Pick up just about any book on dinosaurs, and it will say that dinosaurs lived from 230 million to 65 million years ago. That’s always stated as a fact, but how do they know? The ages of dinosaur fossils are determined by the layer of rock in which they are found. How do they know how old the rock layer is?

   It is usually the case that when layers of rock are piled up upon each other, the bottom one is the oldest, and the top one is the youngest, because the bottom one had to be there before the other ones formed on top of it. The exceptional case is the one in which liquid rock squeezes up through some layers. In that case, the liquid rock hardens into a solid rock formation that is younger than the layers above and below it. It is also possible that an earthquake might slide some bottom layers up over some top layers. But these unusual cases don’t happen very often, and it is generally possible to detect when it does happen.

   So, one can make a cross-sectional cut through a rock formation, examine the layers, and be reasonably confident that the lower layers formed before the upper layers. The questions are, “How long did it take for each layer to form?” and, “How much time elapsed between layers?” Traditionally, geologists have used the “geologic column” to answer these questions.

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