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an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rock are at the bottom.
A unit of geologic time that includes two or more periods.
A unit of geologic time that is longer than an epoch but shorter than an era.
A subdivision of geologic time that is longer than an age but shorter than an period.
The process of change by which new species develop from preexisting species over 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.