Geologic time describes the immense span of time—billions of years—revealed in the complex rock surface of Earth.Geologists have devised a geologic time scale that divides Earth's history into units of time. A unit is defined in terms of the fossils or rock types found in it that makes it different from the other units.Eras, the four largest time blocks in the scale, are named to indicate the fossils they contain: Precambrian (before ancient life), Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life).
Since that time, geologists have learned to study the strata, or the thousands of layers of sedimentary rock that make up the Earth's crust.
Over the course of history, the natural weathering of Earth's surface created sediments (rock debris) that settled in layers on the surface.
As the layers built up, the underlying layers were compressed together to form sedimentary rock.
The two most recent periods are further subdivided into seven epochs.
Before the eighteenth century, ideas about time and the history of Earth came mostly from religious theories.
Many people believed Earth was only a few thousand years old.
They also believed that all the physical features of Earth—mountains, valleys, oceans, rivers, continents—were the same as they had always been.
Everything that existed on Earth was the same as it had been in the beginning.
In the eighteenth century, geologists began to theorize that Earth's lifetime was immense.
However, since they lacked sophisticated scientific measuring devices, they could only offer educated guesses.
They compared the rock record from different parts of the world and estimated how long it would take natural processes to form all the rocks on Earth.