Earth’s history: time scale, layers and half-life

Sediment settles in layers, so in a cliff that has not been disturbed, the bottom layer was laid down first and is the oldest, while the top layer is the youngest. This is the law of superposition, and it gives the relative age. Folds and faults can disturb the layers, so scientists check carefully. Fossils help to match layers in different places. An index fossil is the fossil of a species that lived for a short time but was widespread, such as some small sea animals with shells. If two layers in different places hold the same index fossil, they are about the same age. Relative age puts the layers in order but gives no number of years; for numbers we need radiometric dating, in the next lesson.