Principles of relative dating of geologic events


Smith learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage (the group of fossil species) that they contained.He also realized that a particular assemblage can be found only in a limited interval of strata, and not above or below this interval.Lateral continuity: The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region.If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon.They then go further by interpreting the formation of each feature to be the consequence of a specific geologic event.Examples of geologic events include: Deposition of sedimentary beds; erosion of the land surface; intrusion or extrusion of igneous rocks; deformation (folding and/or faulting); and episodes of metamorphism.



Example: Bed 1 at the base contains fossil species A, Bed 2 contains fossil species A and B, Bed 3 contains B and C, Bed 4 contains C, and so on.Building from the work of Steno, Hutton, and others, the British geologist Charles Lyell (1797–1875) laid out a set of formal, usable geologic principles.These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.

Original horizontality: The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field.

As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species.



Principles of relative dating of geologic events comments


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