Geologic Timescale

The geologic timescale divides the 4.5-billion-year history of Earth into unequal divisions of time. Originally, these divisions were based on sedimentary rocks and their corresponding fossils. Each time interval has its own unique fossil assemblage, and the boundaries between many intervals of geologic time correspond to mass extinction events. The most famous of these was at the boundary between the Mesozoic and the Cenozoic eras. At that time, land-dwelling dinosaurs became extinct, likely due in part to the impact of a large asteroid near the present-day Yucatan Peninsula.

The intervals of the geologic timescale are hierarchic—eons include eras, eras include periods,

Table 1 Geologic timescale

Source: National Park Service.

and then periods are divided into epochs. Epochs can be further subdivided into stages. In developing the geologic ...