10. Is the ditch continuous?

No. In two sectors – in the centre, where the Wall runs along the crags, and west of Carlisle, where it is located on the terrace above the river Eden – the curtain wall was not accompanied by a ditch, since the steep nature of the terrain made it unnecessary. However, even in that central stretch, along the Whin Sill escarpment, short lengths of ditch were thought desirable across the nicks, gaps originally formed by glacial meltwater.

Further reading: Breeze and Dobson 2000; Breeze 2006