Fossil radioactive dating
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.
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.
There are some radioactive elements in rock that decay by giving off energy and turning into different, more stable elements.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.
Although this was a major breakthrough, Boltwood's dating method made it possible to date only the oldest rocks.