THE LARGE ENCYCLOPAEDIA
How do we divide geologic time?
The world’s rocks are like countless volumes of an enormous encyclopaedia. Each tells its own story, and together they weave a marvellous tale: the story of Earth.
Earth is 4.6 billion years old. The events that we can recognise in the rocks serve to mark transitions between different eras and periods.
The golden spikes of deep time
The International Commission on Stratigraphy (ICS) compiles the official chart of Earth’s ages by combining information from around the world.
When an event is marked as a boundary between two periods, this committee of experts chooses the place in the world that best represents this transition. This point is called a stratotype and is marked with a golden spike.
Zumaia is the only place in the world that has two stratotypes.