16. What was the Hadrian’s Wall curtain wall built from?

The curtain wall and associated features (forts, milecastles, and turrets) was almost exclusively built from sandstone. Although the central sector was built on the Whin Sill, that stone (a dolerite) is too hard to dress easily so it normally only appears undressed in the core of the curtain wall. One exception to this is the Peel Gap tower, where whinstone facing stones were extensively used. They are found elsewhere, as at Sycamore Gap, but otherwise sandstone was the main source of dressed stone for the wall faces.

The core was made up from rubble and boulders from rivers beds (where convenient) in a matrix of either clay or mortar. It is thought the bulk (but not all) of the original scheme of work was constructed as a clay-bonded wall with mortar pointing of the facing stones. In many cases it was necessary to repair this, possibly under Severus, and mortar bonding was used.

Further reading: Hill 2006; Picket et al. 2006; Symonds and Mason 2009