Fossil dating maps
The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Other radiometric dating techniques are available for earlier periods.
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.
Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.
Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating.
Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14.
Potassium-40 is a radioactive isotope of potassium that decays into argon-40.Each location includes a reference number which relates to the adjoining geology map.