Biology - Relative dating
Relative ages of fossils  can be determined according to the principle
of_ superposition_ that states that more recent sediments (and therefore
the fossils in them) are laid on top of earlier sediments. Even if rock
strata are folded, faulted and eroded, it is usually possible to work out
the sequence and therefore the relative ages of the fossils they contain.
In some parts of the world erosion has resulted in the exposure of many
strata. The 2 kilometre deep _Grand Canyon_ in the USA represents 500
million years of geological history. Railway and road cuttings expose rock
Two strata are said to correlate if they appear to be of the same age. In
many cases similar fossils are found in similar strata. If a fossil appears
only in specific strata, indicating a short time span before extinction, it
can be used as an index fossil. The index fossils help in correlating rock
strata in different locations.
The amount of fluoride in a fossil can be used in relative dating.
Fluoride ions in soil water replace ions in bone. Fossil bones of the same
age will contain the same amount of fluoride ions. The older the fossil,
the more fluoride ions it contains.
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