The relative age of a rock is determined by its order in the geologic
column. Consider the following set of layered rocks.
surface
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layer 1
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layer 2
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layer 3
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layer 4
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layer 5
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bottom
The relative age of these rocks is easily determinable by who is on top.
Sediments can not be deposited underneath a previously deposited rock. All
sediment falls to the ocean floor from above and it does not burrow into the
pre-existing sediments.
Thus, we can conclude absolutely that layer 1 is the youngest and layer 5 is
the oldest rock layer.
Where do fossils fit in?
Assume that certain fossils, called index fossils are found all over the
world in the same vertical order. Layer 1 has fossil a1, Layer 2 has fossil
a2, Layer 3 has fossil a3, etc.
If all over the world these fossils are found in that order, when I run into
a new area and find fossil a3 in a given rock layer, then I know that the
age of that fossil is equivalent to layer 3. If I dig below layer 3, I am
likely to find fossil a4, and if I can find a layer above this new layer, I
am likely to find either fossil a1 or fossil a2 or both. The circularity
that ICR constantly claims is escaped by having the vertical order of
fossils observed around the world, and this is based upon the vertical order
of the rock layers.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm