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Determines exact ages using radioactive decay.

Time needed for half of radioactive atoms to decay.

Rock layers studied for sequence and history.

Study of prehistoric life using fossils.

Sediments are deposited in horizontal layers.

Oldest layers are at the bottom in undisturbed sequence.

Younger features cut across older rocks.

Included rock fragments are older than the rock they are in.

Tilted rocks beneath horizontal layers.

Parallel rock layers above and below a missing time gap.

Sedimentary rocks overlie metamorphic or igneous rocks.

Largest unit of geologic time, lasting half a billion years or more.

Time unit representing several hundred million years.

Unit representing 100 million years.

Time unit lasting tens of millions of years.

Smallest division of geologic time.

Oldest eon; Earth’s formation, atmosphere, oceans.

Eon with first continents and microbial life.

Eon with oxygen, eukaryotes, multicellular life.

Era with marine life, fish, amphibians, reptiles, and coal.

Era of dinosaurs and the breakup of Pangaea.

Era with mammals, birds, trees, ice ages.

Entire organism preserved in tar, ice, etc.

Hollow impressions left in rock.