I'm sure some of the UK folks will chime in on this, but I believe that Hadrian's wall largely follows the course of the Great Whin Sill (a quartz dolerite that strikes E-W and dips S) which not only provided a physical obstacle to the barbarians to the north (Rob?) but also provided the building material of the wall itself. The construction is impressive - both outer and inner ditches (excavated from the dolerite providing material for the standard 10 foot thick wall faced with finished cut stone), a parallel military road and a civilian one.
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