description below
Geologists used fossils to divide geologic time. The most primitive fossils are found in the oldest rocks, the rocks of the Archean eon. Younger are the more complex but still simple fossils of the Proterozoic eon, such as Dickinsonia. The Paleozoic era marked the first complex organisms, such as the trilobites. The Mesozoic era was the time of dinosaurs. The Cenozoic era is the age of mammals, such as the Eohippus (or dawn horse) of the Paleogene period. By Quaternary time, the fossils are of animals that are almost, but not quite, like animals that are alive today, such as the mammoth and the saber-toothed cat of the Pleistocene epoch.


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