25. Why was the Turf Wall built?

Some scholars have suggested that the reason for the use of a turf rampart may have been the difficulty in obtaining good building stone on the western side of the country or even the difficulty in obtaining limestone to make lime for mortar. Nevertheless, the stone curtain wall was later extended all the way to Bowness, using St Bees sandstone, probably after the final abandonment of the Antonine Wall in the mid-AD 160s, so perhaps the problem was that these materials could not be obtained and assembled quickly enough to complete the project in good time. The Emperor Hadrian himself commented to troops in North Africa that it normally took longer to construct a stone wall than an equivalent turf rampart.

Further reading: Breeze and Dobson 2000