The course of the Wall not only exploited the Tyne/Solway isthmus for its comparatively short distance, but also used the escarpment of the Whin Sill in the central sector to enhance its capabilities as an obstacle and reduce the amount of ditch that had to be dug. Additionally, to the west, on the coastal plain south of the Solway, the low drumlinoid hills that are a feature of the glacial landscape were exploited to provide slightly elevated platforms for the forts of Bowness, Drumburgh, and Burgh-by-Sands. In the central sector, the requisite building materials – sandstone for the facing stones and lime for mortar – were readily available not far from the line of the Whin Sill. Further east we have less information about the availability of material but we do know that sandstone could be found to the west, just not so easily.
Further reading: Pickett et al. 2006