What holds up the Forth Bridges?
Tough igneous rocks on either side of the Firth of Forth create a narrow crossing point for its three bridges to span.
The geological jigsaw of the central belt of Scotland comprises two very different rocks: tough igneous rocks formed during an episode of volcanic activity, and much softer sedimentary rocks, such as sandstone, limestone and coal. It’s that geological mix that creates viewpoints such as Salisbury Crags (link) and sites for castles including Stirling, Edinburgh and Dumbarton. The geology mix also creates the varied coastline of the Firth of Forth and the natural point of narrowing at Queensferry.
The igneous rock at North Queensferry is called quartz dolerite. It formed 300 million years ago as part of a huge body of magma stretching east towards the North Sea. The magma was injected sideways into layered sedimentary rock, deep underground, and formed a series of interlinked pods that today create isolated outcrops of hard rock.
To the south of the Forth the shape of the coastline is controlled by similar igneous rock outcrops at Whitehouse Point and Hound Point, and also forms the high ground of Mons Hill.
The narrow crossing point at Queensferry probably had ferries as early as Roman times. Queen Margaret, wife of King Malcolm III, established a free ferry crossing in the 11th century to assist pilgrims on their way to St Andrews.
The Forth Bridge opened in 1890 to carry trains from Edinburgh to Dundee and Aberdeen and cost £3,000,000, extraordinary for its day. Its remarkable cantilever design remains the world’s second-longest single cantilever span (521m) and it was the first major steel construction in Britain. The bridge towers are made of granite from Aberdeen; to span the estuary it makes use of islands of igneous rock, including Inchgarvie.
The Forth Road Bridge replaced a vehicle and pedestrian ferry across the Forth in 1964. With a main span of 1006 metres, when it opened it was the longest suspension bridge outside the United States. Traffic volumes greatly exceeded projections and corrosion weakened the cables leading to the design and construction of a new road bridge, the Queensferry Crossing, which opened in 2017. The opportunity to call it the Third Forth Bridge was not grasped.
The three bridges crossing the Firth, clustered together in a splay from North Queensferry, demonstrate changing times and styles of innovative bridge building. Their location, and the preference to bridge instead of tunnel infrastructure, owes much to the surrounding geology.
‘Thole is a good Scottish word. It means to endure or bear. You might thole the death of a father or brother. The Forth Bridge was built to thole. Its 53,000 tonnes of steel are strong but are also intended to have the appearance of strength, thereby convincing its first nervous rail passengers that it would not collapse, as the Tay Bridge had done.’ Peter Ross
Image: the Forth Bridges from Port Edgar
by Jason Gilchrist jasongilchrist.co.uk
Geodiversity case study: The Queensferry Crossing – Geodiversity in a major infrastructure project
The Forth Bridge, Peter Ross