Relative dating of rock strata

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So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.

The atoms in some chemical elements have different forms, called isotopes.

Prior to the late 17th century, geologic time was thought to be the same as historical time.

Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.

Index fossils are fossils that are known to only occur within a very specific age range.

Using relative dating the fossil is compared to something for which an age is already known.

For example if you have a fossil trilobite and it was found in the Wheeler Formation.

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?

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