Monday, April 07, 2008

OSL dates "Red Snake" Wall to 5th-6th century CE

When I first studied archaeology a number of years ago we spent several weeks discussing various dating methods. But at that time, optically stimulated luminescence was not among them. I found this application of the technique very interesting.

"It is longer than Hadrian's Wall and the Antonine Wall taken together. It is over a thousand years older than the Great Wall of China as we know it today. It is of more solid construction than its ancient Chinese counterparts. It is the greatest monument of its kind between central Europe and China and it may be the longest brick, or stone, wall ever built in the ancient world. This wall is known as ‘The Great Wall of Gorgan’ or ‘the Red Snake’. An international team of archaeologists has been at work on the snakelike monument and here they report on their findings.

The ‘Red Snake’ in northern Iran, which owes its name to the red colour of its bricks, is at least 195km long. A canal, 5m deep or more, conducted water along most of the Wall. Its continuous gradient, designed to ensure regular water flow, bears witness to the skills of the land-surveyors responsible for marking out the Wall's route. Over 30 forts are lined up along this massive structure. Their combined size is about three times that of those on Hadrian's Wall. Yet these forts are small in comparison with contemporary fortifications in the hinterland, some of which are around ten times larger than the largest Wall forts. The 'Red Snake' is unmatched in so many respects and an enigma in yet more.

Who built this defensive barrier of awesome scale and sophistication, when, and for what reason? Even its length is unclear: its western terminal was flooded by the rising waters of the Caspian Sea, while to the east it runs into the unexplored mountainous landscape of the Elburz Mountains.

An Iranian team, under the direction of Jebrael Nokandeh, has been exploring this Great Wall since 1999. In 2005 it became a joint Iranian and British project. Our aim: to answer the fundamental questions of when, who, and why.

No ancient textual source refers to the Wall, no inscription, and no coin has ever been found on it. With respect to the ‘when’ question, rather than basing our dating on historical guesswork, we felt that we needed to obtain independent scientific dating.

Dating the Enigma

So when was the Wall built? Some thought it was erected under the Macedonian king Alexander, who reached the area in 330 BC, but died seven years later - indeed the Wall is also known as 'Alexander's Barrier'. Others suggested it was built as late as the 6th century AD under the great Persian king Khusrau I. (AD 531-579). Owing to his 1970s fieldwork, Muhammad Yusof Kiani, and many scholars thereafter, have favoured a 2nd or 1st century BC construction. Who was right?

Fortunately the Wall's engineers had used construction techniques eminently suitable to modern dating techniques. Running mostly through a landscape of windblown loess and, in sections, treeless steppe, there was no sufficient supply of stone or timber for construction purposes. The loess, however, was an ideal material to produce tens, if not hundreds, of millions of fired bricks. Each of them was square and of standardised size: 37cm diameter in the west of the Wall, 40cm in the east and some 8cm to 11cm thick. These huge bricks were produced on an industrial scale. Our surveys indicate that brick kilns line most of the Wall. In some areas we found kilns under 40m apart, in others almost 100m. Overall there were probably several thousand brick kilns built for the sole purpose of creating the ancient Near East's greatest linear barrier.

Could the kilns yield the evidence we needed to date the monument? If they used wood fuel they would have left charcoal, a material suitable for radiocarbon dating. Furthermore, a kiln seemed a promising candidate for a second independent technique: optically stimulated luminescence (or OSL) dating. Each time sediments are exposed to direct sun light or, in our case, heated up by fire, the luminescence clock is set back to zero. This allows for them to be OSL dated, which in turn promised to reveal when the kilns had last been used.

With these possibilities in mind, in September 2005 we ventured to the vicinity of the Wall's easternmost known point in the foothills of the Elburz Mountains, where a kiln had been located in a previous survey. Our chosen kiln seemed particularly suitable: it was just 13-20m away from the Wall, and it was on a slope without traces of settlement of any other period and so steep that it was sometimes difficult to gain a foothold when excavating it; we could thus be certain that it had been constructed specifically for burning bricks for the Wall - and it is unlikely anybody would have re-used it at a later date. Soon we established that it had virtually identical dimensions to a kiln excavated in the 1970s over 60km further west and also next to the Wall. Our kiln and the others known so far were designed for 10 stacks of bricks sideways, and 17 to 18 lengthwise. They were all replicas of a single prototype - powerful evidence that the Wall-builders were behind the standardised design.

Sediments washed down the steep slope had preserved our kiln remarkably well. Its eleven arches survived on the hillside to their full height of two metres, not counting another metre of superstructure. Two collapsed arches offered an opportunity to dig a sondage into the interior without destroying any preserved architecture. Eventually we reached a dark layer of charcoal and, immediately underneath, the kiln's fire-reddened bottom. We had achieved our goal. Dr Jean-Luc Schwenninger and Dr Morteza Fattahi, of the Universities of Oxford and Tehran, flew in to take OSL samples in October 2005. They also sampled various sections of the Wall itself and of a second shorter wall further west (the Wall of Tammishe) as well as a kiln next to it that we had also excavated. We impatiently awaited the results. The OSL and radiocarbon samples demonstrated conclusively that both walls had been built in the 5th or, possibly, 6th century AD."