Until fairly recently, it was more or less impossible to accurately date prehistoric sites, artefacts or events. Like historians, archaology had to rely on calendrical dates. In the 1950s Libby invented a method of dating organic deposits up to 40.000 years old. Since then a multitude of dating methods habe been developed. Today it is possible to determine the age of deposits several million years old with some accuracy. Some methods are briefly described below.

Radio-carbon (C-14) dating: All living things contain a tiny amount of radioactive carbon (C-14), which decays slowly and at a known rate after death. In C-14 dating the amount of residual radioactive carbon is measured. From this it is possible to calculate how long ago something died. Modern C-14 dating requires only a very small amount of carbon (taken from bone, wood, charcoal, etc.). The accuracy of C-14 dating depends on the age and quality of the sample. It is not possible to date inorganic matter this way, or to obtain dates more than 40.000 years ago. Radio-carbon dating exposed the turin shroud as a medieval forgery.

Dendrochronology (tree ring dating): Most trees grow in annual cycles and lay down one growth ring per year. The rate of growth varies from year to year, and the rings form a pattern which is alike for trees of the same species growing in the same area. By comparing and overlapping the pattern from successsively older timbers (using trees, buildings, fossilised timbers, etc) dendrochronologists have build up tree ring sequences. If prehistoric timber, retaining a cross section of the rings, is found on a site, the precise date of its felling can be estabilshed by comparison to the sequence. The results of dendrochronology can be spectacular: this method has, for example, dated a prehistoric timber 'road' in Somerset, England to 3806/3807 BC. Unfortunately dendro-dating relies on good preservation of wood, a phenemenon which is very rare and requires permanently wet (waterlogged) conditions. Furthermore, it is impossible to date wood, if no master sequence exists. At present master sequences go back: 8000 years (North America, Bristle Cone Pine), 9000 years (Europe, Oak), 8500 (Ireland, Oak).

Other methods: Aside from the two described above, there are a whole host of new scientific methods of dating the past. Most rely on knowledge of the rate at which radioactive substances decay. Some, such as potassium-argon dating can determine the age of inorganinc deposits several million years old. Other exploit the fact that the earth's magnetic field went through several reversals in the past. by far the most common and cheapest method of dating does, however, rely on the archaeologists' knowledge of what they dig up.

Stratigraphy: The archaeology on sites tends to be laid down in layers (strata). Archaeologists also call these layers contexts. In order to understand a site as well as possible, it is very important that these contexts and their relationships are recorded as fully as possible. Some layers can be dated accurately from the artefacts they contain. Studying the contexts also allows an overall chronology of a site to be established. All these activities are contained in the term stratigraphy, which also refers to the sum of the layers in general. An undisturbed stratigraphy is very much like a document, from which the archaeologists can read much about a site. For this reason, if you do own a metaldetector: please do not go hunting for artefacts on archaeological sites, never dig below the plough-soil anywhere. Disturbing a protected site is illegal in most countries. You might also destroy valuable information.