The rocks under our feet are stacked like the pages of a history book. Today you'll learn to read them — then you will predict the age of fossils before we check.
Before we start — you've seen stripes in a cliff or a road cut into a hill — what do you think those layers are?
Bits of sand and mud settle to the bottom of seas and lakes in flat sheets. Over a very long time they pile up and harden into layers of rock — one on top of another.
Because each new layer lands on top, the deepest layer is the oldest and the top layer is the youngest. Scientists call this the law of superposition.
A fossil's layer tells us its age. An index fossil is a creature that lived for only a short time but spread far and wide — so wherever you find it, you know that layer is the same age.