an ordered arrangement of rock layers that is based on the relative ages of the rocks and which the oldest rocks are at the bottom
ear
a unit of geologic time that includes two or more periods
period
a unit of geologic time that is longer than an epoch but shorter than an era
epoch
a subdivision of geologic time that is longer than an age but shorter than a period
evolution
the process of change by which new species develop from preexisting species over time.
precambrian time
the interval of time in the geologic time scale from the Earth's formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago
Paleozoic Era
the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago
mass extinction
an episode during which large numbers of species become extinct
Mesozoic Era
The geological era that lasted from 251 million to 65.5 million years ago. Also called the age of reptiles
Cenozoic Era
The current geological era that began 65.5 million years ago, also called the age of mammals