| A | B |
|---|
| catastrophism | the principle that all geologic change occurs suddenly |
| uniformitarianism | geologic processes at work today have been at work throughout Earth’s history |
| relative age | the approximate age of fossils or other objects in rock layers determined by comparing whether the surrounding rock layers are younger or older |
| relative dating | any method of determining whether an event or object is older or younger than other events or objects |
| geologic column | an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom. |
| absolute dating | any method of measuring the age of an event or object in years. |
| paleontology | the scientific study of fossils |
| fossil | the trace or remains of an organism that lived long ago |
| trace fossil | a fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment |
| mold | a mark or cavity made in a sedimentary surface by a shell or other body |
| cast | a type of fossil that forms when sediments fill in the cavity left by a decomposed organism |
| index fossil | a fossil that is used to establish the age of a rock layer because the fossil is distinct, abundant, and widespread and the species that formed the fossil existed for only a short span of geologic time. |
| geologic time scale | the standard method used to divide Earth's long natural history into manageable parts. |
| eon | the largest division of geologic time |
| era | a unit of geologic time that includes two or more periods |
| period | a unit of geologic time that is longer than an epoch but shorter than an era |
| epoch | a subdivision of geologic time that is longer than an age but shorter than a period |
| extinction | the death of every member of a species |
