[[Evidence]] from [http://en.wikipedia.org/wiki/Radiometric_dating radiometric dating] indicates that the [[Earth]] is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to [[events]] which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as [[Catastrophism|mass extinctions]]. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the [http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Tertiary_extinction_event Cretaceous–Tertiary extinction event], which marked the demise of the [http://en.wikipedia.org/wiki/Dinosaur dinosaurs] and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. | [[Evidence]] from [http://en.wikipedia.org/wiki/Radiometric_dating radiometric dating] indicates that the [[Earth]] is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to [[events]] which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as [[Catastrophism|mass extinctions]]. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the [http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Tertiary_extinction_event Cretaceous–Tertiary extinction event], which marked the demise of the [http://en.wikipedia.org/wiki/Dinosaur dinosaurs] and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. |