One thing I like about looking at the LIDAR images is that they remind you about things I have forgotten (or perhaps never knew).
In this sector along Hadrian's Wall for example the wall itself lies under the modern road (Military Road). The north wall ditch is often extant and, when not ploughed out, so are the impressive earthworks of the Vallum, here closely parallel to the line of the Wall and to its south. The ditch of the Vallum in my enhanced LIDAR is the dark line between the two white dashed lines of the marginal mounds. These were built from the spoil of the ditch and were continuous so why do they now appear as regular segments?
When constructed the Vallum only had very few crossing points aligned with the Wall forts. However, when Hadrian's Wall was abandoned around 140AD for a new frontier in Scotland (the Antonine Wall) there was a systemmatic slighting of the Vallum creating a large number of crossings across the ditch using soil taken from the linear mounds. 35 beaks were made every Roman mile providing an interval of crossings of around 40m. "We can surely agree that the very regularity of the action of slighting the Vallum points to the hand of the army" (Breeze 2015).
When the Romans returned to Hadrian's Wall a few years later they clearly didn't bother to fill all the gaps in the linear mounds and there seems to be conflicting evidence about whether the Vallum ditch was completely reinstated as originally built.. I wonder if the LIDAR has enough resolution to see the remains of ditch crossings?
Breeze, D. J. (2015). The Vallum of Hadrian's Wall. Archaeologia Aeliana Series 5. Vol 44, pp. 1-30.
https://doi.org/10.5284/1061298

































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