In this video, instructor discuss the idea that older layers underlie younger layers. However, the age of individual grains can be significantly older than the age of the layer they were deposited in because the grains have to exist before they are deposited. In the special case of layers of volcanic ash, the ages of crystals in the ash might be only slightly older than the age of the layer they are in, which provides excellent opportunities for measuring the absolute ages of sediments.
Lecture on age of Sediments. Individual Sediments may be older than time of deoposition
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