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The sediment of this area was laid down after ammonite A appeared 199 million years ago, and before ammonite B became extinct 195 million years ago.This narrows the date of the delta beds to the four million years between these dates.Scientists know exactly how long it will take for half the quantity of the element to change, and this state is known as its half-life.After another half-life has passed, the element will have decayed to a quarter of its original amount.This is because new sediments are always laid down on top of sediments that have already been deposited.So, when looking at the history of a cliff face, it is important to read the story it tells from the bottom layer up.

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The best examples are fossils of animals or plants that lived for a very short period of time and were found in a lot of places.

Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.

These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.

Because of their unique decay rates, different elements are used for dating different age ranges.

For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.

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

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