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For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.

Not surprisingly, these methods resulted in wildly different estimates, from a few million years to "quadrillions of years".

Using logs recovered from old buildings and ancient ruins, scientists have been able to compare tree rings to create a continuous record of tree rings over the past 2,000 years.

Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 million years have passed since the appearance of the first animals with shells.Today scientists know his estimate was too young; we know that this occurred about 530 million years ago.Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.These tree ring variations will appear in all trees growing in a certain region, so scientists can match up the growth rings of living and dead trees.Another example of yearly layers is the deposition of sediments in lakes, especially the lakes that are located at the end of glaciers.

Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.Radioactive materials in Earth's interior provide a steady source of heat.Calculations of Earth's age using radioactive decay showed that Earth is actually much older than Thomson calculated.The thin, dark part of each ring represents slow autumn and winter growth.Several other processes result in the accumulation of distinct yearly layers that can be used for dating.But determining the absolute age of a substance (its age in years) is a much greater challenge.