Relative dating does not provide actual numerical dates for the rocks.

Fossils are important for working out the relative ages of sedimentary rocks.

## Relative dating principles geology

So we can infer that coal seam is younger than the faults (because it disrupts them), and of course the upper sandstone is youngest of all, because it lies on top of the coal seam.

Exercise 8.1 Cross-Cutting Relationships The outcrop shown here (at Horseshoe Bay, B. Buff/pink felsic intrusive igneous rock present as somewhat irregular masses trending from lower right to upper left 2. A 50 cm wide light-grey felsic intrusive igneous dyke extending from the lower left to the middle right – offset in several places Using the principle of cross-cutting relationships outlined above, determine the relative ages of these three rock types.

Suppose you find a fossil at one place that cannot be dated using absolute methods.

That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.

Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.

The method of reading the order is called stratigraphy (layers of rock are called strata).

Some fossils, called index fossils, are particularly useful in correlating rocks.

For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.

(The near-vertical stripes are blasting drill holes.

The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.

Throughout the history of life, different organisms have appeared, flourished and become extinct.

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