Fossil - The preserved remains or traces of an organism that lived in the past, Index Fossil - A fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found, Relative Dating - Method of determining the age of a fossil by comparing its placement with that of fossils in other layers of rock, Cross-cutting Relationship - The principle that an intrusion or fault is younger than the rock it cuts across, Fault - A crack in the earth's crust, Law of Horizontality - The geologic law that states that all rock layers are deposited flat (horizontally), Igneous Intrusion - Forms when magma cuts through formed sedimentary rock layers, which cools and hardens. It is younger than the surrounding rock layers., Law of Superposition - The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it., Unconformity - A gap in the geologic record that shows where rock layers have been lost due to erosion., Absolute Dating - A method used to determine the actual age of a fossil or rock in years, Radioactive Decay - The process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element. It can be used to determine the Absolute Age of a rock or fossil., Geologic Time - A record of the geologic events and life forms in Earth's history., Eon - The largest division of geologic time, Epoch - A division of time that is a subdivision of a period and is itself subdivided into ages, Era - A division of an eon on the geologic time scale, Period - A division of geologic time into which eras are subdivided,

Geologic Time

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