"the erection of a wall cutting the island in two"

Here Yourcenar's Hadrian reflects on the British wall which bears his name to this day, a westward extension, in effect, of the Limes Germanicus. It is 80 Roman miles (120 Km) in length and, in its eastern portion, follows the edge of a natural escarpment, the Great Whin Sill. The 2nd, 6th and 20th Legions were all involved in its construction, beginning in 122 AD. Once built, however, it was manned not by legionaries (Roman citizens) but by auxilliaries (who became citizens only upon completion of their service). Along the length of the wall, there are mile-castles 493 metres apart. There are also around 15 larger forts (although these were built a little later than the wall itself), and a number of logistical outposts set behind the wall, of which Vindolanda is the best known. It is a UNESCO World Heritage Site, and there is a footpath that can be followed along its length (http://www.nationaltrail.co.uk/HadriansWall).  

Map of Hadrian's Wall
GNU Free Documentation LicenseMap of Hadrian's Wall - Credit: Norman Einstein
Mile-castle No.39, near Steel Rigg
Public DomainMile-castle No.39, near Steel Rigg - Credit: Adam Cuerden