Ask a visitor at any ruin how old it is and they will usually point to the sign. Behind that number sits a whole toolkit — and most of it is cleverer, and messier, than people expect.
No single method gives a perfect date. Archaeologists build an answer from several independent lines of evidence, and they are always happiest when those lines agree.
Relative vs absolute dating
Relative dating tells us what came first: this floor is older than that wall, this pot style came before that one. Absolute dating puts a number on it, usually as a range of years.
The two work together. Relative dating gives the order of events; absolute dates pin that sequence to the calendar.
Stratigraphy: the layer cake
Soil and debris build up over time. If nothing has disturbed them, deeper layers are older than the layers above — the principle of superposition. Excavators record every layer, every pit cut into it and every object found in it.
A trench is a book read backwards — the top page was written last.
Real sites are rarely that tidy. Pits, burrowing animals, later building work and floods all mix things up, which is why careful recording matters so much.
Radiocarbon dating
Living things take in carbon, including a tiny amount of radioactive carbon-14. When they die, that carbon-14 decays at a known rate, halving roughly every 5,730 years. Measuring how much is left in charcoal, seeds or bone tells us how long ago the organism died.
The method works for material up to around 50,000 years old. Raw results must be calibrated against records such as tree rings, because the amount of carbon-14 in the atmosphere has changed over time — that is why dates are given as ranges rather than single years.
Tree rings and other clocks
Dendrochronology counts and matches annual growth rings in timber. Where long regional sequences exist, a well-preserved beam can sometimes be dated to the exact year the tree was felled.
Other methods include luminescence dating, which estimates when sediment or pottery was last exposed to light or heat, and coins or inscriptions that carry their own dates.
Pottery as a clock
Pottery breaks easily, survives well and changes in style over time. Once the sequence of styles in a region is known, a handful of sherds can date a layer surprisingly precisely — no laboratory needed.
Putting it all together
A good date is an argument: the layer sequence, the radiocarbon results, the pottery and any written evidence should all point the same way. When they disagree, that is often where the most interesting archaeology begins.