How to read a rock column (relative dating)

Suppose a cliff has five undisturbed layers from top to bottom: sandstone (contains fossil Y), shale, limestone (contains fossil X), shale, and conglomerate.

1. Number the layers from the bottom: 1 = conglomerate, 2 = shale, 3 = limestone with X, 4 = shale, 5 = sandstone with Y.
2. By the law of superposition, layer 1 is the oldest and layer 5 the youngest.
3. Fossil X is in a lower layer than fossil Y, so the organism X lived before organism Y.
4. If a second cliff far away has a layer with fossil X, that layer is the same age as layer 3 and the layers above it are younger.
5. If layer 5 held fossils of more complex organisms than layer 3, the layers also show the order in which these groups appeared.

• Be careful: the law of superposition applies to undisturbed sequences.
• If the crust has been folded or tilted, the order has to be worked out with more care; the question will tell you if the sequence is undisturbed.