Scientists divide geologic time into eras, measured in millions of years. Within each named era are periods, and within each period are epochs. Each vast unit of time is defined by the appearance and disappearance of various life forms and climactic conditions on Earth's ever-changing face. Geologists, paleontologists, archaeologists, anthropologists, and historians read the book of time in Nebraska's land. Their shared insights and interpretations can paint a detailed picture of life ten thousand years.

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