Arguably, most of Hadrian’s Wall is still there, it has just been spread around the landscape a bit. After the Roman period, it became a prime source of dressed building stone for local communities and of especial importance on the western coastal plain where there was no other easily accessible source of such material. Even in areas where building stone was available in local quarries, the saving in time offered by reusing Roman stone rather than dressing freshly quarried rock was an important factor in ensuring a continuous process of robbing of the fabric of the Wall and its features.
This means that the buildings and field walls around the line of Hadrian’s Wall contain most of its original fabric, a fact that is betrayed by the frequent incorporation of inscriptions and sculptures within them.
When the Military Road was constructed in Northumberland, the contractors broke up what survived on the curtain wall to provide roadstone and material for the drystone wall that needed constructing on either side of the road, leaving the foundations on occasion intact within the body of the road.
Earthworks like the Wall ditch and the Vallum can still survive upstanding in places, but have largely been reduced by ploughing, although excavation will always make the Vallum ditch apparent except where it has been completely removed by later disturbance.
Further reading: Whitworth 2000