March 15, 1996. Copyright, 1996, Graphic News. All rights reserved

UNROLLING THE HISTORY OF THE STONES
By Nicholas Booth, Science Editor

LONDON, March 15, Graphic News- This week, archaeologists working for English Heritage will reveal that Stonehenge, perhaps BritainÕs most famous archaeological site, is some 5,000 years old. Hitherto, it could only be aged to within 500 years, but improvements to radiodating techniques and enhanced analysis of the data have allowed a more precise determination Ð to within 80 years. And this week (MARCH 17 Ð 23), details of how the stones were transported to the site and how the site can be looked after in the future will be discussed at a scientific meeting in London, to coincide with the third annual Science Week. 

The fixing of the age of the stones comes as part of English HeritageÕs desire to publish completely all the results from excavations and scientific examinations carried out this century. Improved mathematical analyses of the data followed from very precise archaeological reconstruction of the site and carbon dating from tools, bones and antlers found in and around Stonehenge. These, and other results, will be discussed in greater detail at this weekÕs conference ÔScience & StonehengeÕ (MARCH 20-21).

Carbon is one of natureÕs most important elements and can be used to date material of archaeological interest. It exists in three forms, or isotopes, the most common of which is Carbon 12 which is chemically stable. Another isotope, Carbon 14, is not stable and undergoes radioactive decay, albeit very slowly. Often termed radiocarbon, it takes 5,370 years for half the atoms in any given sample to decay Ð the number of years has been determined by laboratory experiments. Measuring the ratios of Carbon 12 to Carbon 14 in a particular material allows scientists to estimate its age.

Carbon 14 is produced continuously in the upper atmosphere by the action of energetic particles from space. It then trickles down towards the Earth and is absorbed by plants and animals in their food. The ratio of carbons 14 and 12 remains more or less constant in living material but after death, the radiocarbon is not replaced. The time of death can be estimated from measuring the amount of Carbon 14 present today and calculating back from the known rate at which it decays. Fossilised material in and around the site is then examined and dated to reconstruct a chronology of how the stones were assembled.

This glib statement belies the complexity of calculation because the production of radiocarbon has not been constant through history and has to be gauged from tree rings whose ages are known. Other calibrations are required and the Stonehenge results have been compared and contrasted from two separate methods of radiocarbon dating.This checking has allowed scientists to determine the age of artefacts to within 80 years and enabled archaeologists to estimate the relative ages of the various sites and how they evolved over time. One surprise thrown up is from examination of animal bones which reveals that they were buried as relics, perhaps centuries after being used.

Sources: English Heritage, Intel, Virtual Solutions

EDITORÕS NOTE: A press call at Salisbury Museum from 12 noon to 2pm on Tuesday, March 19, will allow journalists to meet scientists involved with this work. For further information, please call Janet Bell on 01722 332151. The meeting, ÔScience And StonehengeÕ on March 20-21, is jointly organised by English Heritage and the Royal Society. For further details, call English Heritage Press Office on 0171 973 3250