This post is the second part of the topic of HOW DO GEOLOGISTS | Geology of the world and the Environment

This post is the second part of the topic of HOW DO GEOLOGISTS KNOW HOW OLD A ROCK IS? ( )

Faunal Succession: Similar to the law of superposition is the law of faunal succession, which states that groups of fossil animals and plants occur throughout the geologic record in a distinct and identifiable order. Following this law, sedimentary rocks can be “dated” by their characteristic #fossil content. Particularly useful are index fossils, #geographically widespread fossils that evolved rapidly through time.
Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as #igneous #intrusions or #faults are younger than the units they cut across.
Inclusions: #Inclusions, which are #fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating. Inclusions are useful at contacts with igneous rock bodies where #magma moving upward through the crust has dislodged and engulfed pieces of the older surrounding rock.
Gaps in the geologic record, called #unconformities, are common where deposition stopped and #erosion removed the previously deposited material. #Fortunately, distinctive features such as index fossils can aid in matching, or #correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating. Relative dating techniques provide geologists abundant #evidence of the incredible vastness of geologic time and ancient age of many rocks and formations. However, in order to place absolute dates on the relative time #scale, other dating methods must be considered.
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