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carbonaceous film

carbon deposit

relative age

approximate age

fossil

remains of a once living organism

radioactivity

breakdown of unstable isotopes into stable atoms

mold

imprint of a once living organism

isotope

variation in number of neutrons

original remains

amber or frozen remains

petrified remains

remains replaced by minerals

law of superposition

oldest rock layers are under younger ones

cast

sediments fill a mold

half-life

amount of time for half of a substance to breakdown

trace fossil

footprints or animal tracks

fossil fuels

natural gas, oil and coal

index fossil

organism that lived in a wide area for a short time

absolute age

exact age

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