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Once the relative ages of a number of fossils have been determined, the fossils can be used to determine the relative age of the beds containing them.
For example, if a bed contains Fossil F (from the succession speciﬁed above), geologists can say the bed is older than a bed containing Fossil A, even if the two beds do not crop out in the same area.
From these data, we can deﬁne the range of speciﬁc fossils in the sequence, meaning the interval in the sequence in which the fossils occur.
The sequence contains a deﬁnable succession of fossils (A, B, C, D, E, F), that the range in which a particular species occurs may overlap with the range of other species, and that once a species vanishes, it does not reappear higher in the sequence.
These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.
Such species are called index fossils (or guide fossils), because they can be used by geologists to associate the strata with the speciﬁc time interval.
The most obvious feature of sedimentary rock is its layering.
Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever.
Smith’s observation has been repeated at millions of locations around the world, and has been codiﬁed as the principle of fossil succession.
If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified.