system for categorizing prehistory
The three-age system is how some historians and archaeologists divide up prehistoric times. The three ages are named after what early humans were making tools out of during each one. The first was the Stone Age. The second was the Bronze Age. The third was the Iron Age. 
- ↑ Kipfer, Barbara Ann (2000-04-30). Encyclopedic Dictionary of Archaeology. Springer Science & Business Media. ISBN 978-0-306-46158-3.