Sample of boulder clay used during the original construction – to infill the spandrel removed from the Laigh Milton Viaduct on the Kilmarnock & Troon Railway during its restoration in 1995. The use of clay for this purpose was one of the factors which led to the generally poor state of the viaduct. This highlights that the design of the viaduct followed the normal late 18th rather than the best early 19th century practice in which an improved hollow form with longitudinal cross-tied walls which by 1809 were being adopted increasingly by Telford and others. The clay was replaced by concrete during the restoration works.
For an article on the viaduct and its restoration see http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=1243.
For a detailed paper on the viaduct and its restoration see here.