Only a couple of hundred years ago most Europeans believed the age of the Earth could be calculated from dates given in the bible – it was believed to be about 6000 years old. Observations of fossils however, made by a Scot called James Hutton, implied a much older age for the Earth.
James Hutton travelled England and Scotland, studying rock formations and collecting samples. He observed the fossils of sea creatures near the tops of mountains, and fossils of animals belonging to species extinct at the time.
His studies led him to the view that the Earth was much older than men had calculated from considering the bible. He considered the rocks as belonging to a cycle of formation and erosion.
These processes are extremely slow, but over enormous periods of time they can produce huge changes in the Earth's surface – exactly the sort needed to elevate part of the sea floor to the height of a mountain, as implied by the presence of fossils there.
It took another century and the support of Hutton's theory by an eminent British geologist, Charles Lyell, before Hutton's theory was widely accepted.
Gradually, geologists learnt to read the history hidden in rocks, following James Hutton. In general
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deeper is older. This is because the newest rock is generally formed at the surface, as a result of volcanic activity or sedimentation.
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fossils can be used to estimate the age of rock. Particular species existed between particular periods of time. Before a certain time they had not evolved, and after a certain time, they had died out.
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If one type of rock cuts across another, it is younger.
These principles tell us the relative ages of rocks, but not their actual ages. To find actual ages of rocks, we can use the method of radioactive dating, which gives an estimation for the Earth's oldest rocks of about 3900 million years.