Stratigraphy and dating

The essentials

To understand the past, archaeologists must know the age of finds. Stratigraphy is the study of layers (strata) of soil and deposits at a site. According to the law of superposition, in undisturbed deposits the lower layers are older than the upper ones.

Relative dating tells whether something is older or younger than something else, without giving an age in years. Absolute (chronometric) dating gives an age in years, usually with a margin of error.

Key points

• Relative methods: stratigraphy; typology (comparing tool or pottery styles); fluorine test on bones (bones absorb fluorine from groundwater over time, so buried bones from the same layer should have similar fluorine content; this helped expose the Piltdown forgery).
• Radiocarbon (carbon-14) dating (Willard Libby): living organisms take in C-14; after death it decays. Half-life about 5730 years. Useful for organic materials (wood, charcoal, bone, shell) up to about 50,000 years.
• Potassium-argon (K-Ar) dating: potassium-40 in volcanic rocks decays to argon-40 with a half-life of about 1.3 billion years. Useful for volcanic layers older than about 100,000 years; it dated volcanic layers at Olduvai Gorge, bracketing early hominin fossils.
• Dendrochronology (tree-ring dating), thermoluminescence (fired pottery) and varve analysis are other absolute methods.
• Fraction remaining after n half-lives = (½)ⁿ.

Exam tips

• Match material to method: organic and young → C-14; volcanic and very old → K-Ar; burnt pottery → thermoluminescence.
• Practise simple half-life sums.

Common mistakes

• Trying to date stone tools directly with C-14; stone is not organic.
• Forgetting that superposition applies only if layers are undisturbed.