Ponte S. Gregorio

The original version of the Anio Vetus bridge crossing the Fosse de Mola stood downstream of
the present bridge. Hadrian shortened the route with the Ponte S. Gregorio. This type of arrangement
was described by Frontinus (18.5): 'But now, whenever a conduit is beyond repair because of its age,
in certain places, to save length, the circuitous underground route is abandoned and substructures
and arches are used to cross valleys'. Engineers of Emperor Hadrian tried to compensate for the
precipitous slope by putting the bridge nearly at right angles to the tunnel which followed
and by making the descent as short as possible (translation and note from R.H. Rodgers 2003).
Drawing from Ashby 1935.