113) 'Each winter's snowfall creates a distinct layer, and the annual layers have been counted back almost 6000 years in a core more than two kilometres in depth, with an excellent level of reliability within around 50 years...The thickness of each layer varies, as do the proportions of different oxygen isotopes whose formation is known to reflect temperature; thus, long-term patterns of variation reflect changes in climatic conditions. Some layers of ice contain high levels of dust and acidity caused by volcanic eruptions.Volcanoes known from historical records, such as Krakatoa (1883) or Vesuvius (AD 79), can be correlated with ice-cores; further undocumented eruptions in prehistoric times may also be detected.' (p.113) '..1950, a number of dating techniques had emerged that could offer chronological frameworks for the study of prehistory at least as reliable as those used by historical archaeologists.This ratio is called the Oxidizable Carbon Ratio, or OCR.' (Douglas S.
114) 'Several scientific dating techniques exploit the phenomenon of radioactive decay, including those first used to date the age of the Earth in the early years of the twentieth century.
A pattern of climatic variation is derived from temperature-sensitive species of marine fauna and from measurements of oxygen isotopes.
It correlates with geological evidence for cold and warm periods that are dated according to deviations in the Earth's orbit around the sun.' (p.
'Dating is the key to organising all archaeological evidence.
Furthermore, the development of dating methods, whether 'traditional' or scientific, illustrates the ingenuity and lateral thinking that make archaeological problem-solving such a fascinating exercise. At the beginning of the twentieth century it must still have been inconceivable that reliable dates could ever be established for European prehistory, other than those that depended on tenuous connections between Egypt and the Aegean in the second millennium BC. Not until 1950 did absolute dates become a reality for prehistoric archaeology in areas outside Scandinavia and the south-west of the United States, where varves and tree-rings had begun to provided a locally applicable dating method some decades earlier.' (p.However, the feature of carbon-14 that makes it exceptionally important is that it is absorbed naturally by all living organisms, but ceases to enter them when they die.' (p.