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".
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
For example, layers form within glaciers because there tends to be less snowfall in the summertime, allowing a dark layer of dust to accumulate on top of the winter snow (Figure 11.23).
To study these patterns, scientists drill deep into ice sheets, producing cores hundreds of meters long.
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, an especially warm summer might result in a very thick layer of sediment deposited from the melting glacier.
Thinner varves can indicate colder summers, because the glacier doesn’t melt as much and carry as much sediment into the lake.
In 1892, William Thomson (later known as Lord Kelvin) calculated the age of Earth in a systematic fashion (Figure 11.24).
He assumed that the Earth began as a ball of molten rock, which has steadily cooled over time.
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.
Another example of yearly layers is the deposition of sediments in lakes, especially the lakes that are located at the end of glaciers.