www.jeffthefish.com

I found this on a message board I recently discovered.  The quote is in response to the claim that rock layers are dated according to the fossils they contain, and fossils are dated according to the rock layers they are found in, as made famous by Kent Hovind and “Dr.” Ron Carlson. This claim, as you may have guessed, is simply false. Theology Web user JonF explains how fossils are actually dated:

Strata are dated relative to each other by the principle of superposition and the principle of cross-cutting (I may have that name wrong); unless there is evidence to the contrary, the deeper layers are older, and layers that intrude into other layers are newer than the layers into which they intrude. Strata are dated absolutely by radiometric methods and a few non-radiometric methods.

Index fossils are used to correlate strata between different locations. Index fossils are those that appear in many places across the globe, and appear in relatively thin sections of the geologic column.

A made-up but realistic example may make this clearer. We find a layer of lava, which dates to 77 million years ago by several independent methods. Under that layer we find a layer of sedimentary rock. In that layer of sedimentary rock we find a dinosaur fossil and a foraminifera fossil, and the foraminifera fossil is a well-known index fossil. We know from the principle of superposition that the dinosaur fossil is older than the lava, and is therefore older than 77 million years.

Somewhere far away we find a layer of sedimentary rock containing the same foraminifera fossil, and below that sedimentary rock is a layer of lava. The lava dates to 79 million years ago by several independent methods.

So, with absolutely no reference to fossils we know that the dinosaur fossil is more than 77 million years. But we can do better. We use the index fossil that is found in the two places to correlate the two sedimentary layers; they must be approximately the same age. Therefore the dinosaur fossil is also less than 79 million years old.

Good work! This is a very good explanation of the process, written for someone who does not really seem to understand how science works.

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Posted on Tuesday, August 12th, 2008 in Category Evolution. Please comment, ok?