There is no rigorous definition to distinguish between a 'bridge' and a 'viaduct'. All viaducts are bridges, but viaducts must have more than one span.
A masonry viaduct is usually thought of has having numerous arches, or of having great height, but this category simply relates to bridges made predominantly of stone or/and brick construction and having three or more arches.
Sources of Information
Pages in category "Masonry Viaducts"
The following 47 pages are in this category, out of 47 total.