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Reading the Rocks: Law of Superposition

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This first rule in reading the rocks is really a very simple one, just basic common sense. To see how it works, you will need:

  • several books
  • a flat surface

Start by placing one of the books on a table or some other flat surface. Then stack another book on top of it. Place another book on top of that one, and keep stacking them until you have at least 5 or 6 books.

OK, now look at the stack. Which book was put in place first? The bottom one of course. You know that because you just stacked them there, but what if someone else had stacked the books. As long as you knew that the books were placed one at a time, and that the pile had not been lifted or turned over, you would still know that the book on the bottom was there first. It had to be in place before the second book could be put on top of it. And the second book had to be in place before the third book was put there.

The same is true for layers of rock. As long as the layers have not been shifted by earthquakes or other geologic events, the bottom layer will be the oldest, and the layers will get younger as you move towards the top.