Dating the past
Knowing when something happened is as important as knowing what happened.
Relative dating
Relative dating puts things in order without giving exact years. The law of superposition states that, in undisturbed deposits, lower layers are older than those above them. Cross-cutting relationships help too: a pit dug through several layers must be younger than the layers it cuts. Changes in artifact styles over time, called seriation, also help order sites.
Absolute dating
Absolute dating gives an age in years. Radiocarbon dating measures the decay of carbon-14 in once-living material, such as wood, bone, or charcoal, and works for roughly the last 50,000 years. Dendrochronology counts and matches tree rings, which can date wooden objects to the exact year the tree was cut. Other methods, such as potassium-argon dating, date volcanic layers millions of years old.
Worked example: dating a site
Suppose a hearth lies two layers below a layer of broken pottery. Superposition tells us the hearth is older than the pottery. Radiocarbon dating of the hearth's charcoal gives about 3,000 years ago. Now archaeologists know the pottery layer is younger than 3,000 years, and they can compare its styles with dated sites nearby to narrow the range further.
Flashcard
Lower layers are older than higher ones
Check your understanding
An archaeologist finds a stone tool in a layer below a layer containing bronze tools. Without other evidence, what can be concluded?