Most were either rebuilt in situ or by a replacement viaduct built immediately alongside, and in the latter case many of the original piers still remain today. Between Saltash and St Germans, a deviation line was built in 1908, eliminating the wooden viaducts on by-passed section of line. Those on the Falmouth branch were all replaced between 1923 and 1934. Constructional features The distinctive timber viaducts were constructed using yellow pine which was preserved by Kyanising (using chloride of mercury), or sometimes by Burnettising (using chloride of zinc, a process patented by William Burnett) in the workshops at Lostwithiel. The various timbers were seated in cast iron chairs, and wrought iron was used for parts of the bridge which would be in tension, such as tie rods between spans. Five distinct types of viaduct were built to suit local conditions at each site. Peter John Margary, the Cornwall Railway engineer from 1868 to 1891, classified them as classes A to E. Class A: The majority of viaducts were constructed on stone piers that rose to about below track level. From the tops of these radiated three fans of timber struts to support the beams beneath the track; one fan on each side and the third beneath the centre of the track. The struts radiated at around 55, 75, 105 and 125 degrees from horizontal (resulting in a \\// form), although there were some slight variations. This gave piers spaced at about centres. Class B: A stronger support was given to St Pinnock, Largin, and Ponsanooth viaducts by replacing the central set of fan struts with a pair. These joined the outer fans at the top of the pier and met each other at the top, giving a W form when viewed across the width of the piers. Class C: The soft ground of the tidal valleys at Weston Mill, Forder, Wivelscombe, and Nottar required a lighter structure. This was achieved by dispensing with the stone piers and replacing them with vertical timber trestles built on top of timber piles driven into the mud. There were no fans supporting the track, instead timber trusses were built directly on top of the timber trestles. Class D: Coombe (by Saltash) and Moorswater viaducts had only two fans on each pier – one on each side – but the bottoms of these were joined by laminated timber beams between each set of fans, which