Condercum Vallum Crossing

The Vallum was a wide ditch flanked with earthworks that ran behind the wall.  It limited rear access to the area behind the wall, and crossing points were established at the rear of most forts.   This crossing was discovered and the stone culvert as well as about 25 meters of the Vallum have been restored.
            
See: http://www.northofthetyne.co.uk/HWC2.html