are layers of the gological column are dated by the fossles and the fossils by the layers?

Question

isnt this circular reasoning?

Answer 1

No. The basic geological column was first realised by creationists such as Rev. Sedgewick and William Smith, following logical rules such as the principle of superposition (sediments were deposited on top of older sediments). They observed that layers of different ages contained some different fossils which could aid in recognising the layers, without always having to physically trace them along. This was well before the Origin of Species.

Nowdays, the geological column is dated by absolute radiometric methods (eg. Uranium-Lead and Potassium-Argon dating). There are hundreds of radiometrically-dated tie-points in the geological column. Index fossils, along with other methods such as magnetostratigraphy, are used to correlate, relatively, between different locations, and to the radiometrically-determined time scale.

Answer 2

The fossils are much more important to dating layers than the layers themselves--in fact, I think it's safe to say that usually the fossils come first. In an intensely deformed sedimentary sequence in which say, the strata have been overturned, fossils are the best way to determine which layers are actually older.

The linchpin of using fossils for dating is the fact that each period and era has its own distinctive fossil assemblage. The fossils are dated according to what other fossil species they "share" the geological column with; that they often fall in similar layers is only incidental. Trilobites existed all through the Paleozoic; by themselves they aren't very useful, but if you find trilobites with archaeocyathids, you know that you're dealing with an early to mid-Cambrian assemblage.

Answer 3

Fossils are used to determine the "Relative" ages of rocks--older versus younger, Paleozoic versus Mesozoic, Permian versus Triassic.

The "Absolute" age of some rock layers can be determined by radiometric decay rates--example: 165 million years old plus/minus 3 million years.

When fossil-bearing layers are interbedded with layers that can be radiometrically dated, then geologists can link the two types of dating methods.
Which is how they have determined, for example that the boundary between rocks with Cretaceous fossils (end of age of dinosaurs) and rocks with Tertiary fossils is about 65 million years old.

This has allowed earth scientists to put together a geologic time scale linked to fossil-based events.

Answer 4

It would be, but the layers were dated by the radioactive decay of radioactive substances.