The Mongolian Arc

Why this is extraordinary

In Sükhbaatar and Dornod, a line about 405 kilometres long runs through the eastern steppe, part of a much longer frontier of walls and ditches between Mongolia and northern China. A field survey counted 34 rectangular enclosures along it. Thirty-two sit with the line. One rectangular enclosure lies about 17 kilometres off it and held far more objects than the others. The line is not continuous. Twenty-one gaps add up to about 105 kilometres, a quarter of the length. The gaps average about 2.7 kilometres and run from 0.3 to 17.9. Of the missing wall, about 83 kilometres look as if they were never built, because the line stops cleanly rather than fading into sand, and about 22 kilometres are ambiguous. The deer stones are a different century and a different kind of monument. This is a frontier.

What the line is made of surprised the dig. Enclosure MA03, in Erdenetsagaan district of Sükhbaatar, is about 30 by 31 metres. Its walls, stone and rammed earth, still stand up to about a metre. An outer trench is about 4 metres wide and 2 metres deep. The diagram is those measurements. It is not a photograph, and no openly licensed photograph of the trench was found for this room. Along the line itself, in the stretch that was dug, there is no high wall. There is a relatively shallow ditch and a pile of earth. A ditch that shallow is a poor shield against an army. It is a clear mark on the ground, and it can steer people, animals, and goods toward gates where someone is watching. Structures along the arc sit closer together than on a more northerly line, often around 8 kilometres apart, a median of 7.8 and an average of 11.87. They do not see one another well. A viewshed puts them off the ridge, where a fort that wanted a view would have stood.

The date is one enclosure, used for the whole arc with that caution. Nine radiocarbon samples from MA03, modelled with the historical bracket of the Jin dynasty, put the start of habitation between cal AD 1150 and 1216 and the abandonment between cal AD 1180 and 1242, at 95.4 percent. At 68.3 percent the start is 1163–1210 and the end is 1199–1226. The Jin years are AD 1115 or 1125 to 1234. A grave inside the enclosure is later, cal AD 1433–1452, between 199 and 264 years after the fort was given up. Wood and charcoal can be older than the fire that burned them. The samples group tightly enough that the study treats old wood as unlikely to be the whole story. The pin is the town of Erdenetsagaan, the district of MA03. It is not a nail in the ditch. The line is 405 kilometres long.

Why build it is the split that remains. One reading, a century old in the histories, is a hasty defence against Mongol armies at the end of the Jin. A court memorandum around AD 1200 describes a different section, about 300 kilometres, built in a great hurry. The gaps, especially a long empty stretch, are offered as what haste looks like in the ground. The other reading is a civilian frontier: a symbol of control, a way to channel crossings, a watch on small raids and on trade, not a wall for a pitched battle. The shallow ditch is the evidence that reading uses. A symbol does not have to stop a cavalry charge. It has to be visible, and a ditch with forts every few kilometres is visible. No sunrise, star, or azimuth of the line is published in these studies. The direction of the arc follows the border country, not a reported solstice.