Methods of dating rock layers

Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.

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A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.

In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.

This dating method is also known as Archaeological Dating or Historical Chronology. These methods were relied on especially prior to the introduction of scientific methods of dating.

But, even when the scientific methods of absolute dating are available, this method of dating has not lost its importance, as many a time we have to depend solely on relative dating.

Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived.

Geologists call this the principle of lateral continuity.

The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.

But, how can we determine how old a rock formation is, if it hasn’t previously been dated?

New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great ‘tree of life’.

Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since 1859, nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.

There are two main methods determining a fossils age, relative dating and absolute dating.

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