A defensive fortification about 59 km (37 miles) long, built across the narrowest part of central Scotland between the Firth of Forth and the Firth of Clyde c.140 ad, in the time of Antoninus Pius. It was intended to mark the frontier of the Roman province of Britain, and consisted of a turf wall with a broad ditch in front and a counterscarp bank on the outer edge, with 29 small forts linked by a military road. The Romans, however, were unable to consolidate their position and in c.181 the wall was breached and the northern tribes forced a retreat from the Forth–Clyde frontier, eventually to that established earlier at Hadrian’s Wall.