These thick layers alternate with thin, clay-rich layers deposited during the winter.
The resulting layers, called varves, give scientists clues about past climate conditions.
But determining the absolute age of a substance (its age in years) is a much greater challenge.
To accomplish this, scientists use a variety of evidence, from tree rings to the amounts of radioactive materials in a rock.
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
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.
As we learned in the previous lesson, index fossils and superposition are effective methods of determining the relative age of objects.
In other words, you can use superposition to tell you that one rock layer is older than another.
Thomson's calculations, however, were soon shown to be flawed when radioactivity was discovered in 1896.
Radioactivity is the tendency of certain atoms to decay into lighter atoms, emitting energy in the process.
For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.