1.8.1 Relative dating

Relative dating is simply figuring out which rock came first. In some circumstances that’s pretty simple, like knowing that the sandstone in the cliff below must have been there before the igneous intrusion that has been forced into it.

Figure 1.6 Dyke cross cutting sandstone

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A photograph of a large sandstone cliff with an igneous rock intrusion.

Figure 1.6 Dyke cross cutting sandstone

However, sometimes what looks straightforward can be quite complicated. As discussed earlier this week, sediments are laid down on top of each other, one after the other like the pages of a book, so you might think that in this picture the rock on the top should be younger than the rock at the bottom.

Figure 1.7 Nappe fold zoomed in

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A close-up photograph of a nappe fold

Figure 1.7 Nappe fold zoomed in

In this case, however, something else has happened – the whole pile has been inverted by processes during plate tectonics, and from further away, you can see that the whole lot has been folded over.

Figure 1.8 Nappe fold

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This photograph shows the same rocks as Figure 1.7, but seen from further away.

Figure 1.8 Nappe fold