I'd never noticed the castle motif on the keystone, nor actually read the inscription above it:
QUÆ DEERANT ADEUNT : SOCIANTUR DISSITA : MERCES
FLUMINA CONVENIUNT ARTE DATURA NOVAS
A.D.MDCCXCVII.I?G.J.RENNIE.EXTRUX.A.STEVENS.P.ET.F.
On mouseover, the highlighted words show my own amateurish translation; I've since discovered it should be something like:
Things that were wanting: brought together; things remote: connected
Rivers meet with the assistance of art
To afford new objects of commerce