Text lesson

Unit 4 — Dating Planet Earth

Science — Third Preparatory

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 = conglomerate, 2 = shale, 3 = limestone with X, 4 = shale, 5 = sandstone with Y.
By the law of superposition, layer 1 is the oldest and layer 5 the youngest.
Fossil X is in a lower layer than fossil Y, so the organism X lived before organism Y.
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.
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.

Unit 4 — Dating Planet Earth · 20 / 28

How to read a rock column (relative dating)