The Perth Bridge is constructed of local "Old Red Sandstone" of Lower Devonian age (about 300 million years old). Rocks like this extend over the low country between the volcanic rocks of the Sidlaw Hills to the south and the metamorphic rocks of the Highlands to the north.
The sandstone contains water-rounded pebbles, mainly of rocks derived from a Highland rather than a southern source. The pebbles are scattered randomly, suggesting deposition from turbulent rivers – probably flash floods. During the Devonian, land plants were rare and the water run-off after rain would have been a powerful agent of erosion.
Nearer to the Highlands the embedded pebbles are larger and boulders become common. This confirms that the source was in the high mountains that were the forerunners of the present Highland hills.
The stones for the Perth Bridge came from Quarrymill, about 3/4 of a mile upstream. They were brought downstream to the site in rafts.
See the Menubar for further information on geology and stones.