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Those laid down during the fall and winter have a dark color because of the presence of dead vegetation; those deposited during the rest of the year have a light color.
The stratigraphy may also reflect seasonal variation in the velocity of stream flow.
Since 1950 the physical sciences contributed a number of absolute dating techniques that have had a revolutionary effect on archaeology and geology.
Fission track dating is based on the fact that when uranium-238 atoms fission within a solid medium such as a mineral or a glass, they expel charged particles that leave a trail of damage (known as fission tracks) preserved in the medium.
The number of tracks per unit area is a function of time and the uranium concentration.
The varved-clay method is applied with fair accuracy on deposits up to 12,000 years old.
Streams flowing into still bodies commonly deposit layers (varves) of summer silt and winter clay through the year.