| A | B |
|---|
| absolute age | actual age of an object |
| amber | hardened tree sap in which fossilsmay be preserved |
| angular unconformity | boundary between horizontal and tilted layers of rock |
| bedding plane | boundary between two sedimentary rock layers |
| coprolite | fossilized waste material from an animal |
| disconformity | boundary between layers of rock that have not been deposited continuously |
| evolution | change of living things over time |
| gastrolith | fossilized stone found within the digestive system of a dinosaur or other reptile |
| half-life | time required for half the mass of a radioactive element to decay into its daughter elements |
| index fossil | guide fossil; fossil found in the rock layers of only one geologic age and is used to establish the absolute age of the rock layers |
| law of crosscutting relationships | principle that a fault or intrusion is always younger than the rock layers it cuts through |
| law of superposition | principle that a sedimentary rock layer is older than the layers above it and younger than the layers below it |
| mummification | preservation of a dead organism by drying |
| nonconformity | unconformity in which stratified rock rests on unstratified rock |
| paleontologist | scientist who studies fossils |
| paleontology | study of fossils |
| petrification | process in which organic materials are replaced by new materials |
| principle of uniformitarianism | theory that geologic processes at work in the pesent were also at work in the past |
| relative age | age of an object compared with the ages of other objects |
| trace fossil | fossil trace left by an ancient organism, such as a track, footprint, boring, or burrow |
| unconformity | break in the geologic record created when rock layers are removed by erosion |
| varve | annual layer of sedimentary deposit on a lake bed |
