A geological bed or stratification is a layer of sediment or volcanic material that is distinctly separate from other layers. Beds can vary in thickness from 1 cm thick to over 3 meters thick. Beds vary in texture and their resistance to weathering from one bed to another. The bedding plane separates beds and is a area easily fractured. There are a number of geologic laws related to stratification.
Formations
A formation is an outcrop of similar sediments forming a series of beds. A formation must be different from the surrounding rocks. The beds must be thick and wide enough to cover an area on a map to be considered a formation.
Law of Original Horizontality
There are a few laws concerning geologic stratification. The law of original horizontalitly states that all beds were flat and horizontal when they were formed. The beds may be tilted or folded after formation.
Law of Superposition
The law of superposition states that new rock beds are formed on top of existing rock beds. Therefore, when looking at a geologic column or outcrop, the older rocks are at the bottom of the column, the newer rocks are at the top of the column.
Law of Cross-Cutting Relationship
The law of cross-cutting relationships states that any fault or intrusion into the rock occurred after the beds were formed. Therefore, the fault or the intrusion are younger than the beds that they cut.
Law of Faunal Succession
The law of faunal succession states that fossils in geologic bedding follows a predictable fashion. Life has become more complex over time. Therefore, fossils in older geologic beds will be simpler than fossils found in more recent geologic beds. Furthermore, two fossils that lived during different time periods will not be found in the same geologic bed, as organisms are deposited in the rock forming when they died.
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