The River Tay is tidal and Perth is as far up the River as medieval ships could sail, and as far down as a bridge could be built.
The discharge of the Tay exceeds that of any other British river – three earlier bridges were swept away when the Tay was in flood. The Perth Bridge was built between 1766 and 1772 by John Smeaton (1724-1792) – the first to call himself a Civil (as opposed to a Military) Engineer.
A contemporary traveller wrote: “The stones of which it is chiefly composed are a reddish sand-stone [click for a picture] containing in it’s [sic] substance a variety of round polished stones of different kinds and sizes. The quarry from whence the stone was brought, lies about a quarter of a mile farther up the river, and its strata dip obliquely towards the North.” [D.M. Henderson and J.H. Dickson, A Naturalist in the Highlands: James Robertson: His Life and Travels in Scotland, 1767-1771, Scottish Academic Press, 1994, p.151-152 (June 2nd 1771). I am grateful to Dr Iain Robertson for this reference]. Stone from Kincarrathie [Quarrymill] was mentioned by Robert the Bruce in 1328. The spandrels (between the arches) are filled with dark-coloured whinstone [basalt]. Whinstone and red sandstone – Perth's two local rocks – are Lower Devonian in age.
Footpaths were attached to the original bridge in 1869. The original parapets were removed and the stone used in nearby buildings.
The bridge has nine arches. The westernmost is a dry arch through which there is access to the North Inch. Known flood-levels of the river are marked on the north side of the dry arch, which is a convenient place to examine the building stones of the bridge.