Great Wall of Gorgan: OSL dating of part of the wall

The Great Wall of Gorgan in Northern Iran was around 200 km long from the Elburz Mountains into the south-east of the Caspian Sea and inculded over 33 forts. A canal at least 5 m deep، conducted water along most of the wall. The main research questions included how، when and why did people construct a wall of such enormous dimensions? To make such large quantities of bricks، it was essential to have a supply of flowing water. A water canal was fed by a complex system of supplier canals that ran along most of the wall. To ensure regular water flow، the route of the canal and the wall had to have a constant gradient. To achieve such a constant gradient over more than 100 km is the most impressive evidence for the high standards of Persian engineering. Until 2005، there had been no consensus on the age of the Gorgan Wall. Some thought it had been built by the Macedonian king Alexander in the 4th c. BC; the wall is also known as ‘Alexander’s Barrier’. Others suggested it was built as late as the 6th c. AD under the great Persian king Khusrau I. Majority opinion، since the important fieldwork project by Dr Kiani (1982a and b) in the 1970s، favoured a 2nd or 1st c. BC construction. In order to solve the question that which of these dating proposals is correct، the best way was to determine the absolute date of the wall. Among other dating methods carbon-14 and luminescence were the best for this purpose. 14C is one of the most accurate methods but requires organic material. However، Gorgan wall is made of bricks. There are quartz and feldspar in loess that is the main material of the bricks. Quartz and feldspar are two main luminescence dosimeters. As bricks are cooked in ovens، the luminescence clock of quartz and feldspar inside the bricks will reset due to being exposed to a temperature more than 300°C. The sediments contain quartz and feldspar. When sediments are exposed to day light، their luminescence clock will reset. As soon as bricks cool down or the sediment that was exposed to light is buried، the luminescence clock will start again. The minerals inside the bricks and the sediment will be exposed to a radiation (alpha، beta and gamma). Such radiations are emitted by radioactive materials such as uranium، thorium and potassium-40. They damage the crystal structure and the atoms are ionised. Electrons separated from the atoms are trapped and their number are accumulated as time passes. We sampled one brick from the best-preserved section in Trench E (near the excavated Gorgan Wall brick kiln in Trench A) near the eastern terminal of the Gorgan Wall (Figs 5 and 6). Another brick was taken from the Gorgan Wall near Gonbad-e Kavus. We also collected one OSL sample sediment from below the foundations of the Gorgan Wall near Gonbad-e Kavus that predated the Gorgan wall (Fig 7). One bone was also collected from a trench close to Fort 9 which post-dated the construction of wall (Fig 9). All four samples were transferred to and dated in Oxford University luminescence lab. The results of the OSL samples are presented in Table 1. The optically stimulated luminescence and radiocarbon dating conclusively proved that the Gorgan Wall had been built in during the Sasanian Empire. We hope to be able to narrow down this date further in the future.