How do we know?

With this new millennium still in its infancy, even archaeologists are starting to ponder the significance of all those zeros. I say 'even' archaeologists, as we very often find ourselves thinking in millennia, especially when talking about Lincolnshire's later stone age inhabitants (what we call the Neolithic period) who were around in the third millennium BC - 5000 years ago - and even their more modern counterparts in the bronze age, who lived in this county during the second and early first millennia BC. So, for us, all this millennia business slips off the tongue rather easily. But HOW do we know?? How can we confidently state the ages of things that are three four, five thousand, or even more thousand years old?

In fact, it's only in the past four or five decades that archaeologists have been able to use scientific methods that enable them to date the objects that they find. Before that, we could only make a reasonable guess at things that had come from literate societies - that is societies who were writing things down, and whose writings have survived. So, it was possible to work out the history of medieval England from the writings of monks and scholars, and even in the Dark Ages (those years of Saxon and Viking invasions) some writings have survived to help us piece together how people lived. The Romans, who were here from 43 AD to 410AD, were great writers, and have left us loads of detailed histories and descriptions of life at the start of the first millennium.

Over in Egypt, written records go back to about 3000BC but here in England anything older than Roman - that is, anything more than 2000 years old - posed something of a problem.

A Sequence Game

At first, the best that archaeologists could do was to work out what was older than what - a sort of sequence game. They realised that stone and flint implements were in use long before the first metals were discovered. They worked out that bronze tools followed stone and that knowledge of working iron followed on from bronze. Thus we invented the stone age, bronze age and iron age, in that sequence - names that we still use, although our knowledge has now extended far beyond that simple sequence.

So far, so good then, but how could we find out when these things were actually made and used? It was all very well to say the iron age followed the bronze age, but WHEN. Through the 1930s, 40s and 50s archaeologists drew up grand schemes to try and date these things .... but then .... in 1949 came a breakthrough..... Radiocarbon dating, a method of telling the age of things which had developed out of nuclear physics - the science that gave us the atomic bomb. A man called Willard Libby, in New York, had discovered that there were different types of carbon, some of which were radioactive. More importantly, living things only absorbed the radioactive types of carbon while they were alive. From the point that they died, be they plant or animal, a clock started ticking as the radioactive carbon in them (which was known as carbon 14) began to decay back to ordinary carbon. By a wonderful piece of mathematics and some enormous assumptions about the earth's outer atmosphere and the intensity of the sun, Libby managed to come up with a formula that could date anything containing carbon, up to about 60,000 years old.

Linking Dates to the Ages

The first problem was that the thing to be dated had to contain carbon - so bones, wood, charcoal - even peat, were fine, but pottery, metals and stone (which are the commonest archaeological materials) were not. So the importance of finding charcoal or bone associated with more common archaeological materials of pottery or metal artefacts suddenly rose enormously. Archaeologists found they could now fasten dates onto the stone age, the bronze age and the iron age .... and what a surprise that was. The achievements of the human race suddenly began to extend not to just a few thousand years BC as had been though but tens of thousands of years.

Up until the mid 1960s, archaeologists were blissfully happy with their fabulous invention .... then a few errors began to be noticed and some archaeologists began to question Libby's rather sweeping assumptions about the way the method worked.

It's all in the Rings

At about that time another dating method was being developed. Known as dendrochronology, it depended on the fact that trees normally make one tree ring each year and that the width of a tree ring is affected by temperature and rainfall. Over the centuries, complex sequences could be recognised which showed in tree trunks throughout the land. Thus, once the main sequence or pattern had been established over many centuries, any large enough piece of timber - whether from a church tower or a shipwrecked boat, could be compared with the master tree ring pattern, and the date of the timber found. This method was proving particularly useful for dating buildings and structures in the last 1000 years and slowly the sequences were being extended backwards as archaeologists found more timbers in their excavations.

This dating system did not impinge on carbon 14 dating until someone realised that, high in the White Mountains of California, there lived a type of tree known as the Californian bristlecone pine, which could live to the incredible age of 4900 years old! By radiocarbon dating the rings of this ancient tree the dating method could be 'calibrated'. It was discovered that Willard Libby's assumptions about carbon 14 had been close, but not that close. The bristlecone pine allowed the radiocarbon clock to be reset so that it now gave the right time - and it still does today.

A Revolution Starts in Lincolnshire?

Through the 1980s and 1990s archaeologists and scientists have worked closer together to develop a wide range of dating techniques. We now have techniques with such fine sounding names like thermoluminescence, thermo remanent magnetic dating, Amino Acid dating, Fission track dating and potassium argon dating, to name but a few. In the last twelve months a new dating method has been on trial here in Lincolnshire. The method, known as viscous remanent magnetic dating, is being pioneered by Professor Graham Borrodaile of Lakehead University, Canada (who is in fact a Lincoln man) using samples from several Lincolnshire monuments. The method has the potential to tell us just when the stones of a building were quarried and laid, and even if they were subsequently re-used in a new building. Who knows - perhaps the dating revolution of the next millennium will start here in Lincolnshire!

Dave Start
Director, Heritage Lincolnshire